Heavy Metal Contamination and Bod Kinetics of Septic Tank Fecal Sludge: Implications for Environmental Risk

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This study assessed the physico-chemical characteristics, pollution potential, and biodegradation kinetics of septic tank fecal sludge in Nsukka, Nigeria. Thirty-two samples were collected from eight septic tanks, stratified into four fractions: top of liquid (TL), bottom of liquid (BL), top of sludge (TS), and bottom of sludge (BS). Heavy metal concentrations (Fe, Pb, Cr, Cu, Ni, Cd, Zn, and Mn), Biological Oxygen Demand (BOD₁–BOD₉), and BOD degradation rate constants (k) were analyzed using standard methods. Pollution and ecological risks were evaluated using the Nemero Comprehensive Pollution Index (NCPI), Pollution Load Index (PLI), and Geo-accumulation Index (Igeo). Results showed pronounced fraction-specific variability. Heavy metals were generally enriched in sludge layers, with iron exhibiting the highest mean concentration (up to 1271.38 mg/L in BS), while manganese showed the lowest levels. Most metals, particularly Pb, Cd, Cu, Cr, and Ni, exceeded Federal Environmental Protection Agency (FEPA), South African (DWAF), and US-EPA permissible limits. NCPI values ranged from 55.9 to 209.6, classifying the sludge as heavily polluted, while PLI (6.37–28.22) and Igeo (2.09–4.23) indicated strong to extreme contamination, posing severe risks to soil, groundwater, and surface water systems. Correlation analysis revealed significant positive associations among Cu–Ni, Cu–Cr, and Cd–Pb, suggesting common anthropogenic sources and co-mobilization behavior. BOD consumed increased with incubation time and depth, following the order TL < BL < BS < TS, confirming effective solids settling and partial stabilization within septic tanks. BOD degradation rate constants varied widely from 0.0004–0.0673 d⁻¹, with higher k values in liquid fractions indicating greater biodegradability and immediate environmental risk, while low to near-zero k values in bottom sludge reflected highly stabilized, mature material. These findings demonstrate that septic tanks function as partial anaerobic digesters with increasing stabilization with depth and age. Overall, the study highlights the critical need for fraction-specific characterization in fecal sludge management. Fecal sludge heavy metal contamination kinetics pollution indices Environmental impact Figures Figure 1 Figure 2 Figure 3 1.0 INTRODUCTION Though a simple cost-effective, nature-based and energy-efficient sewage treatment system, the septic tank system developed in the 19th century, has retained a key role in sewage treatment and management, especially in developing countries. Despite widely reported cases of failure due to poor design and lack of maintenance, it is still considered the most effective onsite wastewater treatment system due to its robustness, ease of construction, auto-pilot operation and minimal maintenance requirement. (Withers, et al. 2013 ). In several developing countries where socio-economic and demographic factors limit the use of centralized sewage treatment systems, the septic tank system has largely played a vital role in containing environmental degradation and health risk associated with sewage treatment. The high degree of dependence on septic tanks has been corroborated by studies in Nigeria, Rwanda and Somalia (Jacques, 2024 ; Omohwovo, 2024 ; Oluseyi & Nweke, 2020 ). Other studies have also highlighted the prominent role of the septic system in sewage in developed countries. In the United States, 20–25% of households use the septic tanks system, with a higher rate of adoption among newer homes (Capps et al., 2020 ). Sprouse et al ( 2024 ) reported that half of households in North Carolina use the septic tank system. A significant level of adoption has also been reported in Ireland, Australia and France (Dubber and Gill, 2014 ; Beal et al, 2005 ; Dubois and Boutin, 2018 ), further highlighting the global penetration of this simple system. Long detention times associated with the septic tank system enable a high degree of waste stabilization. However, due to time and space stratification, septic tank sludge still contains high Biological Oxygen Demand (BOD) which can lead to oxygen depletion in water bodies when discharged untreated (Imansyah and Karnaningroem, 2020 ; Rasawula et al., 2021). While onsite treatment systems like the septic tanks have proved effective in the stabilization of organic matter, the sludge layer forms a sink of inorganic pollutants and toxic substances, specifically heavy metals. Though originally conceived for organic waste, the increasing concentration of chemical substances in modern wastewater appears to be overwhelming the system, posing severe environmental risk. The physical and biochemical processes within the system are inefficient to cater for persistent chemical substances. The primary contributors to the toxicity of fecal sludge includes; heavy metals, microplastics and organic pollutants. Several studies have reported the presence of high-imapct heavy metals, specifically zinc (Zn), copper (Cu), lead (Pb), cadmium (Cd), chromium (Cr), nickel (Ni), arsenic (As), and mercury (Hg) (Xiao et al., 2023 ; Islam et al., 2023 ; Eliyan et al., 2024 ; Kyayesimira et al., 2019 ). Polycyclic Aromatic Hydrocarbons (PAHs), hexachlorohexane (HCH) and polybrominated diphenyl ethers (PBDEs) have also been reported (Tomczyk et al., 2020 ; Stiborova et al., 2017 ). Exposure to fecal sludge can lead to adverse effects on various organisms, indicating its potential hazards to both human health and the environment. While the environmental risks associated with fecal sludge might be minimal during its residence time in the onsite system, post-disposal impacts have proved to be far reaching. Exposure pathways include uncontrolled application of sewage sludge in crop production, leaching of septic tank effluent into groundwater, inhalation of suspended sludge particulate matter and stream discharge. In Jakarta, fecal sludge from various sources did not comply with wastewater quality standards, risking water body pollution (Suryawan et al., 2022 ). Utilizing fecal sludge as a solid fuel is a potential alternative, as it can reduce environmental burdens while providing energy, with calorific values reaching up to 2,853 kcal/kg (Suryawan et al., 2022 ), but concerns due to heavy metal concentrations, which can accumulate in crops and contaminate soil and water sources limits its usage (Rosik-Dulewska, 2003 ). Sewage sludge has been implicated in toxicity-related sensitivity, oxidative stress and histopathological changes in earthworms even at low concentrations (Babić et al. ( 2016 ); Rastetter & Gerhardt, 2016 ) as well as tissue damage, reduced survival rates and mobility in aquatic organisms (Rastetter & Gerhardt, 2017; da Silva et al., 2020). While direct ingestion of contact with human is considered minimal, significant health impacts have been reported due to consumption of crops grown with sewage and occupational exposure among sewage workers (Sklar et al., 2019 ; Tiwari, 2008 ) Effective fecal sludge management is crucial for public health and environmental protection, reduces the risk of waterborne diseases by preventing fecal contamination of water bodies, mitigates pollution and protects ecosystems from untreated waste (Sisay et al., 2024 ). Evolution in design of onsite sewage treatment such as the multi-compartment septic tanks and incorporation of zeolite filters in septic tanks have shown significant improvements in effluent quality (Liu and Lo, 2001 ), but without corresponding mitigation of sludge pollution potential. Also, there is insufficient data regarding the management of fecal sludge and this complicates the planning and public health efforts (Simiyu et al., 2021 ). For a sustainable management of fecal sludge as well as effective planning and construction of onsite treatment systems, there is need for an in-depth understanding of the local context which includes the existing infrastructures and the characteristics of the fecal sludge. 2.0 METHODOLOGY 2.1 Study Area Nsukka is a town and LGA in Enugu State, Nigeria, which is home to Nigeria's first indigenous University, the University of Nigeria, Nsukka (UNN). It is located in latitude 6.8429° N and longitude 7.3733° E in the South Eastern section of Nigeria. It lies in the humid tropical climate zone, with two distinct wet and dry seasons. It has an average temperature of 25 degrees Celsius, an average annual precipitation of 1,695.4mm, an average relative humidity of 56.1%, and an average daily sunlight of 5.6 hours per day. (National Centers for Environmental Information, 2024 ). Like many other urban settlements in Nigeria, Nsukka Urban area relies heavily on the septic tanks, making it an ideal location to study the environmental impact of fecal sludge management. Due to the absence of a centralized sewage system, fecal sludge from these containment systems is often disposed of improperly, posing significant environmental challenges (Maqbool et al 2022 ;). A key focus for studying the environmental impact of fecal sludge in Nsukka Urban area involves evaluating the levels of Biological Oxygen Demand (BOD) and heavy metals in septic tank fecal sludge with a view to predicting environmental impact. 2.2 Study Design and Procedures The samples were collected at the Top of Liquid (TL), Bottom of Liquid (BL), Top of Sludge (TS) and Bottom of Sludge (BS) of eight residential septic tanks in bungalow type buildings desludged within the last ten (10) years. In other to ensure proper collection of samples, a sludge sampler was designed, constructed and tested according to (Nnaji and Ozota, 2025 ). The samples were carefully collected ensuring that there was no mixing between the different fraction and carefully labelled. The various fractions were taken to the laboratory for analysis of BOD and heavy metals. The concentrations of these parameters in each fraction were determined using the standard methods which are the winkler iodometric method for BOD determination and aqua regia digestion method followed by Atomic Absorption Spectrophotometry (AAS) for heavy metal determination. These methods are described in Velkushanova et al. ( 2021 ). The BOD rate constant and ultimate BOD were obtained by the method of least square. The BOD of the samples was determined on days 1, 3, 5, 7 and 9 to determine biodegradation trends and pattern in different layers. The mean values and standard deviation of the BOD in each fraction was calculated and was used to analyze the trend across each fraction. The Biological Oxygen Demand (BOD) rate constant (k) was calculated using the least square method. It was applied to determine the first-order BOD degradation rate constant (k) and the ultimate BOD (L u ) of septic tank fecal sludge samples. The least squares approach fits a theoretical model to experimental BOD data by minimizing the sum of the squared deviations between observed and predicted values. The BOD exertion follows the first-order reaction model (Eq. 1 ). $$\:\frac{\text{dy}}{\begin{array}{c}\text{d}\text{t}{\left.\text{}\right|}_{{\text{t}}_{{\text{=}}_{\text{n}}}}\\\:\end{array}}\text{=}\text{k}\text{(}{\text{L}}_{\text{0}}\text{-}{\text{y}}_{\text{n}}\text{)}$$ 1 Where y is the BOD exerted at time t, L o is the ultimate BOD and k is the BOD rate constant. Following the results obtained, other kinetic parameters were obtained as follows. The ultimate BOD (L u ), characteristic time ( \(\:\text{τ}\) ), being the time it will take to achieve complete biodegradation and the biodegradation half life, being the length of time it will take to reduce half of the ultimate BOD were obtained as average values for each septic tank strata. Heavy metals were determined using Flame Atomic Absorption Spectroscopy (FAAS). Sludge samples (5 mL) were digested with 10 mL of aqua regia (HCl:HNO₃, 3:1 v/v) by heating for 30 minutes. After cooling, the digest was filtered and diluted to 50 mL with deionized water. Metal concentrations were analyzed using a Buck Scientific Atomic Absorption Spectrometer (Model 210 VGP) equipped with element-specific hollow cathode lamps and an air–acetylene flame. Absorbance measurements were carried out at the characteristic analytical wavelengths of each metal: Cu (324.7 nm), Cr (357.9 nm), Ni (232.0 nm), Pb (283.3 nm), Zn (213.9 nm), Fe (248.3 nm), Cd (228.8 nm) and Mn (279.5 nm). Calibration curves were prepared using standard solutions in the range of 0.00–0.80 mg/L, with regression coefficients (R² ≥ 0.999) to ensure analytical accuracy. Absorbance was detected using a photomultiplier tube, and metal concentrations were quantified according to Beer–Lambert’s law. Statistical Analysis and Determination of Environmental Hazards Experimental results were subjected to inferential and diagnostic statistical analyses to explore possible relationship between various parameters. The Pollution Indices (P i ) Nemero Comprehensive Pollution Index (NCPI), Geoaccumulation Index (Igeo) method and Pollution Load Index (PLI) method were used to assess the potential pollution levels in the various fecal sludge fractions as shown in Equations 2 to 5. P i = C i /B i (2) Where: C i = Concentration of the indicator in the sample; B i = Standard or permissible limit for that indicator (United States Environmental Protection Agency (USA-EPA) under the National Pollutant Discharge Elimination System (NPDES) reports from waste water treatment plants); P i = Pollution indices for each indicator. The NCPI was then calculated using the formular below; NCPI = √ (P 2 max + P 2 avg )/2 (3) Where: P 2 max is the maximum value among all pollution indices; P 2 avg = Average of all pollution indices. The NCPI values are rated as follows: NCPI < 0.7 - clean, 0.7 ≤ NCPI < 1.0 - slightly polluted, 1.0 ≤ NCPI < 2.0 - moderately polluted, NCPI ≥ 2.0 - heavily polluted. Igeo quantifies the degree of heavy metal pollution by comparing current concentrations with pre-industrial levels and is given as follows. I geo = log 2 (C i /1.5B i ) (4) The classification of I geo values is as follows: I geo ≤ 0 - unpolluted, 0 < I geo ≤ 1 - npolluted to moderately polluted, 1 < I geo ≤ 2: moderately polluted, 2 < I geo ≤ 3 - moderately to strongly polluted, 3 < I geo ≤ 4: strongly polluted, 4 5: extremely polluted. The Pollution Load Index (PLI) is a tool used to assess the overall level of heavy metal contamination in a given medium, such as soil, water, or fecal sludge. It provides a cumulative measure of heavy metal pollution and is calculated to determine whether the medium is contaminated or within safe limits. PLI = (P 1 * P 2 * P 3 *………. * P n ) 1/n (5) Where P n = Contamination Factor for each metal and n number of metals. PLI 1 - contamination exists, where progressively higher values indicate greater pollution). 3.0 RESULTS AND DISCUSION 3.1 Concentration and Vertical Fractionation of Heavy Metals in Septic Tank Sludge In this study, the highest concentration of heavy metals was observed in the sludge layer (exception of chromium with its highest concentration at the bottom of the liquid) with iron (Fe) having the highest mean concentration of 1271.38mg/L in the bottom of the sludge (BS) and manganese having the least concentration of 0.2544mg/L at the top of the liquid (TL). The concentration of heavy metal in the fecal sludge is in this decreasing order: Fe > Pb>Cr > Cu>Ni > Cd > Zn > Mn. The mean values of the concentration of heavy metals in the septic tanks gave a high standard deviation some being twice the mean values. These high variations reflect the highly variable nature of septic tank influent. Sources of heavy metal are; cleansing materials, soap, detergents, flush water etc. Figure 1 is a graphical representation of the mean values of the heavy metal concentrations in each layer of the fecal sludge. The mean values for Lead (Pb) concentration ranged from 7.53mg/L at the top of liquid layer to 28.74mg/L at the top of sludge layer. These values are lower than the concentration of Pb (34.56-102.83 mg/L) in municipal sewage sludge in South Africa (Shamuyarira and Gumbo, 2014 ), However, this value is much higher than the maximum permissible lead (Pb) concentration of 1 mg/L in wastewater as stipulated by the Department of Water Affairs and Forestry (DWAF, 1996 ) and the Federal Environmental Protection Agency (FEPA), Nigeria. Also, the United States Environmental Protection Agency (EPA) regulates the concentration of lead in waste water under the National Pollutant Discharge Elimination System (NPDES) limits for lead in waste water. According to these regulations, the concentration for lead in waste water before discharge into water bodies range from 0.05 to 0.1mg/L. The sources of lead (Pb) in septic tank are the inputs into the septic tank like cleaning materials, detergents and soap. Lead (Pb) is a toxic heavy metal and can accumulate in the environment through improper management of fecal sludge which is predominant in Nsukka urban area and this has serious environmental implications due to the risk of accumulation in water, soil and plants (Kyayesimira et al., 2019 ). The concentration of copper (Cu) in all samples varied widely and very much higher than the DWAF ( 1996 ) guideline value of 0.2 mg/L and the FEPA limit of 1.0 mg/L. The concentrations ranged from 0.0078-75.25mg/L both in the Top liquid and bottom sludge fraction of the septic tank. This is lower than the value of 280 mg/L in Cambodia (Eliyan et al., 2024 ). The presence of copper in the sewage sludge could be due to the corrosion of water supply pipes or the use of brass which is alloy of copper and zinc or abrasives in washing pots and cleaning the kitchen (Shamuyarira and Gumbo, 2014 ). Just like Lead (Pb), cadmium which enters the environment via wear and tear of the rubber tires and high traffic maybe washed into the septic tank by the storm water drains. Cadmium (Cd) range of below detection limit (BDL) − 6.8471 mg/L which is above the DWAF ( 1996 ) guideline for cadmium (0.01 to 0.05 mg/L) as well as BDL − 3.79 mg/L and BDL − 2.3 mg/L reported by Liu and Sun (2013) and Eliyan et al. ( 2024 ) respectively. Manganese (Mn) distribution in various fractions showed a number of different patterns for all fraction with mean concentrations ranging from 0.2544mg/L at the top of liquid layer to 1.241mg/L at the bottom of the sludge layer. The top liquid exhibited a relatively low mean concentration (0.25 mg/L) but high variability (SD = 0.22 mg/L), indicating uneven Mn distribution in the liquid phase. The bottom liquid showed a higher mean concentration (0.40 mg/L) with very high variability (SD = 0.52 mg/L), reflecting strong heterogeneity likely influenced by interactions with settled sludge. The top sludge fraction recorded a moderately elevated mean Mn concentration (0.43 mg/L) with moderate variability (SD = 0.34 mg/L), suggesting more consistent Mn retention compared to liquid fractions. In contrast, the bottom sludge exhibited the highest mean concentration (1.24 mg/L) and extreme variability (SD = 1.76 mg/L), indicating substantial Mn accumulation with localized enrichment. Manganese concentrations in various types of sludge have been reported to range from 3.6 to 17.6 mg/kg (Liu et al 2012 ). Environmental factors such as composition of urban waste water, surface run-off of contaminants onto artificialized areas and the presence of persistent compounds can lead to Mn accumulation in sewage sludge. The mean concentration of nickel ranging from 1.243mg/L at the top of the liquid layer to 13mg/L at the bottom of the sludge layer was higher than the DWAF and FEPA guideline values of 2 and 1.0 mg/L in effluent wastewater, but within the same range as that reported by Shamuyarira and Gumbo, ( 2014 ). The mean values of Chromium (Cr) concentrations (9.82mg/L at the top of the liquid layer to 20.72mg/L at the bottom of the liquid layer) are above the DWAF 0.05 mg/L maximum permissible limit but less than that reported by Shamuyarira and Gumbo, ( 2014 ) (35.07–134.48mh/kg). Mean Zinc (Zn) concentrations (0.8467mg/L at the top of the liquid layer to 1.9706mg/L at the bottom of the sludge layer) were generally within the 1 mg/L and 2 mg/L FEPA and DWAF guidelines for effluent water samples respectively. Table 1 shows Heavy metal limit values in Waste Water Effluent (mg/L) and Table 2 shows a descriptive statistic of the various parameters. Heavy metal concentration significantly impacts septic tank efficiency. Research in Nigeria highlighted that heavy metals like iron, lead, chromium, and zinc were found in septic tank effluents, affecting microbial remediation processes (Oyem and Oyem 2022 ). In Hanoi, the presence of copper, zinc, chromium, and lead hindered anaerobic co-digestion, reducing substrate degradation and biogas production in sludge mixtures (Nguyen et al 2019 ). Heavy metal concentrations in septic tanks vary significantly based on the location and conditions. Studies have shown that heavy metals like cadmium, copper, lead, nickel, and zinc can be present in varying amounts in different layers of septic tanks, with concentrations being highly variable (Iverson et al 2022 ). Table 1 Heavy metal limit values in Waste Water (mg/L). Metal USA-EPA under NPDES Regulation South African (DWAF Guideline) Nigeria (FEPA ( 2001 ) Guideline) This Study(mg/L) Cd 0.005–0.01 0.01–0.05 0.1 0.6661 TL , 1.956 BL , 1.940 TS , 2.053 BS Ni 0.1–0.5 2.0 1.0 480.14 TL , 387.14 BL , 864.9 TS , 1271 BS Zn 1.0–2.0 2.0 1.0 0.8467 TL , 1.289 BL , 1.616 TS , 1.97 BS Cu 0.5-1.0 0.2 1.0 1.6452 TL , 3.885 BL , 4.664 TS , 15.86 BS Cr 0.1–0.5 0.05 0.5 9.8086 TL , 20.724 BL , 9.859 TS , 11.74 BS Pb 0.05–0.1 1.0 1.0 7.5342 TL , 23.061 BL , 28.74 TS , 25.75 BS TL = Top of Liquid, BL= Bottom of Liquid, TS = Top of Sludge, BS=Bottom of Sludge Table 2 Descriptive statistics for all parameters (for the various layers sampled) Parameters Top of Liquid (TL) Bottom of Liquid (BL) Top of Sludge (TS) Bottom of Sludge (BS) Mean SD Max Min Mean SD Max Min Mean SD Max Min Mean SD Max Min BOD BOD1(mg/L) 52.50 38.48 112.9 40.6 62.20 35.34 112.9 60.28 115.88 87.00 225 66.8 77.16 49.42 177.4 76 BOD3(mg/L) 187.46 85.30 261.25 80.6 195.90 91.27 261.5 80.6 199.58 91.06 241.95 90 213.73 136.44 161.25 78 BOD5(mg/L) 325.4 116.90 342 182.4 378.46 193.54 349 180.2 480.27 259.87 560.5 166.8 424.61 273.56 322.5 180.6 BOD7(mg/L) 323.6 322.87 822.75 204 324.64 205.16 512.9 380.2 428.54 293.00 674.44 564.5 375.18 267.58 682.82 322.5 BOD9(mg/L) 587.10 525.24 1,120.2 443.85 653.87 578.84 1340.4 492.2 784.15 602.99 1,524 921 720.27 618.24 1,440 403.1 Heavy Metals Pb (mg/L) 7.5342 6.260 17.62 2.74 23.061 33.380 41.10 2.891 28.74 36.23 106.96 2.290 25.7525 27.8612 58.73 0.34 Fe (mg/L) 480.14 879.38 2356.3 2.07 387.14 717.47 837.5 3.819 864.9 1305 3080.8 1.605 1271.38 2015.72 4967.3 6.44 Mn (mg/L) 0.2544 0.224 0.61 0.02 0.398 0.522 0.779 0.025 0.430 0.340 0.911 0.027 1.2410 1.7608 5.34 0.04 Ni (mg/L) 1.2435 1.164 3.09 0.00 1.496 1.508 4.190 0.020 1.175 1.262 3.650 0.020 13.0053 19.6870 47.80 0.00 Cr (mg/L) 9.8086 10.941 27.92 0.00 20.724 32.793 87.95 0.069 9.859 10.10 25.848 0.000 11.7485 16.7351 48.34 2.32 Cu (mg/L) 1.6452 2.879 6.51 0.03 3.885 8.807 25.25 0.034 4.664 11.42 32.750 0.025 15.8689 29.6977 75.25 0.01 Cd (mg/L) 0.6661 0.709 1.64 0.00 1.956 2.358 6.847 0.000 1.940 1.920 5.303 0.000 2.0532 2.0410 5.14 0.00 Zn (mg/L) 0.8467 0.627 1.77 0.04 1.289 0.972 2.669 0.276 1.616 0.988 2.961 0.000 1.9706 1.1138 3.54 0.28 3.2 Potential Pollution and Ecological Impact Of Septic Tank Sludge The pollution potential index of the various fecal sludge fractions was determined as discussed earlier and presented in Table 3 . The NCPI values ranged from 55.9 to 209.6 thus classifying the fecal sludge as heavily polluted. The PLI values ranged from 6.37 to 28.22 thus the fecal sludge is greatly contaminated. Also, the Lgeo values ranged from 2.07 to 4.23, this imply that the sampled fecal sludge is strongly to extremely polluted. Hence, the fecal sludge samples pose a significant environmental threat for land and water disposal considering the selected indicators. Table 3 NCPI, PLI, L geo values for the various indicators in the different fractions Sample Pb(mg/l) Ni Cr Cu Cd Zn NCPI PLI L geo (Avg) TL 7.53 0.78 8.58 1.65 0.48 0.85 55.90 6.37 2.09 BL 23.06 1.50 20.72 3.89 1.22 1.29 169.81 14.36 3.26 TS 28.75 1.17 8.63 4.66 1.21 1.62 209.60 13.22 3.14 BS 25.75 13.01 11.75 15.87 2.05 1.97 192.48 28.22 4.23 3.2.1 Environmental pollution risks of septic tank fecal sludge disposal When disposed on land or discharged into water bodies, septic tank effluent contaminates both the ground and surface water through leaching and runoff and ca also accumulate in edible plants. In this study, the NCPI values ranged from 55.9 to 209.6 (Table 3 ), indicating an exceptionally high contamination potential and suggesting that the surrounding environment is highly vulnerable to pollutant migration. Such elevated NCPI levels imply that heavy metals and associated contaminants can readily leach through the soil profile, increasing the likelihood of groundwater pollution, including elevated dissolved metal concentrations, increased salinity, and alterations to natural hydrogeochemical conditions (Fatta-Kassinos et al., 2011 ). High NCPI values also signify a strong risk of contamination of surface water bodies through runoff and seepage, which may result in metal enrichment, sediment contamination, reduced ecological integrity, and potential bioaccumulation across aquatic food webs (Ali et al., 2019 ). These effects collectively shows that the observed NCPI values far exceed typical risk thresholds and present a substantial threat to the environment. The Pollution Load Index (PLI) values ranged from 6.367 to 28.22 (Table 3 ), indicating a severe level of contamination far above the threshold value of PLI > 1, which typically signifies progressive environmental degradation. Such elevated PLI values suggest that the concentration of heavy metals in the study area has surpassed the natural geochemical background levels, increasing the likelihood of contaminant mobility into groundwater and surface water systems. High PLI values imply that soils and sediments may no longer act as effective sinks, thereby facilitating the leaching of toxic metals into underlying aquifers, where they can deteriorate groundwater quality, increase salinity and electrical conductivity, and pose potential health risks to dependent communities. Similarly, surface water bodies receiving runoff or seepage from contaminated sites are at heightened risk of heavy-metal enrichment, sediment contamination, and subsequent ecological stress, including reduced biodiversity and bioaccumulation across aquatic food webs. The Geo-accumulation Index (Igeo) values for the septic tank fecal sludge ranged from 2.086 to 4.236 (Table 3 ), indicating contamination levels from moderately to heavily polluted to heavily polluted according to Müller’s classification. Such elevated Igeo values demonstrate that the concentrations of heavy metals in the sludge are significantly above natural background levels, implying strong anthropogenic enrichment. Disposing this sludge directly into the environment poses a substantial risk to groundwater because high metal loads can leach through the soil profile, especially under conditions of high permeability, low organic matter, or acidic pH, leading to long-term deterioration of aquifer quality. Likewise, runoff and surface deposition of this contaminated sludge can introduce heavy metals into surface water bodies, where they may accumulate in sediments, increase metal bioavailability, and disrupt aquatic ecosystems through toxicity, bioaccumulation, and reduced biodiversity. Extremely high pollution indices in fecal sludge reflect substantial heavy metal enrichment in sludge fractions (Table 3 ), with values far exceeding safe baseline levels for Pb, Ni, Cr, Cu, Cd, and Zn. When used for soil enrichment, it enhances the bioavailability of metals to plants through altered soil chemistry (e.g., pH changes and increased labile metal pools), which has been linked with elevated uptake and translocation of heavy metals into edible plant parts in multiple recent studies (e.g., excessive Cd, Pb, Cr and Ni in leafy and root vegetables grown on contaminated soils) (e.g., high bioaccumulation and health risk indices in crops irrigated with polluted water sources) (Balkhair and Ashraf, 2016 ). When soils are amended with metal-rich sludge, increased heavy metal concentrations in root zones promote uptake into crops, as demonstrated by recent work showing significant metal levels in edible portions of vegetables grown on polluted soils, frequently exceeding safe limits and raising potential health risks upon consumption (e.g., elevated Hazard Index values) (Abd-elhalim et al, 2025 ). These processes not only threaten crop safety and productivity but also risk groundwater and surface water contamination through leaching and runoff, particularly under high rainfall or irrigation scenarios. Therefore, although fecal sludge offers nutrient benefits for soil fertility, its use without adequate treatment and monitoring could increase heavy metal mobilization into the food chain and aquatic environments. High NCPI, PLI, and Lgeo values observed in the septic tank fecal sludge fractions (Table 3 ) indicate severe heavy-metal contamination and imply significant risks beyond soil and water pathways, particularly through the mobilization of dry sludge particles. When fecal sludge dries during handling, storage, transportation, or land application, contaminated fine particles may become airborne and form metal-laden dust. Such particulates can transport toxic metals including Pb, Cr, Ni, Cd, and Cu, which are commonly associated with fine particulate matter and readily inhaled into the respiratory tract. This inhalation pathway represents an important but often underestimated exposure route, particularly for sanitation workers and nearby populations. The inhalation of heavy-metal-contaminated dust poses significant human health risks, as fine particles can penetrate deep into the lungs and facilitate systemic absorption of metals into the bloodstream. Chronic exposure to inhaled Pb, Cd, and Cr has been associated with respiratory inflammation, neurotoxicity, renal impairment, and increased carcinogenic risk, even at relatively low airborne concentrations. Studies have shown that particulate-bound metals originating from contaminated soils and waste residues contribute substantially to non-dietary human exposure, especially in dry and windy environments common in many low- and middle-income regions. Given the elevated pollution indices observed in this study, uncontrolled drying and dispersal of septic tank fecal sludge could therefore increase airborne metal exposure and compound public health risks, underscoring the need for controlled handling practices, moisture management, and protective measures during sludge reuse or disposal (Ferreira-Baptista & De Miguel, 2005 ; Alloway, 2013 ). Overall, the combined evidence from the NCPI, PLI, and Igeo assessments clearly demonstrates that the fecal sludge examined in this study poses a substantial environmental risk, with the potential to severely degrade both groundwater and surface water resources. The exceptionally high NCPI values, the critically elevated PLI values, and the Igeo classifications showing moderate to heavy pollution collectively confirm that the sludge contains heavy metals far above natural background concentrations and well beyond internationally recognized contamination thresholds. These indicators highlight not only the current pollution burden but also the high mobility and persistence of these contaminants in the environment, increasing the likelihood of leaching, runoff, and long-term ecological damage. If disposed of without adequate treatment or containment, the sludge can act as a continuous source of heavy-metal loading to hydrological systems, resulting in groundwater deterioration, surface-water enrichment, sediment contamination, and potential bioaccumulation in aquatic organisms. 3.2.2 Toxicological effects of heavy metals in septic tank fecal sludge disposal on soil microorganisms . In the study area, septic tank sludge is usually disposed on land without further treatment and subsequently used by vegetable farmers to boost soil fertility. The NCPI values ranged obtained from 55.9 to 209.6 (Table 3 ) indicating an extremely high contamination potential with pronounced negative impact on soil microorganisms, which are highly sensitive indicators of soil health. At moderate NCPI levels, soils experience reduced microbial biomass and enzyme activities but may retain partial resilience. Studies show that microbial dehydrogenase (DH) and urease activities decline as metal concentrations rise, reflecting inhibited respiration and nitrogen cycling (Farsang et al., 2020 ). However, at very high NCPI values (Table 3 ), soils exhibit acute toxicity, causing a collapse in microbial diversity and enzymatic functionality (Hu et al., 2014 ; Zhao et al., 2020 ). Heavy metals such as Cu, Pb, Zn, Cd, and Cr interact with microbial cell membranes, enzymes, and DNA, leading to oxidative stress and metabolic inhibition (Ansari et al., 2022 ). High NCPI soils tend to favor metal-tolerant taxa such as Pseudomonas, Bacillus, and certain Actinobacteria, while sensitive nitrifiers and decomposers decline. The resulting community shift reduces soil fertility and organic matter turnover (Mkhinini et al., 2020 ). Moreover, the elevated NCPI range corresponds to contamination levels comparable to industrial mining zones, where soil enzymes such as phosphatase and catalase are severely inhibited, indicating irreversible microbial damage (Xu et al., 2018 ). These impacts compromise soil ecosystem services, including nutrient cycling, organic matter decomposition, and pathogen suppression. The PLI values ranging 6.367 to 28.222 and the Lgeo values ranging from 2.086 to 4.234 indicates an extremely polluted fecal sludge which have detrimental effects on soil microorganisms. Soil microorganisms are responsible for the breakdown of organic matters into forms that it can be easily absorbed by plants and also used as energy source for other microorganisms. Hence, the disposal of septic tank fecal sludge containing elevated concentrations of heavy metals poses serious toxicological threats to soil microorganisms, it exerts inhibitory effects on microbial metabolic functions even at relatively low concentrations. These metals can bind to microbial cell membranes, disrupt membrane permeability, inhibit enzyme activity, and interfere with essential cellular processes such as respiration, DNA replication, and protein synthesis (Giller et al., 2009 ). 3.3 Correlation Between the Heavy Metals The Heavy metal parameters showed significant correlation value suggesting a significant level of interaction and source commonality. Copper (Cu) and Nickel (Ni) showed a significant positive correlation value of 0.876. This shows a strong level of interaction and commonality of sources, which is consistent with recent soil contamination studies showing that metals with similar geochemical behavior often co-occur due to shared anthropogenic influences (e.g., industrial effluents, domestic pollution) and common binding mechanisms in soil matrices (Ma et al., 2023 ), and also, copper and Nickel bioavailability is significantly influenced by pH and organic matter. Lower pH increases both metal solubility and presence of organic matter can complex with the metals reducing their free ion availability. Also, a strong Cu–Ni correlation (r = 0.876) indicates co-mobilization and common origin, most likely from domestic corrosion, personal care products, and household chemical use. Their similar ionic behavior results in parallel accumulation in sludge and co-binding to organic matter. Ecologically, Cu–Ni combination poses heightened risk to soil microorganisms when sludge is land-applied. Copper (Cu) and chromium (Cr) also showed a significant positive correlation value of 0.597 and this is because their bioavailability is also influenced by pH and organic matter. All the metals with significant correlation like iron (Fe) and Lead (Pb), Cadmium (Cd) and Lead (Pb) have a positive correlation value and this is because their availability in nature is directly been influenced by factors such as; pH which affect the solubility of heavy metals, high pH values increase the solubility of heavy metals and lower pH values reduce the solubility of heavy metals. Organic matter also affects the bioavailability of heavy metals, heavy metals bond to organic matters through complexation their-by affecting its solubility and mobility, these complexes can either increase or decrease their bioavailability depending on their stability and environmental conditions. Ecologically, co-occurrence poses heightened risks to soil microorganisms and plant uptake, as multiple heavy metals can have synergistic toxic effects on microbial communities and disrupt key soil functions, consistent with findings that heavy metal mixtures impair microbial enzyme activity and soil nutrient cycling (Yu et al., 2023 ). These emphasizes the need for source control and pre-treatment to limit their concentrations. 3.4 Organic Pollution Potential of Sewage Sludge The mean value of BOD consumed across the days for day nine (9) ranged from 587.1mg/L at the top of liquid layer (TL) to 784.15mg/L at the top of sludge layer (TS). The mean values of the BOD consumed for all the layers were on the increase from Day 1 to 5 (carbonaceous biochemical oxygen demand), a slight decrease was observed in Day 7 which signifies a decrease in the carbonaceous biochemical oxygen demand and a resumption of the nitrogenous biochemical oxygen demand which is responsible for nitrogen fixation (Young, and Vanrolleghem, 2021 ). The range of the mean values of BOD consumed for the various fractions are: Top of Liquid: 52.5 ± 38.48mg/L to 587.11 ± 525.24mg/L, Bottom of Liquid: 62.2 ± 35.34mg/L to 653.88 ± 578.85mg/L, Top of Sludge: 115.88 ± 87.0mg/L to 784.15 ± 602.99mg/L, Bottom of Sludge 77.17 ± 49.43 to 720.28 ± 618.24mg/L. See Fig. 2 for these observed trends. A trend was observed across the various fractions. The BOD consumed increased from the top of liquid to the bottom of the fecal sludge as shown in Fig. 3 . The BOD consumed at the top of the fecal sludge was lower than that at the bottom of the fecal sludge because the bottom of the sludge contains older organic solids which has undergone anaerobic digestion indicating stabilization and sequestration of stabilized organic matter. The BOD consumed is in this ascending order: TL < BL<BS < TS, showing that septic tank system aid in the removal of BOD through settling of solids. The BOD values obtained from this study are higher than and quite variable from the BOD values reported by SANDEC from different regions (Heinss et al, 1994 ) as shown in Table 4 . This can be linked to factors such as; storage duration, admixture to FS such as grease, organic waste from kitchen, temperature, performance of septic tank, tank emptying technology and pattern, common treatment of black and greywater. Also, as fecal sludge is very heterogeneous consisting of scum, liquid, partly settled and settled layers each layer with its own distinct characteristics, collection of representative samples from septic tanks is one of the greatest challenges in fecal sludge characterization studies. Table 4 BOD concentration in fecal sludge from other studies and this study Author Study location Number of Onsite sanitation system sampled BOD 5 concentration(mg/L) Type of Fecal Sludge This Study Nsukka-Nigeria 8 325.4 TL , 378.46 BL , 480.27 TS , 424.61 BS Septic Tank SANDEC (Heinss et al, 1994 ) Accra-Ghana 60 600–1500 Septage SANDEC (Heinss et al, 1994 ) Accra-Ghana - 7600 Public Toilet Generally, in alignment with established guidelines like those from WHO and the Environmental Conservation Rules (ECR) of various regions the BOD standard in effluent before disposal into water bodies should be less than 30mg/L (Tahri et al., 2016 ), but variations in local environmental conditions and pollution sources influence this standard. Some water bodies because of its characteristics such as temperature, flow rate, pH and so on might be able to take higher concentration of BOD while some because of its characteristics will not be able to handle the 30mg/L bench mark so it’s important to know the characteristics of the receiving water before discharging fecal sludge effluent into it. Untreated or poorly treated fecal sludge with high BOD when it enters water bodies accelerates microbial activity that consumes dissolved oxygen. This leads to oxygen depletion, which can create hypoxic (low-oxygen) or anoxic (no-oxygen) zones, harming aquatic life and potentially causing fish kills (Prasanna et al., 2021 ). High-BOD septic sludge that infiltrates soils from the soak pit can lead to nutrient and pathogen leaching, contaminating groundwater sources critical for drinking water, especially in areas dependent on wells and aquifers (Chun et al., 2014 ). It’s important that septic tanks are well located away from ground water sources to avoid contamination. 3.5 Biological Oxygen Demand Rate Degradation and ultimate BOD of the Sampled Septic Tanks Studies on BOD degradation rate in fecal sludge are essential for understanding organic matter stabilization kinetics, optimizing treatment system design, predicting oxygen demand in receiving environments, and assessing microbial activity and environmental risks associated with fecal sludge disposal and reuse. Table 4 shows the calculated values of the BOD degradation constant (k) for the various septic tanks and fractions (TL, BL, TS and BS). The BOD degradation rate constants (k) varied markedly among the top of liquid (TL), bottom of liquid (BL), top of sludge (TS), and bottom of sludge (BL) fractions. The liquid layer exhibited higher average rate constant (0.021 d − 1 TL and 0.021 d − 1 for BL) than the sludge layer (0.013 d − 1 for TS and 0.012 d − 1 for BS), indicating that the liquid layer contains more readily biodegradable substances than the sludge layer. Gravitational separation is the primary and fastest process within the septic tank, leading to partial clarification of the liquid layer and leaving highly soluble, dilute and highly dispersed content that are easily accessible to microorganisms. On the other hand, the densification of high molecular weight and partially soluble materials in the sludge layer tend to slow down the biedegradation process. Though the septic tank system is generally over-simplified as an anaerobic reactor, the intermittent input of oxygen from influent wastewater generates micro-aerobic conditions with better mixing in the liquid layer, thus improving reaction kinetics. However, the sludge layer operates under strict anaerobic or facultative anaerobic conditions, which inherently exhibit slower organic matter degradation rates due to limited diffusion, slow hydrolysis and dominance of methanogens with associated slow kinetics. Anaerobic biochemical reactions proceed at lower energy yields and slower microbial growth rates, which explains the low k values even when substantial amounts of BOD are ultimately consumed (Burton et al., 2013 ). Table 4 BOD rate degradation in the various fractions Using Least square Method Septic Tank Effluent Kinetics Top Liquid Layer (TL) Bottom Liquid Layer (BL) Top Sludge Layer (TS) Bottom Sludge Layer (BS) BOD constant (day − 1 ) 0.021 0.026 0.013 0.012 Characteristic time, \(\:\text{τ}\) = 1/k (days) 47.51 39.01 75.51 85.65 Ultimate BOD, L u (mg/L) 1116.63 1337.50 9770.57 15669.0 Biodegradation half life, t 1/2 (days) 32.93 27.04 52.34 59.37 While the BOD degradation rate constant (k) describes the speed at which biodegradable organic matter is oxidized, the ultimate BOD (L u ) represents the total oxygen demand exerted when all biodegradable organic matter is fully stabilized. The L u values obtained in this study varied widely across septic tanks and sludge fractions (Table 4 ), indicating pronounced heterogeneity in organic matter content, degree of stabilization, and biodegradability. Generally, higher L u values were observed in the sludge fractions (TS and BS) compared to the liquid fractions (TL and BL), reflecting the accumulation of particulate and slowly biodegradable organic matter within the sludge matrix. This is consistent with the role of septic tanks as partial anaerobic digesters, where solids retention promotes the buildup of long-term biodegradable organic reserves even as short-term BOD is reduced (Burton et al., 2013 ; Strande et al., 2014 ). The elevated L u values observed in older septic tanks and deeper sludge layers suggest the presence of substantial residual organic loading, despite low or near-zero k values in some fractions. A persistent increase in ultimate BOD with depth from 1116.63 mg/L in the top liquid layer to 15,669.0 mg/L in the bottom sludge layer indicates a clearly defined stratification of the septic tank content occasioned by a combination of physical/gravitational separation which leads to the accumulation of denser and more resistant materials near the tank bottom. The accumulation of slowly biodegradable and particulate organic matter in the sludge layer is responsible for the high ultimate BOD (Lu). The top of sludge (TS) contains partially degraded organic matter, recently settled solids, and active fermentative microbes while the bottom sludge contains older, compacted, highly stabilized material, recalcitrant complex organics (lignocellulosic residues, microbial biomass remnants) and reduced availability of biodegradable carbon. Very low or negative BOD rate constants indicate very low biodegradation rates, consistent with aged or mature fecal sludge and anaerobic condition which exist in the sludge fraction of septic tanks (Strande et al., 2014 ; Grady et al., 2011 ). The characteristic time ( \(\:\text{τ}\) ) for the sludge layer is about twice (75.5 days for top sludge and 85.6 days for bottom sludge) that of the liquid layer with 47.5 days and 39.01 days for top liquid and bottom liquid layers respectively. This indicates that although biodegradation proceeds slowly in stabilized or aged sludge, a large fraction of organic matter remains capable of exerting oxygen demand over extended periods if remobilized. Such conditions pose a significant environmental risk when sludge is excavated, land-applied, or discharged into aquatic environments, as the slow oxidation of this residual organic matter can lead to prolonged oxygen depletion, sediment oxygen demand, and secondary anaerobic conditions. From a fecal sludge management (FSM) perspective, the combined interpretation of k, L u and \(\:\text{τ}\) is critical for treatment system design and environmental risk assessment. Fractions with high k, moderate L u and lower \(\:\text{τ}\) values present immediate oxygen demand risks and require rapid stabilization, while fractions with low k but high \(\:\text{τ}\) and L u necessitate longer treatment times or post-stabilization processes to prevent delayed pollution impacts. Ignoring L u in FSM planning may underestimate the long-term oxygen demand and environmental footprint of disposed sludge, particularly under conditions that enhance re-aeration or microbial reactivation. Therefore, incorporating ultimate BOD alongside degradation kinetics provides a more comprehensive basis for selecting appropriate treatment technologies, retention times, and disposal strategies for septic tank fecal sludge (Strande et al., 2014 ). 4.0 CONCLUSION Heavy metals are critical parameters in fecal sludge characterization due to their persistence, bioaccumulation, and well-documented links to chronic human health conditions such as cancer, cardiovascular, kidney, and liver diseases. Their presence also directly affects septic tank performance by inhibiting microbial activity and reducing biodegradation efficiency. Previous studies have shown that metals such as iron, lead, chromium, copper, zinc, and cadmium can interfere with anaerobic digestion processes, lower substrate degradation rates, and reduce biogas production, with concentrations varying widely across septic tank layers depending on location and input sources. These variations highlight the necessity of routinely assessing heavy metal concentrations prior to sludge disposal or reuse to ensure compliance with environmental regulations and to minimize ecological and public health risks. In this study, heavy metals were predominantly concentrated in the sludge layers, with iron showing the highest mean concentration (1271.38 mg/L in bottom sludge) and manganese the lowest (0.2544 mg/L in top liquid), following the general order Fe > Pb > Cr > Cu > Ni > Cd > Zn > Mn. Most metals, particularly lead, copper, cadmium, nickel, and chromium, exceeded national (FEPA, DWAF) and international (US-EPA/NPDES) permissible limits for wastewater discharge, indicating significant pollution potential. The high standard deviations observed reflect strong variability in household inputs such as detergents, cleaning agents, plumbing materials, and surface runoff. These elevated concentrations pose serious environmental concerns, as improper fecal sludge management can facilitate heavy metal accumulation in soils, crops, aquatic systems, and ultimately the food chain. Overall, the findings emphasize the need for stricter control of sludge disposal practices, targeted treatment strategies, and regulatory enforcement to mitigate the environmental and health risks associated with heavy metals in fecal sludge. The pollution and ecological risk assessment using NCPI, PLI, and Igeo clearly demonstrates that the fecal sludge across all fractions is severely to extremely polluted with heavy metals. NCPI values (55.9–209.6), PLI values (6.37–28.22), and Igeo classifications (moderately to extremely polluted) far exceed accepted safety thresholds, indicating strong anthropogenic enrichment and high contamination potential. These elevated indices imply that disposal of untreated fecal sludge poses a substantial threat to both groundwater and surface water systems through leaching, runoff, and sediment accumulation, with risks of long-term water quality degradation, metal bioaccumulation in aquatic ecosystems, and subsequent human health impacts. The results confirm that soils and sediments in contact with such sludge may no longer function as effective contaminant sinks, increasing the likelihood of pollutant mobility and persistence in the environment. Beyond hydrological impacts, the high pollution indices also indicate acute toxicological risks to soil microbial communities, which are essential for nutrient cycling and organic matter decomposition. Extremely high NCPI, PLI, and Igeo values are associated with strong inhibition of microbial biomass, enzyme activities, and metabolic functions, leading to reduced soil fertility and impaired ecosystem services. The observed significant correlations among heavy metals, particularly Cu–Ni and Cu–Cr, suggest common anthropogenic sources and co-mobilization mechanisms influenced by pH and organic matter, further amplifying ecological risks when sludge is land-applied. Overall, the combined pollution indices and metal correlations emphasize that septic tank fecal sludge in the study area represents a critical environmental hazard, underscoring the urgent need for source control, targeted treatment, and strict regulatory enforcement before disposal or reuse to prevent irreversible damage to soil and water ecosystems. This study demonstrated that septic tank fecal sludge in Nsukka exhibits high organic strength and strong spatial heterogeneity across liquid and sludge fractions. Mean BOD concentrations increased both with incubation time and depth, following the order TL < BL < BS < TS, indicating effective settling of organic solids and progressive stabilization with depth. The higher BOD values in sludge layers, particularly at the top of sludge, reflect the presence of partially degraded organic matter, while lower BOD at the bottom of sludge confirms more advanced digestion and maturity. Although BOD concentrations observed were lower than those reported in some SANDEC studies, they remain far above recommended discharge limits, highlighting the significant risk posed by untreated or poorly treated fecal sludge to surface water and groundwater through oxygen depletion, nutrient loading, and pathogen transport. The BOD degradation rate constants (k) further revealed marked fraction-specific biodegradation behavior within septic tanks. Higher k values in liquid fractions, especially at the bottom of the liquid layer, indicate greater biodegradability, higher microbial activity, and rapid oxygen demand, whereas very low or near-zero k values in bottom sludge signify highly stabilized, mature material with limited remaining biodegradable carbon. These kinetic differences confirm that septic tanks function as partial anaerobic digesters, with stabilization increasing with depth and age. Importantly, the results show that fecal sludge management strategies must account for fraction-specific kinetics, as high-k fractions pose immediate environmental risks upon disposal, while low-k fractions require longer retention times and more intensive treatment. Overall, the findings emphasize that ignoring these kinetic variations can lead to treatment system under-performance or overload, underscoring the need for tailored design, appropriate treatment selection, and controlled disposal to protect soil and water resources. Declarations Ethical Approval: • The research work followed all ethical compliance. All participants involved during the sample collection from onsite sanitation systems (septic tanks) were pre-informed before the collection and the samples were collected ensuring that the environment was not polluted. Consent to Participate: • All participants that were involved during the collection of preliminary data were properly informed. Consent to Publish: • All contributing authors has read the manuscript and proper approval for publication was given Competing Interests: • The authors have no relevant financial or non-financial interests to disclose. Funding: The authors declare that no funds, grants, or other support were received during the preparation of this manuscript. Author Contributions: All authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by Ozota, Chidera Emmanuel and under the supervision of Nnaji, Chidozie Charles . The first draft of the manuscript was written by Ozota, Chidera Emmanuel and Nnaji, Chidozie Charles commented and made all necessary corrections on the manuscript. All authors read and approved the final manuscript for submission. ACKNOWLEDGMENTS The authors sincerely appreciate the support and contributions that made this research possible. We extend our gratitude to the Department of Civil Engineering, University of Nigeria Nsukka, Enugu, Nigeria and Faculty of Engineering and the Built Environment, University of Johannesburg, Johannesburg, South Africa , for providing the necessary facilities and resources for this study. We are also grateful to all individuals and institutions that assisted in data collection and fieldwork, including septic tank owners who granted access to their facilities. Special thanks go to colleagues and research assistants for their valuable input and technical support during the study. Finally, we acknowledge the reviewers and editorial team of Environmental Science and Pollution Research for their insightful feedback and guidance in improving this manuscript. 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Int J Environ Chem Ecotoxicol Res 4(1):15–35. https://doi.org/10.37745/10.37745/ijecer.16/vol4n11532 Prasanna K, Annadurai R, Godson MD, Murali A, Ashok I, Krishnan MV (2021) Treatment of septic tank effluent using sequencing batch reactor along with the incorporation of rice husk and Bael pericarp as a natural adsorbent in reducing BOD and COD. IOP Conference Series: Materials Science and Engineering, 1101(1), 012021. https://doi.org/10.1088/1757-899x/1101/1/012021 Rasawula Lukman R, Eka Pratiwi Y, Rosdiana R (2021) Evaluation of Operational Techniques on the Performance of Fecal Sludge Treatment Plants in Kendari City. TELUK Journal: Environ Eng Univ Muhammadiyah Kendari 1(1):1–7. https://doi.org/10.51454/teluk.v1i1.119 Rastetter N, Gerhardt A (2016) Toxic potential of different types of sewage sludge as fertiliser in agriculture: ecotoxicological effects on aquatic, sediment and soil indicator species. J Soils Sediments 17(1):106–121. https://doi.org/10.1007/s11368-016-1468-4 Rosik-Dulewska C (2003) Environmental Impact of Sewage Sludge Application for Non-Industrial Purposes. In Environmental Engineering Studies. Springer US 259–271. https://doi.org/10.1007/978-1-4419-8949-9_25 Shamuyarira K, Gumbo J (2014) Assessment of Heavy Metals in Municipal Sewage Sludge: A Case Study of Limpopo Province, South Africa. Int J Environ Res Public Health 11(3):2569–2579. https://doi.org/10.3390/ijerph110302569 Simiyu S, Chumo I, Mberu B (2021) Fecal Sludge Management in Low Income Settlements: Case Study of Nakuru, Kenya. Front Public Health 9. https://doi.org/10.3389/fpubh.2021.750309 Sisay SF, Gari SR, Ambelu A (2024) Fecal Sludge Management and Sanitation Safety: An Assessment in Addis Ababa, Ethiopia. Environ Health Insights 18. https://doi.org/10.1177/11786302241267187 Sklar R, Zhou Z, Zalay M, Muspratt A, Hammond SK (2019) Occupational Exposure to Endotoxin along a Municipal Scale Fecal Sludge Collection and Resource Recovery Process in Kigali, Rwanda. Int J Environ Res Public Health 16(23):4740. https://doi.org/10.3390/ijerph16234740 Sprouse L, Kryston A, Lebu S, Muoghalu C, Woods C, Manga M (2024) Septic systems in North Carolina: A neglected half of the state? PLOS Water 3(10):e0000304. https://doi.org/10.1371/journal.pwat.0000304 Stiborova H, Kolar M, Vrkoslavova J, Pulkrabova J, Hajslova J, Demnerova K, Uhlik O (2017) Linking toxicity profiles to pollutants in sludge and sediments. J Hazard Mater 321:672–680. https://doi.org/10.1016/j.jhazmat.2016.09.051 Strande L, Ronteltap M, Brdjanovic D (2014) Faecal Sludge Management. IWA Publishing. https://doi.org/10.2166/9781780404738 Suryawan IWK, Lim J-W, Ramadan BS, Septiariva IY, Sari NK, Sari MM, Zahra NL, Qonitan FD, Sarwono A (2022) Effect of sludge sewage quality on heating value: case study in Jakarta, Indonesia. Desalination Water Treat 249:183–190. https://doi.org/10.5004/dwt.2022.28071 Tahri M, Bachiri B, Larif M, Taky M, Elamrani M, Midaoui E, Benazouz A, K., Khimani M (2016) Physicochemical and microbiological quality of the treated wastewater of the Marrakech WWTP for irrigation. Moroccan J Chem 4(3). https://doi.org/10.48317/IMIST.PRSM/MORJCHEM-V4I3.5368 Tiwari R (2008) Occupational health hazards in sewage and sanitary workers. Indian J Occup Environ Med 12(3):112. https://doi.org/10.4103/0019-5278.44691 Tomczyk B, Siatecka A, Jędruchniewicz K, Sochacka A, Bogusz A, Oleszczuk P (2020) Polycyclic aromatic hydrocarbons (PAHs) persistence, bioavailability and toxicity in sewage sludge- or sewage sludge-derived biochar-amended soil. Sci Total Environ 747:141123. https://doi.org/10.1016/j.scitotenv.2020.141123 United States Environmental Protection Agency (USEPA) (2024) : A Guide for Land Appliers on the Requirements of the Federal Standards for the Use or Disposal of Sewage Sludge, 40 CFR Part 503 Velkushanova K, Strande L, Ronteltap M, Koottatep T, Brdjanovic D, Buckley C (eds) (2021) Methods for Faecal Sludge Analysis. IWA Publishing. https://doi.org/10.2166/9781780409122 Withers PJ, Jordan P, May L, Jarvie HP, Deal NE (2013) Do septic tank systems pose a hidden threat to water quality? Front Ecol Environ 12(2):123–130. https://doi.org/10.1890/130131 Xiao D, Li H, Wang Y, Wen G, Wang C (2023) Distribution Characteristics of Typical Heavy Metals in Sludge from Wastewater Plants in Jiangsu Province (China) and Their Potential Risks. Water 15(2):313. https://doi.org/10.3390/w15020313 Xu Z, Wu J, Li H, Chen Y, Xu J, Xiong L, Zhang J (2018) Characterizing heavy metals in combined sewer overflows and its influence on microbial diversity. Sci Total Environ 625:1272–1282. https://doi.org/10.1016/j.scitotenv.2017.12.338 Young JC, Vanrolleghem PA (2021) Carbonaceous vs. total biochemical oxygen demand as a basis for WRRF design and performance monitoring. Water Environ Res 93(9):1510–1515. https://doi.org/10.1002/wer.1541 Yu H, Li C, Yan J, Ma Y, Zhou X, Yu W, Kan H, Meng Q, Xie R, Dong P (2023) A review on adsorption characteristics and influencing mechanism of heavy metals in farmland soil. RSC Adv 13(6):3505–3519. https://doi.org/10.1039/d2ra07095b Zhao X, Sun Y, Huang J, Wang H, Tang D (2020) Effects of soil heavy metal pollution on microbial activities and community diversity in different land use types in mining areas. Environ Sci Pollut Res 27(16):20215–20226. https://doi.org/10.1007/s11356-020-08538-1 Cite Share Download PDF Status: Under Review Version 1 posted Reviewers agreed at journal 21 Apr, 2026 Reviewers invited by journal 04 Apr, 2026 Editor invited by journal 23 Feb, 2026 Editor assigned by journal 17 Feb, 2026 First submitted to journal 15 Feb, 2026 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-8824149","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":617641518,"identity":"d19d221f-acf0-4e22-bf5c-de2080b3ab88","order_by":0,"name":"Chidozie Charles Nnaji","email":"","orcid":"","institution":"University of Nigeria Faculty of Engineering","correspondingAuthor":false,"prefix":"","firstName":"Chidozie","middleName":"Charles","lastName":"Nnaji","suffix":""},{"id":617641519,"identity":"e7f036dc-abb0-4d5b-8b51-f7ef8c4fb85d","order_by":1,"name":"Chidera Emmanuel Ozota","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAABBklEQVRIiWNgGAWjYBADGQYJxgYJIEMOxDvwgAgtPDAtxmAtCcRpYQAhhsQGEBefFn6x04mfC9tsePhnNzfe+LjHLn1+2OGHQFvs5HQbsGuRnJ27WXpmWxqPxJ2DzZYzniXnbrydZgDUkmxsdgC7FoPbuRukedsO8xhIJLZJ8xxgzt04OwGk5UDiNhxa7G/nbv7N2/YfouXPgfp0w9npH/BqMZDO3Qa05QBEC8OBwwny0jn4bZG4nbvNmudcMo/EjcRmy54Dxw03SOcUHEgwwO0XfqD3b/OU2cnxz0h/eOPHgWp5+dnpmz98qLCTw6UFi1PBKg2IVQ4C8g2kqB4Fo2AUjIKRAACdGWGD6Q56hgAAAABJRU5ErkJggg==","orcid":"https://orcid.org/0009-0002-5236-0414","institution":"University of Nigeria Faculty of Engineering","correspondingAuthor":true,"prefix":"","firstName":"Chidera","middleName":"Emmanuel","lastName":"Ozota","suffix":""}],"badges":[],"createdAt":"2026-02-08 21:06:45","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-8824149/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-8824149/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":106779956,"identity":"e59a7d6f-acae-401d-a3da-35951bbc1737","added_by":"auto","created_at":"2026-04-13 11:28:02","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":115857,"visible":true,"origin":"","legend":"\u003cp\u003eWhiskers and Box plot of Heavy Metal Concentrations in each fraction of the fecal sludge\u003c/p\u003e","description":"","filename":"Picture1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8824149/v1/31118edbf33d6f384a55afbf.jpg"},{"id":106779957,"identity":"7b9525f3-2376-4d13-82e9-c95ab4a2540b","added_by":"auto","created_at":"2026-04-13 11:28:02","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":130574,"visible":true,"origin":"","legend":"\u003cp\u003emean value of BOD consumed across the days at various fraction\u003c/p\u003e","description":"","filename":"Picture2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8824149/v1/a9ea524c3f7bb41b4ed440f7.jpg"},{"id":106779954,"identity":"e7263608-aa1d-428f-8412-cf6e65e0c28d","added_by":"auto","created_at":"2026-04-13 11:28:02","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":117402,"visible":true,"origin":"","legend":"\u003cp\u003emean value plot of BOD for concentration in the different fractions\u003c/p\u003e","description":"","filename":"Picture3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8824149/v1/de6298729ff10fee31afc6f5.jpg"},{"id":106780000,"identity":"4d2f1b03-7ec4-4d0b-b3b4-a0bfde23684b","added_by":"auto","created_at":"2026-04-13 11:28:11","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1660175,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8824149/v1/b35e4772-b5ac-4223-8177-bbf3b6a8c370.pdf"}],"financialInterests":"","formattedTitle":"\u003cp\u003eHeavy Metal Contamination and Bod Kinetics of Septic Tank Fecal Sludge: Implications for Environmental Risk\u003c/p\u003e","fulltext":[{"header":"1.0 INTRODUCTION","content":"\u003cp\u003eThough a simple cost-effective, nature-based and energy-efficient sewage treatment system, the septic tank system developed in the 19th century, has retained a key role in sewage treatment and management, especially in developing countries. Despite widely reported cases of failure due to poor design and lack of maintenance, it is still considered the most effective onsite wastewater treatment system due to its robustness, ease of construction, auto-pilot operation and minimal maintenance requirement. (Withers, et al. \u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e2013\u003c/span\u003e). In several developing countries where socio-economic and demographic factors limit the use of centralized sewage treatment systems, the septic tank system has largely played a vital role in containing environmental degradation and health risk associated with sewage treatment. The high degree of dependence on septic tanks has been corroborated by studies in Nigeria, Rwanda and Somalia (Jacques, \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2024\u003c/span\u003e; Omohwovo, \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e2024\u003c/span\u003e; Oluseyi \u0026amp; Nweke, \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Other studies have also highlighted the prominent role of the septic system in sewage in developed countries. In the United States, 20\u0026ndash;25% of households use the septic tanks system, with a higher rate of adoption among newer homes (Capps et al., \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Sprouse et al (\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e2024\u003c/span\u003e) reported that half of households in North Carolina use the septic tank system. A significant level of adoption has also been reported in Ireland, Australia and France (Dubber and Gill, \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Beal et al, \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2005\u003c/span\u003e; Dubois and Boutin, \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2018\u003c/span\u003e), further highlighting the global penetration of this simple system.\u003c/p\u003e \u003cp\u003eLong detention times associated with the septic tank system enable a high degree of waste stabilization. However, due to time and space stratification, septic tank sludge still contains high Biological Oxygen Demand (BOD) which can lead to oxygen depletion in water bodies when discharged untreated (Imansyah and Karnaningroem, \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Rasawula et al., 2021). While onsite treatment systems like the septic tanks have proved effective in the stabilization of organic matter, the sludge layer forms a sink of inorganic pollutants and toxic substances, specifically heavy metals. Though originally conceived for organic waste, the increasing concentration of chemical substances in modern wastewater appears to be overwhelming the system, posing severe environmental risk. The physical and biochemical processes within the system are inefficient to cater for persistent chemical substances. The primary contributors to the toxicity of fecal sludge includes; heavy metals, microplastics and organic pollutants. Several studies have reported the presence of high-imapct heavy metals, specifically zinc (Zn), copper (Cu), lead (Pb), cadmium (Cd), chromium (Cr), nickel (Ni), arsenic (As), and mercury (Hg) (Xiao et al., \u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Islam et al., \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Eliyan et al., \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2024\u003c/span\u003e; Kyayesimira et al., \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). Polycyclic Aromatic Hydrocarbons (PAHs), hexachlorohexane (HCH) and polybrominated diphenyl ethers (PBDEs) have also been reported (Tomczyk et al., \u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Stiborova et al., \u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). Exposure to fecal sludge can lead to adverse effects on various organisms, indicating its potential hazards to both human health and the environment. While the environmental risks associated with fecal sludge might be minimal during its residence time in the onsite system, post-disposal impacts have proved to be far reaching. Exposure pathways include uncontrolled application of sewage sludge in crop production, leaching of septic tank effluent into groundwater, inhalation of suspended sludge particulate matter and stream discharge. In Jakarta, fecal sludge from various sources did not comply with wastewater quality standards, risking water body pollution (Suryawan et al., \u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). Utilizing fecal sludge as a solid fuel is a potential alternative, as it can reduce environmental burdens while providing energy, with calorific values reaching up to 2,853 kcal/kg (Suryawan et al., \u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e2022\u003c/span\u003e), but concerns due to heavy metal concentrations, which can accumulate in crops and contaminate soil and water sources limits its usage (Rosik-Dulewska, \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e2003\u003c/span\u003e). Sewage sludge has been implicated in toxicity-related sensitivity, oxidative stress and histopathological changes in earthworms even at low concentrations (Babić et al. (\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2016\u003c/span\u003e); Rastetter \u0026amp; Gerhardt, \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e2016\u003c/span\u003e) as well as tissue damage, reduced survival rates and mobility in aquatic organisms (Rastetter \u0026amp; Gerhardt, 2017; da Silva et al., 2020). While direct ingestion of contact with human is considered minimal, significant health impacts have been reported due to consumption of crops grown with sewage and occupational exposure among sewage workers (Sklar et al., \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e2019\u003c/span\u003e; Tiwari, \u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e2008\u003c/span\u003e)\u003c/p\u003e \u003cp\u003eEffective fecal sludge management is crucial for public health and environmental protection, reduces the risk of waterborne diseases by preventing fecal contamination of water bodies, mitigates pollution and protects ecosystems from untreated waste (Sisay et al., \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). Evolution in design of onsite sewage treatment such as the multi-compartment septic tanks and incorporation of zeolite filters in septic tanks have shown significant improvements in effluent quality (Liu and Lo, \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2001\u003c/span\u003e), but without corresponding mitigation of sludge pollution potential. Also, there is insufficient data regarding the management of fecal sludge and this complicates the planning and public health efforts (Simiyu et al., \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). For a sustainable management of fecal sludge as well as effective planning and construction of onsite treatment systems, there is need for an in-depth understanding of the local context which includes the existing infrastructures and the characteristics of the fecal sludge.\u003c/p\u003e"},{"header":"2.0 METHODOLOGY","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Study Area\u003c/h2\u003e \u003cp\u003eNsukka is a town and LGA in Enugu State, Nigeria, which is home to Nigeria's first indigenous University, the University of Nigeria, Nsukka (UNN). It is located in latitude 6.8429\u0026deg; N and longitude 7.3733\u0026deg; E in the South Eastern section of Nigeria. It lies in the humid tropical climate zone, with two distinct wet and dry seasons. It has an average temperature of 25 degrees Celsius, an average annual precipitation of 1,695.4mm, an average relative humidity of 56.1%, and an average daily sunlight of 5.6 hours per day. (National Centers for Environmental Information, \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). Like many other urban settlements in Nigeria, Nsukka Urban area relies heavily on the septic tanks, making it an ideal location to study the environmental impact of fecal sludge management. Due to the absence of a centralized sewage system, fecal sludge from these containment systems is often disposed of improperly, posing significant environmental challenges (Maqbool et al \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2022\u003c/span\u003e;). A key focus for studying the environmental impact of fecal sludge in Nsukka Urban area involves evaluating the levels of Biological Oxygen Demand (BOD) and heavy metals in septic tank fecal sludge with a view to predicting environmental impact.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Study Design and Procedures\u003c/h2\u003e \u003cp\u003eThe samples were collected at the Top of Liquid (TL), Bottom of Liquid (BL), Top of Sludge (TS) and Bottom of Sludge (BS) of eight residential septic tanks in bungalow type buildings desludged within the last ten (10) years. In other to ensure proper collection of samples, a sludge sampler was designed, constructed and tested according to (Nnaji and Ozota, \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). The samples were carefully collected ensuring that there was no mixing between the different fraction and carefully labelled. The various fractions were taken to the laboratory for analysis of BOD and heavy metals. The concentrations of these parameters in each fraction were determined using the standard methods which are the winkler iodometric method for BOD determination and aqua regia digestion method followed by Atomic Absorption Spectrophotometry (AAS) for heavy metal determination. These methods are described in Velkushanova et al. (\u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). The BOD rate constant and ultimate BOD were obtained by the method of least square. The BOD of the samples was determined on days 1, 3, 5, 7 and 9 to determine biodegradation trends and pattern in different layers. The mean values and standard deviation of the BOD in each fraction was calculated and was used to analyze the trend across each fraction.\u003c/p\u003e \u003cp\u003eThe Biological Oxygen Demand (BOD) rate constant (k) was calculated using the least square method. It was applied to determine the first-order BOD degradation rate constant (k) and the ultimate BOD (L\u003csub\u003eu\u003c/sub\u003e) of septic tank fecal sludge samples. The least squares approach fits a theoretical model to experimental BOD data by minimizing the sum of the squared deviations between observed and predicted values. The BOD exertion follows the first-order reaction model (Eq.\u0026nbsp;\u003cspan refid=\"Equ1\" class=\"InternalRef\"\u003e1\u003c/span\u003e).\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$\\:\\frac{\\text{dy}}{\\begin{array}{c}\\text{d}\\text{t}{\\left.\\text{}\\right|}_{{\\text{t}}_{{\\text{=}}_{\\text{n}}}}\\\\\\:\\end{array}}\\text{=}\\text{k}\\text{(}{\\text{L}}_{\\text{0}}\\text{-}{\\text{y}}_{\\text{n}}\\text{)}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eWhere y is the BOD exerted at time t, L\u003csub\u003eo\u003c/sub\u003e is the ultimate BOD and k is the BOD rate constant. Following the results obtained, other kinetic parameters were obtained as follows. The ultimate BOD (L\u003csub\u003eu\u003c/sub\u003e), characteristic time (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\text{\u0026tau;}\\)\u003c/span\u003e\u003c/span\u003e), being the time it will take to achieve complete biodegradation and the biodegradation half life, being the length of time it will take to reduce half of the ultimate BOD were obtained as average values for each septic tank strata.\u003c/p\u003e \u003cp\u003eHeavy metals were determined using Flame Atomic Absorption Spectroscopy (FAAS). Sludge samples (5 mL) were digested with 10 mL of aqua regia (HCl:HNO₃, 3:1 v/v) by heating for 30 minutes. After cooling, the digest was filtered and diluted to 50 mL with deionized water. Metal concentrations were analyzed using a Buck Scientific Atomic Absorption Spectrometer (Model 210 VGP) equipped with element-specific hollow cathode lamps and an air\u0026ndash;acetylene flame. Absorbance measurements were carried out at the characteristic analytical wavelengths of each metal: Cu (324.7 nm), Cr (357.9 nm), Ni (232.0 nm), Pb (283.3 nm), Zn (213.9 nm), Fe (248.3 nm), Cd (228.8 nm) and Mn (279.5 nm). Calibration curves were prepared using standard solutions in the range of 0.00\u0026ndash;0.80 mg/L, with regression coefficients (R\u0026sup2; \u0026ge; 0.999) to ensure analytical accuracy. Absorbance was detected using a photomultiplier tube, and metal concentrations were quantified according to Beer\u0026ndash;Lambert\u0026rsquo;s law.\u003c/p\u003e \u003cp\u003e \u003cb\u003eStatistical Analysis and Determination of Environmental Hazards\u003c/b\u003e \u003c/p\u003e \u003cp\u003eExperimental results were subjected to inferential and diagnostic statistical analyses to explore possible relationship between various parameters. The Pollution Indices (P\u003csub\u003ei\u003c/sub\u003e) Nemero Comprehensive Pollution Index (NCPI), Geoaccumulation Index (Igeo) method and Pollution Load Index (PLI) method were used to assess the potential pollution levels in the various fecal sludge fractions as shown in Equations 2 to 5.\u003c/p\u003e \u003cp\u003eP\u003csub\u003ei\u003c/sub\u003e = C\u003csub\u003ei\u003c/sub\u003e/B\u003csub\u003ei\u003c/sub\u003e (2)\u003c/p\u003e \u003cp\u003eWhere: C\u003csub\u003ei\u003c/sub\u003e = Concentration of the indicator in the sample; B\u003csub\u003ei\u003c/sub\u003e = Standard or permissible limit for that indicator (United States Environmental Protection Agency (USA-EPA) under the National Pollutant Discharge Elimination System (NPDES) reports from waste water treatment plants); P\u003csub\u003ei\u003c/sub\u003e = Pollution indices for each indicator. The NCPI was then calculated using the formular below;\u003c/p\u003e \u003cp\u003eNCPI = \u0026radic; (P\u003csup\u003e2\u003c/sup\u003e\u003csub\u003emax\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;P\u003csup\u003e2\u003c/sup\u003e\u003csub\u003eavg\u003c/sub\u003e)/2 (3)\u003c/p\u003e \u003cp\u003eWhere: P\u003csup\u003e2\u003c/sup\u003e\u003csub\u003emax\u003c/sub\u003e is the maximum value among all pollution indices; P\u003csup\u003e2\u003c/sup\u003e\u003csub\u003eavg\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;Average of all pollution indices. The NCPI values are rated as follows: NCPI\u0026thinsp;\u0026lt;\u0026thinsp;0.7 - clean, 0.7\u0026thinsp;\u0026le;\u0026thinsp;NCPI\u0026thinsp;\u0026lt;\u0026thinsp;1.0 - slightly polluted, 1.0\u0026thinsp;\u0026le;\u0026thinsp;NCPI\u0026thinsp;\u0026lt;\u0026thinsp;2.0 - moderately polluted, NCPI\u0026thinsp;\u0026ge;\u0026thinsp;2.0 - heavily polluted. \u003cb\u003eIgeo\u003c/b\u003e quantifies the degree of heavy metal pollution by comparing current concentrations with pre-industrial levels and is given as follows.\u003c/p\u003e \u003cp\u003eI\u003csub\u003egeo\u003c/sub\u003e = log\u003csub\u003e2\u003c/sub\u003e(C\u003csub\u003ei\u003c/sub\u003e/1.5B\u003csub\u003ei\u003c/sub\u003e) (4)\u003c/p\u003e \u003cp\u003eThe classification of I\u003csub\u003egeo\u003c/sub\u003e values is as follows: I\u003csub\u003egeo\u003c/sub\u003e \u0026le; 0 - unpolluted, 0\u0026thinsp;\u0026lt;\u0026thinsp;I\u003csub\u003egeo\u003c/sub\u003e \u0026le; 1 - npolluted to moderately polluted, 1\u0026thinsp;\u0026lt;\u0026thinsp;I\u003csub\u003egeo\u003c/sub\u003e\u0026le; 2: moderately polluted, 2\u0026thinsp;\u0026lt;\u0026thinsp;I\u003csub\u003egeo\u003c/sub\u003e \u0026le; 3 - moderately to strongly polluted, 3\u0026thinsp;\u0026lt;\u0026thinsp;I\u003csub\u003egeo\u003c/sub\u003e \u0026le; 4: strongly polluted, 4\u0026thinsp;\u0026lt;\u0026thinsp;I\u003csub\u003egeo\u003c/sub\u003e \u0026le; 5: strongly to extremely polluted, I\u003csub\u003egeo\u003c/sub\u003e \u0026gt; 5: extremely polluted. The Pollution Load Index (PLI) is a tool used to assess the overall level of heavy metal contamination in a given medium, such as soil, water, or fecal sludge. It provides a cumulative measure of heavy metal pollution and is calculated to determine whether the medium is contaminated or within safe limits.\u003c/p\u003e \u003cp\u003ePLI = (P\u003csub\u003e1\u003c/sub\u003e * P\u003csub\u003e2\u003c/sub\u003e * P\u003csub\u003e3\u003c/sub\u003e *\u0026hellip;\u0026hellip;\u0026hellip;. * P\u003csub\u003en\u003c/sub\u003e)\u003csup\u003e1/n\u003c/sup\u003e (5)\u003c/p\u003e \u003cp\u003eWhere P\u003csub\u003en\u003c/sub\u003e= Contamination Factor for each metal and n number of metals. PLI\u0026thinsp;\u003cb\u003e\u0026lt;\u003c/b\u003e\u0026thinsp;1 is classified as no contamination (low pollution), PLI\u0026thinsp;=\u0026thinsp;1 represents baseline contamination (average pollution), PLI\u0026thinsp;\u0026gt;\u0026thinsp;1 - contamination exists, where progressively higher values indicate greater pollution).\u003c/p\u003e \u003c/div\u003e"},{"header":"3.0 RESULTS AND DISCUSION","content":"\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e3.1 Concentration and Vertical Fractionation of Heavy Metals in Septic Tank Sludge\u003c/h2\u003e \u003cp\u003eIn this study, the highest concentration of heavy metals was observed in the sludge layer (exception of chromium with its highest concentration at the bottom of the liquid) with iron (Fe) having the highest mean concentration of 1271.38mg/L in the bottom of the sludge (BS) and manganese having the least concentration of 0.2544mg/L at the top of the liquid (TL). The concentration of heavy metal in the fecal sludge is in this decreasing order: Fe\u0026thinsp;\u0026gt;\u0026thinsp;Pb\u0026gt;Cr\u0026thinsp;\u0026gt;\u0026thinsp;Cu\u0026gt;Ni\u0026thinsp;\u0026gt;\u0026thinsp;Cd\u0026thinsp;\u0026gt;\u0026thinsp;Zn\u0026thinsp;\u0026gt;\u0026thinsp;Mn. The mean values of the concentration of heavy metals in the septic tanks gave a high standard deviation some being twice the mean values. These high variations reflect the highly variable nature of septic tank influent. Sources of heavy metal are; cleansing materials, soap, detergents, flush water etc. Figure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e is a graphical representation of the mean values of the heavy metal concentrations in each layer of the fecal sludge. The mean values for Lead (Pb) concentration ranged from 7.53mg/L at the top of liquid layer to 28.74mg/L at the top of sludge layer. These values are lower than the concentration of Pb (34.56-102.83 mg/L) in municipal sewage sludge in South Africa (Shamuyarira and Gumbo, \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e2014\u003c/span\u003e), However, this value is much higher than the maximum permissible lead (Pb) concentration of 1 mg/L in wastewater as stipulated by the Department of Water Affairs and Forestry (DWAF, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e1996\u003c/span\u003e) and the Federal Environmental Protection Agency (FEPA), Nigeria. Also, the United States Environmental Protection Agency (EPA) regulates the concentration of lead in waste water under the National Pollutant Discharge Elimination System (NPDES) limits for lead in waste water. According to these regulations, the concentration for lead in waste water before discharge into water bodies range from 0.05 to 0.1mg/L. The sources of lead (Pb) in septic tank are the inputs into the septic tank like cleaning materials, detergents and soap. Lead (Pb) is a toxic heavy metal and can accumulate in the environment through improper management of fecal sludge which is predominant in Nsukka urban area and this has serious environmental implications due to the risk of accumulation in water, soil and plants (Kyayesimira et al., \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). The concentration of copper (Cu) in all samples varied widely and very much higher than the DWAF (\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e1996\u003c/span\u003e) guideline value of 0.2 mg/L and the FEPA limit of 1.0 mg/L. The concentrations ranged from 0.0078-75.25mg/L both in the Top liquid and bottom sludge fraction of the septic tank. This is lower than the value of 280 mg/L in Cambodia (Eliyan et al., \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). The presence of copper in the sewage sludge could be due to the corrosion of water supply pipes or the use of brass which is alloy of copper and zinc or abrasives in washing pots and cleaning the kitchen (Shamuyarira and Gumbo, \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). Just like Lead (Pb), cadmium which enters the environment via wear and tear of the rubber tires and high traffic maybe washed into the septic tank by the storm water drains. Cadmium (Cd) range of below detection limit (BDL)\u0026thinsp;\u0026minus;\u0026thinsp;6.8471 mg/L which is above the DWAF (\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e1996\u003c/span\u003e) guideline for cadmium (0.01 to 0.05 mg/L) as well as BDL\u0026thinsp;\u0026minus;\u0026thinsp;3.79 mg/L and BDL\u0026thinsp;\u0026minus;\u0026thinsp;2.3 mg/L reported by Liu and Sun (2013) and Eliyan et al. (\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2024\u003c/span\u003e) respectively. Manganese (Mn) distribution in various fractions showed a number of different patterns for all fraction with mean concentrations ranging from 0.2544mg/L at the top of liquid layer to 1.241mg/L at the bottom of the sludge layer. The top liquid exhibited a relatively low mean concentration (0.25 mg/L) but high variability (SD\u0026thinsp;=\u0026thinsp;0.22 mg/L), indicating uneven Mn distribution in the liquid phase. The bottom liquid showed a higher mean concentration (0.40 mg/L) with very high variability (SD\u0026thinsp;=\u0026thinsp;0.52 mg/L), reflecting strong heterogeneity likely influenced by interactions with settled sludge. The top sludge fraction recorded a moderately elevated mean Mn concentration (0.43 mg/L) with moderate variability (SD\u0026thinsp;=\u0026thinsp;0.34 mg/L), suggesting more consistent Mn retention compared to liquid fractions. In contrast, the bottom sludge exhibited the highest mean concentration (1.24 mg/L) and extreme variability (SD\u0026thinsp;=\u0026thinsp;1.76 mg/L), indicating substantial Mn accumulation with localized enrichment. Manganese concentrations in various types of sludge have been reported to range from 3.6 to 17.6 mg/kg (Liu et al \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2012\u003c/span\u003e). Environmental factors such as composition of urban waste water, surface run-off of contaminants onto artificialized areas and the presence of persistent compounds can lead to Mn accumulation in sewage sludge. The mean concentration of nickel ranging from 1.243mg/L at the top of the liquid layer to 13mg/L at the bottom of the sludge layer was higher than the DWAF and FEPA guideline values of 2 and 1.0 mg/L in effluent wastewater, but within the same range as that reported by Shamuyarira and Gumbo, (\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). The mean values of Chromium (Cr) concentrations (9.82mg/L at the top of the liquid layer to 20.72mg/L at the bottom of the liquid layer) are above the DWAF 0.05 mg/L maximum permissible limit but less than that reported by Shamuyarira and Gumbo, (\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e2014\u003c/span\u003e) (35.07\u0026ndash;134.48mh/kg).\u003c/p\u003e \u003cp\u003eMean Zinc (Zn) concentrations (0.8467mg/L at the top of the liquid layer to 1.9706mg/L at the bottom of the sludge layer) were generally within the 1 mg/L and 2 mg/L FEPA and DWAF guidelines for effluent water samples respectively. Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e shows Heavy metal limit values in Waste Water Effluent (mg/L) and Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e shows a descriptive statistic of the various parameters. Heavy metal concentration significantly impacts septic tank efficiency. Research in Nigeria highlighted that heavy metals like iron, lead, chromium, and zinc were found in septic tank effluents, affecting microbial remediation processes (Oyem and Oyem \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). In Hanoi, the presence of copper, zinc, chromium, and lead hindered anaerobic co-digestion, reducing substrate degradation and biogas production in sludge mixtures (Nguyen et al \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). Heavy metal concentrations in septic tanks vary significantly based on the location and conditions. Studies have shown that heavy metals like cadmium, copper, lead, nickel, and zinc can be present in varying amounts in different layers of septic tanks, with concentrations being highly variable (Iverson et al \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2022\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eHeavy metal limit values in Waste Water (mg/L).\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eMetal\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"4\" nameend=\"c5\" namest=\"c2\"\u003e\u0026nbsp;\u003c/th\u003e \u003c/tr\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eUSA-EPA under NPDES\u003c/p\u003e \u003cp\u003eRegulation\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eSouth African (DWAF Guideline)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eNigeria (FEPA (\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2001\u003c/span\u003e) Guideline)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eThis Study(mg/L)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCd\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.005\u0026ndash;0.01\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.01\u0026ndash;0.05\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.6661\u003csup\u003eTL\u003c/sup\u003e, 1.956\u003csup\u003eBL\u003c/sup\u003e, 1.940\u003csup\u003eTS\u003c/sup\u003e, 2.053\u003csup\u003eBS\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNi\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.1\u0026ndash;0.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e2.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e1.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e480.14\u003csup\u003eTL\u003c/sup\u003e, 387.14\u003csup\u003eBL\u003c/sup\u003e, 864.9\u003csup\u003eTS\u003c/sup\u003e, 1271\u003csup\u003eBS\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eZn\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e1.0\u0026ndash;2.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e2.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e1.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.8467\u003csup\u003eTL\u003c/sup\u003e, 1.289\u003csup\u003eBL\u003c/sup\u003e, 1.616\u003csup\u003eTS\u003c/sup\u003e, 1.97\u003csup\u003eBS\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCu\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.5-1.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e1.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1.6452\u003csup\u003eTL\u003c/sup\u003e, 3.885\u003csup\u003eBL\u003c/sup\u003e, 4.664\u003csup\u003eTS\u003c/sup\u003e, 15.86\u003csup\u003eBS\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCr\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.1\u0026ndash;0.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.05\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e9.8086\u003csup\u003eTL\u003c/sup\u003e, 20.724\u003csup\u003eBL\u003c/sup\u003e, 9.859\u003csup\u003eTS\u003c/sup\u003e, 11.74\u003csup\u003eBS\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePb\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.05\u0026ndash;0.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e1.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e1.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e7.5342\u003csup\u003eTL\u003c/sup\u003e, 23.061\u003csup\u003eBL\u003c/sup\u003e, 28.74\u003csup\u003eTS\u003c/sup\u003e, 25.75\u003csup\u003eBS\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003ctfoot\u003e \u003ctr\u003e\u003ctd colspan=\"5\"\u003eTL\u0026thinsp;=\u0026thinsp;Top of Liquid, BL= Bottom of Liquid, TS\u0026thinsp;=\u0026thinsp;Top of Sludge, BS=Bottom of Sludge\u003c/td\u003e\u003c/tr\u003e \u003c/tfoot\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eDescriptive statistics for all parameters (for the various layers sampled)\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"17\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c9\" colnum=\"9\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c10\" colnum=\"10\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c11\" colnum=\"11\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c12\" colnum=\"12\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c13\" colnum=\"13\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c14\" colnum=\"14\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c15\" colnum=\"15\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c16\" colnum=\"16\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c17\" colnum=\"17\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eParameters\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"4\" nameend=\"c5\" namest=\"c2\"\u003e \u003cp\u003eTop of Liquid (TL)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"4\" nameend=\"c9\" namest=\"c6\"\u003e \u003cp\u003eBottom of Liquid (BL)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"4\" nameend=\"c13\" namest=\"c10\"\u003e \u003cp\u003eTop of Sludge (TS)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"4\" nameend=\"c17\" namest=\"c14\"\u003e \u003cp\u003eBottom of Sludge (BS)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eMean\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eSD\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eMax\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eMin\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eMean\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eSD\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c8\"\u003e \u003cp\u003eMax\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c9\"\u003e \u003cp\u003eMin\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c10\"\u003e \u003cp\u003eMean\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c11\"\u003e \u003cp\u003eSD\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c12\"\u003e \u003cp\u003eMax\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c13\"\u003e \u003cp\u003eMin\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c14\"\u003e \u003cp\u003eMean\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c15\"\u003e \u003cp\u003eSD\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c16\"\u003e \u003cp\u003eMax\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c17\"\u003e \u003cp\u003eMin\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colspan=\"17\" nameend=\"c17\" namest=\"c1\"\u003e \u003cp\u003e\u003cb\u003eBOD\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBOD1(mg/L)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e52.50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e38.48\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e112.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e40.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e62.20\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e35.34\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e112.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e60.28\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003e115.88\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c11\"\u003e \u003cp\u003e87.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c12\"\u003e \u003cp\u003e225\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c13\"\u003e \u003cp\u003e66.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c14\"\u003e \u003cp\u003e77.16\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c15\"\u003e \u003cp\u003e49.42\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c16\"\u003e \u003cp\u003e177.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c17\"\u003e \u003cp\u003e76\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBOD3(mg/L)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e187.46\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e85.30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e261.25\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e80.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e195.90\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e91.27\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e261.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e80.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003e199.58\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c11\"\u003e \u003cp\u003e91.06\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c12\"\u003e \u003cp\u003e241.95\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c13\"\u003e \u003cp\u003e90\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c14\"\u003e \u003cp\u003e213.73\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c15\"\u003e \u003cp\u003e136.44\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c16\"\u003e \u003cp\u003e161.25\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c17\"\u003e \u003cp\u003e78\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBOD5(mg/L)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e325.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e116.90\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e342\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e182.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e378.46\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e193.54\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e349\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e180.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003e480.27\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c11\"\u003e \u003cp\u003e259.87\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c12\"\u003e \u003cp\u003e560.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c13\"\u003e \u003cp\u003e166.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c14\"\u003e \u003cp\u003e424.61\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c15\"\u003e \u003cp\u003e273.56\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c16\"\u003e \u003cp\u003e322.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c17\"\u003e \u003cp\u003e180.6\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBOD7(mg/L)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e323.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e322.87\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e822.75\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e204\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e324.64\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e205.16\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e512.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e380.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003e428.54\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c11\"\u003e \u003cp\u003e293.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c12\"\u003e \u003cp\u003e674.44\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c13\"\u003e \u003cp\u003e564.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c14\"\u003e \u003cp\u003e375.18\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c15\"\u003e \u003cp\u003e267.58\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c16\"\u003e \u003cp\u003e682.82\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c17\"\u003e \u003cp\u003e322.5\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBOD9(mg/L)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e587.10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e525.24\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e1,120.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e443.85\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e653.87\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e578.84\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e1340.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e492.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003e784.15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c11\"\u003e \u003cp\u003e602.99\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c12\"\u003e \u003cp\u003e1,524\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c13\"\u003e \u003cp\u003e921\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c14\"\u003e \u003cp\u003e720.27\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c15\"\u003e \u003cp\u003e618.24\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c16\"\u003e \u003cp\u003e1,440\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c17\"\u003e \u003cp\u003e403.1\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colspan=\"17\" nameend=\"c17\" namest=\"c1\"\u003e \u003cp\u003e\u003cb\u003eHeavy Metals\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePb (mg/L)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e7.5342\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e6.260\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e17.62\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e2.74\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e23.061\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e33.380\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e41.10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e2.891\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003e28.74\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c11\"\u003e \u003cp\u003e36.23\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c12\"\u003e \u003cp\u003e106.96\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c13\"\u003e \u003cp\u003e2.290\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c14\"\u003e \u003cp\u003e25.7525\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c15\"\u003e \u003cp\u003e27.8612\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c16\"\u003e \u003cp\u003e58.73\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c17\"\u003e \u003cp\u003e0.34\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFe (mg/L)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e480.14\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e879.38\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e2356.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e2.07\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e387.14\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e 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\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMn (mg/L)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.2544\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.224\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.61\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.02\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.398\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e0.522\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e0.779\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e0.025\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003e0.430\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c11\"\u003e \u003cp\u003e0.340\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c12\"\u003e \u003cp\u003e0.911\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c13\"\u003e \u003cp\u003e0.027\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c14\"\u003e \u003cp\u003e1.2410\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c15\"\u003e \u003cp\u003e1.7608\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c16\"\u003e \u003cp\u003e5.34\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c17\"\u003e \u003cp\u003e0.04\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNi (mg/L)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1.2435\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1.164\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e3.09\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e1.496\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e1.508\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e4.190\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e0.020\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003e1.175\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c11\"\u003e \u003cp\u003e1.262\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c12\"\u003e \u003cp\u003e3.650\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c13\"\u003e \u003cp\u003e0.020\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c14\"\u003e \u003cp\u003e13.0053\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c15\"\u003e \u003cp\u003e19.6870\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c16\"\u003e \u003cp\u003e47.80\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c17\"\u003e \u003cp\u003e0.00\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCr (mg/L)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e9.8086\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e10.941\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e27.92\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e20.724\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e32.793\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e87.95\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e0.069\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003e9.859\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c11\"\u003e \u003cp\u003e10.10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c12\"\u003e \u003cp\u003e25.848\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c13\"\u003e \u003cp\u003e0.000\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c14\"\u003e \u003cp\u003e11.7485\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c15\"\u003e \u003cp\u003e16.7351\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c16\"\u003e \u003cp\u003e48.34\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c17\"\u003e \u003cp\u003e2.32\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCu (mg/L)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1.6452\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e2.879\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e6.51\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.03\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e3.885\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e8.807\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e25.25\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e0.034\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003e4.664\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c11\"\u003e \u003cp\u003e11.42\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c12\"\u003e \u003cp\u003e32.750\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c13\"\u003e \u003cp\u003e0.025\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c14\"\u003e \u003cp\u003e15.8689\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c15\"\u003e \u003cp\u003e29.6977\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c16\"\u003e \u003cp\u003e75.25\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c17\"\u003e \u003cp\u003e0.01\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCd (mg/L)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.6661\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.709\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e1.64\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e1.956\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e2.358\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e6.847\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e0.000\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003e1.940\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c11\"\u003e \u003cp\u003e1.920\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c12\"\u003e \u003cp\u003e5.303\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c13\"\u003e \u003cp\u003e0.000\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c14\"\u003e \u003cp\u003e2.0532\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c15\"\u003e \u003cp\u003e2.0410\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c16\"\u003e \u003cp\u003e5.14\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c17\"\u003e \u003cp\u003e0.00\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eZn (mg/L)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.8467\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.627\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e1.77\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.04\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e1.289\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e0.972\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e2.669\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e0.276\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003e1.616\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c11\"\u003e \u003cp\u003e0.988\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c12\"\u003e \u003cp\u003e2.961\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c13\"\u003e \u003cp\u003e0.000\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c14\"\u003e \u003cp\u003e1.9706\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c15\"\u003e \u003cp\u003e1.1138\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c16\"\u003e \u003cp\u003e3.54\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c17\"\u003e \u003cp\u003e0.28\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e3.2 Potential Pollution and Ecological Impact Of Septic Tank Sludge\u003c/h2\u003e \u003cp\u003eThe pollution potential index of the various fecal sludge fractions was determined as discussed earlier and presented in Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. The NCPI values ranged from 55.9 to 209.6 thus classifying the fecal sludge as heavily polluted. The PLI values ranged from 6.37 to 28.22 thus the fecal sludge is greatly contaminated. Also, the Lgeo values ranged from 2.07 to 4.23, this imply that the sampled fecal sludge is strongly to extremely polluted. Hence, the fecal sludge samples pose a significant environmental threat for land and water disposal considering the selected indicators.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eNCPI, PLI, L\u003csub\u003egeo\u003c/sub\u003e values for the various indicators in the different fractions\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"10\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c9\" colnum=\"9\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c10\" colnum=\"10\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSample\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003ePb(mg/l)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eNi\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eCr\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eCu\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eCd\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eZn\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c8\"\u003e \u003cp\u003eNCPI\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c9\"\u003e \u003cp\u003ePLI\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c10\"\u003e \u003cp\u003eL\u003csub\u003egeo\u003c/sub\u003e\u003c/p\u003e \u003cp\u003e(Avg)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTL\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e7.53\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.78\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e8.58\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e1.65\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0.48\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e0.85\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e55.90\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c9\"\u003e \u003cp\u003e6.37\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c10\"\u003e \u003cp\u003e2.09\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBL\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e23.06\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e1.50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e20.72\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e3.89\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e1.22\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e1.29\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e169.81\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c9\"\u003e \u003cp\u003e14.36\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c10\"\u003e \u003cp\u003e3.26\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTS\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e28.75\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e1.17\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e8.63\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e4.66\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e1.21\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e1.62\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e209.60\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c9\"\u003e \u003cp\u003e13.22\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c10\"\u003e \u003cp\u003e3.14\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBS\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e25.75\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e13.01\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e11.75\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e15.87\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e2.05\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e1.97\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e192.48\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c9\"\u003e \u003cp\u003e28.22\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c10\"\u003e \u003cp\u003e4.23\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cdiv id=\"Sec8\" class=\"Section3\"\u003e \u003ch2\u003e3.2.1 Environmental pollution risks of septic tank fecal sludge disposal\u003c/h2\u003e \u003cp\u003eWhen disposed on land or discharged into water bodies, septic tank effluent contaminates both the ground and surface water through leaching and runoff and ca also accumulate in edible plants. In this study, the NCPI values ranged from 55.9 to 209.6 (Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e), indicating an exceptionally high contamination potential and suggesting that the surrounding environment is highly vulnerable to pollutant migration. Such elevated NCPI levels imply that heavy metals and associated contaminants can readily leach through the soil profile, increasing the likelihood of groundwater pollution, including elevated dissolved metal concentrations, increased salinity, and alterations to natural hydrogeochemical conditions (Fatta-Kassinos et al., \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). High NCPI values also signify a strong risk of contamination of surface water bodies through runoff and seepage, which may result in metal enrichment, sediment contamination, reduced ecological integrity, and potential bioaccumulation across aquatic food webs (Ali et al., \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). These effects collectively shows that the observed NCPI values far exceed typical risk thresholds and present a substantial threat to the environment.\u003c/p\u003e \u003cp\u003eThe Pollution Load Index (PLI) values ranged from 6.367 to 28.22 (Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e), indicating a severe level of contamination far above the threshold value of PLI\u0026thinsp;\u0026gt;\u0026thinsp;1, which typically signifies progressive environmental degradation. Such elevated PLI values suggest that the concentration of heavy metals in the study area has surpassed the natural geochemical background levels, increasing the likelihood of contaminant mobility into groundwater and surface water systems. High PLI values imply that soils and sediments may no longer act as effective sinks, thereby facilitating the leaching of toxic metals into underlying aquifers, where they can deteriorate groundwater quality, increase salinity and electrical conductivity, and pose potential health risks to dependent communities. Similarly, surface water bodies receiving runoff or seepage from contaminated sites are at heightened risk of heavy-metal enrichment, sediment contamination, and subsequent ecological stress, including reduced biodiversity and bioaccumulation across aquatic food webs.\u003c/p\u003e \u003cp\u003eThe Geo-accumulation Index (Igeo) values for the septic tank fecal sludge ranged from 2.086 to 4.236 (Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e), indicating contamination levels from moderately to heavily polluted to heavily polluted according to M\u0026uuml;ller\u0026rsquo;s classification. Such elevated Igeo values demonstrate that the concentrations of heavy metals in the sludge are significantly above natural background levels, implying strong anthropogenic enrichment. Disposing this sludge directly into the environment poses a substantial risk to groundwater because high metal loads can leach through the soil profile, especially under conditions of high permeability, low organic matter, or acidic pH, leading to long-term deterioration of aquifer quality. Likewise, runoff and surface deposition of this contaminated sludge can introduce heavy metals into surface water bodies, where they may accumulate in sediments, increase metal bioavailability, and disrupt aquatic ecosystems through toxicity, bioaccumulation, and reduced biodiversity.\u003c/p\u003e \u003cp\u003eExtremely high pollution indices in fecal sludge reflect substantial heavy metal enrichment in sludge fractions (Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e), with values far exceeding safe baseline levels for Pb, Ni, Cr, Cu, Cd, and Zn. When used for soil enrichment, it enhances the bioavailability of metals to plants through altered soil chemistry (e.g., pH changes and increased labile metal pools), which has been linked with elevated uptake and translocation of heavy metals into edible plant parts in multiple recent studies (e.g., excessive Cd, Pb, Cr and Ni in leafy and root vegetables grown on contaminated soils) (e.g., high bioaccumulation and health risk indices in crops irrigated with polluted water sources) (Balkhair and Ashraf, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). When soils are amended with metal-rich sludge, increased heavy metal concentrations in root zones promote uptake into crops, as demonstrated by recent work showing significant metal levels in edible portions of vegetables grown on polluted soils, frequently exceeding safe limits and raising potential health risks upon consumption (e.g., elevated Hazard Index values) (Abd-elhalim et al, \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). These processes not only threaten crop safety and productivity but also risk groundwater and surface water contamination through leaching and runoff, particularly under high rainfall or irrigation scenarios. Therefore, although fecal sludge offers nutrient benefits for soil fertility, its use without adequate treatment and monitoring could increase heavy metal mobilization into the food chain and aquatic environments.\u003c/p\u003e \u003cp\u003eHigh NCPI, PLI, and Lgeo values observed in the septic tank fecal sludge fractions (Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e) indicate severe heavy-metal contamination and imply significant risks beyond soil and water pathways, particularly through the mobilization of dry sludge particles. When fecal sludge dries during handling, storage, transportation, or land application, contaminated fine particles may become airborne and form metal-laden dust. Such particulates can transport toxic metals including Pb, Cr, Ni, Cd, and Cu, which are commonly associated with fine particulate matter and readily inhaled into the respiratory tract. This inhalation pathway represents an important but often underestimated exposure route, particularly for sanitation workers and nearby populations. The inhalation of heavy-metal-contaminated dust poses significant human health risks, as fine particles can penetrate deep into the lungs and facilitate systemic absorption of metals into the bloodstream. Chronic exposure to inhaled Pb, Cd, and Cr has been associated with respiratory inflammation, neurotoxicity, renal impairment, and increased carcinogenic risk, even at relatively low airborne concentrations. Studies have shown that particulate-bound metals originating from contaminated soils and waste residues contribute substantially to non-dietary human exposure, especially in dry and windy environments common in many low- and middle-income regions. Given the elevated pollution indices observed in this study, uncontrolled drying and dispersal of septic tank fecal sludge could therefore increase airborne metal exposure and compound public health risks, underscoring the need for controlled handling practices, moisture management, and protective measures during sludge reuse or disposal (Ferreira-Baptista \u0026amp; De Miguel, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2005\u003c/span\u003e; Alloway, \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2013\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eOverall, the combined evidence from the NCPI, PLI, and Igeo assessments clearly demonstrates that the fecal sludge examined in this study poses a substantial environmental risk, with the potential to severely degrade both groundwater and surface water resources. The exceptionally high NCPI values, the critically elevated PLI values, and the Igeo classifications showing moderate to heavy pollution collectively confirm that the sludge contains heavy metals far above natural background concentrations and well beyond internationally recognized contamination thresholds. These indicators highlight not only the current pollution burden but also the high mobility and persistence of these contaminants in the environment, increasing the likelihood of leaching, runoff, and long-term ecological damage. If disposed of without adequate treatment or containment, the sludge can act as a continuous source of heavy-metal loading to hydrological systems, resulting in groundwater deterioration, surface-water enrichment, sediment contamination, and potential bioaccumulation in aquatic organisms.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section3\"\u003e \u003ch2\u003e\u003cb\u003e3.2.2 Toxicological effects of heavy metals in septic tank fecal sludge disposal on soil microorganisms\u003c/b\u003e.\u003c/h2\u003e \u003cp\u003eIn the study area, septic tank sludge is usually disposed on land without further treatment and subsequently used by vegetable farmers to boost soil fertility. The NCPI values ranged obtained from 55.9 to 209.6 (Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e) indicating an extremely high contamination potential with pronounced negative impact on soil microorganisms, which are highly sensitive indicators of soil health. At moderate NCPI levels, soils experience reduced microbial biomass and enzyme activities but may retain partial resilience. Studies show that microbial dehydrogenase (DH) and urease activities decline as metal concentrations rise, reflecting inhibited respiration and nitrogen cycling (Farsang et al., \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). However, at very high NCPI values (Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e), soils exhibit acute toxicity, causing a collapse in microbial diversity and enzymatic functionality (Hu et al., \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Zhao et al., \u003cspan citationid=\"CR64\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Heavy metals such as Cu, Pb, Zn, Cd, and Cr interact with microbial cell membranes, enzymes, and DNA, leading to oxidative stress and metabolic inhibition (Ansari et al., \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). High NCPI soils tend to favor metal-tolerant taxa such as Pseudomonas, Bacillus, and certain Actinobacteria, while sensitive nitrifiers and decomposers decline. The resulting community shift reduces soil fertility and organic matter turnover (Mkhinini et al., \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Moreover, the elevated NCPI range corresponds to contamination levels comparable to industrial mining zones, where soil enzymes such as phosphatase and catalase are severely inhibited, indicating irreversible microbial damage (Xu et al., \u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). These impacts compromise soil ecosystem services, including nutrient cycling, organic matter decomposition, and pathogen suppression.\u003c/p\u003e \u003cp\u003eThe PLI values ranging 6.367 to 28.222 and the Lgeo values ranging from 2.086 to 4.234 indicates an extremely polluted fecal sludge which have detrimental effects on soil microorganisms. Soil microorganisms are responsible for the breakdown of organic matters into forms that it can be easily absorbed by plants and also used as energy source for other microorganisms. Hence, the disposal of septic tank fecal sludge containing elevated concentrations of heavy metals poses serious toxicological threats to soil microorganisms, it exerts inhibitory effects on microbial metabolic functions even at relatively low concentrations. These metals can bind to microbial cell membranes, disrupt membrane permeability, inhibit enzyme activity, and interfere with essential cellular processes such as respiration, DNA replication, and protein synthesis (Giller et al., \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2009\u003c/span\u003e).\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003e3.3 Correlation Between the Heavy Metals\u003c/h2\u003e \u003cp\u003eThe Heavy metal parameters showed significant correlation value suggesting a significant level of interaction and source commonality. Copper (Cu) and Nickel (Ni) showed a significant positive correlation value of 0.876. This shows a strong level of interaction and commonality of sources, which is consistent with recent soil contamination studies showing that metals with similar geochemical behavior often co-occur due to shared anthropogenic influences (e.g., industrial effluents, domestic pollution) and common binding mechanisms in soil matrices (Ma et al., \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2023\u003c/span\u003e), and also, copper and Nickel bioavailability is significantly influenced by pH and organic matter. Lower pH increases both metal solubility and presence of organic matter can complex with the metals reducing their free ion availability. Also, a strong Cu\u0026ndash;Ni correlation (r\u0026thinsp;=\u0026thinsp;0.876) indicates co-mobilization and common origin, most likely from domestic corrosion, personal care products, and household chemical use. Their similar ionic behavior results in parallel accumulation in sludge and co-binding to organic matter. Ecologically, Cu\u0026ndash;Ni combination poses heightened risk to soil microorganisms when sludge is land-applied. Copper (Cu) and chromium (Cr) also showed a significant positive correlation value of 0.597 and this is because their bioavailability is also influenced by pH and organic matter. All the metals with significant correlation like iron (Fe) and Lead (Pb), Cadmium (Cd) and Lead (Pb) have a positive correlation value and this is because their availability in nature is directly been influenced by factors such as; pH which affect the solubility of heavy metals, high pH values increase the solubility of heavy metals and lower pH values reduce the solubility of heavy metals. Organic matter also affects the bioavailability of heavy metals, heavy metals bond to organic matters through complexation their-by affecting its solubility and mobility, these complexes can either increase or decrease their bioavailability depending on their stability and environmental conditions. Ecologically, co-occurrence poses heightened risks to soil microorganisms and plant uptake, as multiple heavy metals can have synergistic toxic effects on microbial communities and disrupt key soil functions, consistent with findings that heavy metal mixtures impair microbial enzyme activity and soil nutrient cycling (Yu et al., \u003cspan citationid=\"CR63\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). These emphasizes the need for source control and pre-treatment to limit their concentrations.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003e3.4 Organic Pollution Potential of Sewage Sludge\u003c/h2\u003e \u003cp\u003eThe mean value of BOD consumed across the days for day nine (9) ranged from 587.1mg/L at the top of liquid layer (TL) to 784.15mg/L at the top of sludge layer (TS). The mean values of the BOD consumed for all the layers were on the increase from Day 1 to 5 (carbonaceous biochemical oxygen demand), a slight decrease was observed in Day 7 which signifies a decrease in the carbonaceous biochemical oxygen demand and a resumption of the nitrogenous biochemical oxygen demand which is responsible for nitrogen fixation (Young, and Vanrolleghem, \u003cspan citationid=\"CR62\" class=\"CitationRef\"\u003e2021\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThe range of the mean values of BOD consumed for the various fractions are: Top of Liquid: 52.5\u0026thinsp;\u0026plusmn;\u0026thinsp;38.48mg/L to 587.11\u0026thinsp;\u0026plusmn;\u0026thinsp;525.24mg/L, Bottom of Liquid: 62.2\u0026thinsp;\u0026plusmn;\u0026thinsp;35.34mg/L to 653.88\u0026thinsp;\u0026plusmn;\u0026thinsp;578.85mg/L, Top of Sludge: 115.88\u0026thinsp;\u0026plusmn;\u0026thinsp;87.0mg/L to 784.15\u0026thinsp;\u0026plusmn;\u0026thinsp;602.99mg/L, Bottom of Sludge 77.17\u0026thinsp;\u0026plusmn;\u0026thinsp;49.43 to 720.28\u0026thinsp;\u0026plusmn;\u0026thinsp;618.24mg/L. See Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e for these observed trends.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eA trend was observed across the various fractions. The BOD consumed increased from the top of liquid to the bottom of the fecal sludge as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. The BOD consumed at the top of the fecal sludge was lower than that at the bottom of the fecal sludge because the bottom of the sludge contains older organic solids which has undergone anaerobic digestion indicating stabilization and sequestration of stabilized organic matter. The BOD consumed is in this ascending order: TL\u0026thinsp;\u0026lt;\u0026thinsp;BL\u0026lt;BS\u0026thinsp;\u0026lt;\u0026thinsp;TS, showing that septic tank system aid in the removal of BOD through settling of solids.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe BOD values obtained from this study are higher than and quite variable from the BOD values reported by SANDEC from different regions (Heinss et al, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e1994\u003c/span\u003e) as shown in Table\u0026nbsp;\u003cspan refid=\"Tab5\" class=\"InternalRef\"\u003e4\u003c/span\u003e. This can be linked to factors such as; storage duration, admixture to FS such as grease, organic waste from kitchen, temperature, performance of septic tank, tank emptying technology and pattern, common treatment of black and greywater. Also, as fecal sludge is very heterogeneous consisting of scum, liquid, partly settled and settled layers each layer with its own distinct characteristics, collection of representative samples from septic tanks is one of the greatest challenges in fecal sludge characterization studies.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab4\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 4\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eBOD concentration in fecal sludge from other studies and this study\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAuthor\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eStudy location\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eNumber of Onsite sanitation system sampled\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eBOD\u003csub\u003e5\u003c/sub\u003e concentration(mg/L)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eType of Fecal Sludge\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eThis Study\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eNsukka-Nigeria\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e325.4\u003csup\u003eTL\u003c/sup\u003e, 378.46\u003csup\u003eBL\u003c/sup\u003e, 480.27\u003csup\u003eTS\u003c/sup\u003e, 424.61\u003csup\u003eBS\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eSeptic Tank\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSANDEC (Heinss et al, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e1994\u003c/span\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eAccra-Ghana\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e60\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e600\u0026ndash;1500\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eSeptage\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSANDEC (Heinss et al, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e1994\u003c/span\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eAccra-Ghana\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e7600\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003ePublic Toilet\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eGenerally, in alignment with established guidelines like those from WHO and the Environmental Conservation Rules (ECR) of various regions the BOD standard in effluent before disposal into water bodies should be less than 30mg/L (Tahri et al., \u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e2016\u003c/span\u003e), but variations in local environmental conditions and pollution sources influence this standard. Some water bodies because of its characteristics such as temperature, flow rate, pH and so on might be able to take higher concentration of BOD while some because of its characteristics will not be able to handle the 30mg/L bench mark so it\u0026rsquo;s important to know the characteristics of the receiving water before discharging fecal sludge effluent into it. Untreated or poorly treated fecal sludge with high BOD when it enters water bodies accelerates microbial activity that consumes dissolved oxygen. This leads to oxygen depletion, which can create hypoxic (low-oxygen) or anoxic (no-oxygen) zones, harming aquatic life and potentially causing fish kills (Prasanna et al., \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). High-BOD septic sludge that infiltrates soils from the soak pit can lead to nutrient and pathogen leaching, contaminating groundwater sources critical for drinking water, especially in areas dependent on wells and aquifers (Chun et al., \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). It\u0026rsquo;s important that septic tanks are well located away from ground water sources to avoid contamination.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003e3.5 Biological Oxygen Demand Rate Degradation and ultimate BOD of the Sampled Septic Tanks\u003c/h2\u003e \u003cp\u003eStudies on BOD degradation rate in fecal sludge are essential for understanding organic matter stabilization kinetics, optimizing treatment system design, predicting oxygen demand in receiving environments, and assessing microbial activity and environmental risks associated with fecal sludge disposal and reuse. Table\u0026nbsp;\u003cspan refid=\"Tab5\" class=\"InternalRef\"\u003e4\u003c/span\u003e shows the calculated values of the BOD degradation constant (k) for the various septic tanks and fractions (TL, BL, TS and BS). The BOD degradation rate constants (k) varied markedly among the top of liquid (TL), bottom of liquid (BL), top of sludge (TS), and bottom of sludge (BL) fractions. The liquid layer exhibited higher average rate constant (0.021 d\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e TL and 0.021 d\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for BL) than the sludge layer (0.013 d\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for TS and 0.012 d\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for BS), indicating that the liquid layer contains more readily biodegradable substances than the sludge layer. Gravitational separation is the primary and fastest process within the septic tank, leading to partial clarification of the liquid layer and leaving highly soluble, dilute and highly dispersed content that are easily accessible to microorganisms. On the other hand, the densification of high molecular weight and partially soluble materials in the sludge layer tend to slow down the biedegradation process. Though the septic tank system is generally over-simplified as an anaerobic reactor, the intermittent input of oxygen from influent wastewater generates micro-aerobic conditions with better mixing in the liquid layer, thus improving reaction kinetics. However, the sludge layer operates under strict anaerobic or facultative anaerobic conditions, which inherently exhibit slower organic matter degradation rates due to limited diffusion, slow hydrolysis and dominance of methanogens with associated slow kinetics. Anaerobic biochemical reactions proceed at lower energy yields and slower microbial growth rates, which explains the low k values even when substantial amounts of BOD are ultimately consumed (Burton et al., \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2013\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab5\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 4\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eBOD rate degradation in the various fractions Using Least square Method\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSeptic Tank Effluent Kinetics\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eTop Liquid Layer (TL)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eBottom Liquid Layer (BL)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eTop Sludge Layer (TS)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eBottom Sludge Layer (BS)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBOD constant (day\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.021\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.026\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.013\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.012\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCharacteristic time, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\text{\u0026tau;}\\)\u003c/span\u003e\u003c/span\u003e = 1/k (days)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e47.51\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e39.01\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e75.51\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e85.65\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eUltimate BOD, L\u003csub\u003eu\u003c/sub\u003e (mg/L)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e1116.63\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e1337.50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e9770.57\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e15669.0\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBiodegradation half life, t\u003csup\u003e1/2\u003c/sup\u003e (days)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e32.93\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e27.04\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e52.34\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e59.37\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eWhile the BOD degradation rate constant (k) describes the speed at which biodegradable organic matter is oxidized, the ultimate BOD (L\u003csub\u003eu\u003c/sub\u003e) represents the total oxygen demand exerted when all biodegradable organic matter is fully stabilized. The L\u003csub\u003eu\u003c/sub\u003e values obtained in this study varied widely across septic tanks and sludge fractions (Table\u0026nbsp;\u003cspan refid=\"Tab5\" class=\"InternalRef\"\u003e4\u003c/span\u003e), indicating pronounced heterogeneity in organic matter content, degree of stabilization, and biodegradability. Generally, higher L\u003csub\u003eu\u003c/sub\u003e values were observed in the sludge fractions (TS and BS) compared to the liquid fractions (TL and BL), reflecting the accumulation of particulate and slowly biodegradable organic matter within the sludge matrix. This is consistent with the role of septic tanks as partial anaerobic digesters, where solids retention promotes the buildup of long-term biodegradable organic reserves even as short-term BOD is reduced (Burton et al., \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Strande et al., \u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). The elevated L\u003csub\u003eu\u003c/sub\u003e values observed in older septic tanks and deeper sludge layers suggest the presence of substantial residual organic loading, despite low or near-zero k values in some fractions. A persistent increase in ultimate BOD with depth from 1116.63 mg/L in the top liquid layer to 15,669.0 mg/L in the bottom sludge layer indicates a clearly defined stratification of the septic tank content occasioned by a combination of physical/gravitational separation which leads to the accumulation of denser and more resistant materials near the tank bottom. The accumulation of slowly biodegradable and particulate organic matter in the sludge layer is responsible for the high ultimate BOD (Lu). The top of sludge (TS) contains partially degraded organic matter, recently settled solids, and active fermentative microbes while the bottom sludge contains older, compacted, highly stabilized material, recalcitrant complex organics (lignocellulosic residues, microbial biomass remnants) and reduced availability of biodegradable carbon. Very low or negative BOD rate constants indicate very low biodegradation rates, consistent with aged or mature fecal sludge and anaerobic condition which exist in the sludge fraction of septic tanks (Strande et al., \u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Grady et al., \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2011\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThe characteristic time (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\text{\u0026tau;}\\)\u003c/span\u003e\u003c/span\u003e) for the sludge layer is about twice (75.5 days for top sludge and 85.6 days for bottom sludge) that of the liquid layer with 47.5 days and 39.01 days for top liquid and bottom liquid layers respectively. This indicates that although biodegradation proceeds slowly in stabilized or aged sludge, a large fraction of organic matter remains capable of exerting oxygen demand over extended periods if remobilized. Such conditions pose a significant environmental risk when sludge is excavated, land-applied, or discharged into aquatic environments, as the slow oxidation of this residual organic matter can lead to prolonged oxygen depletion, sediment oxygen demand, and secondary anaerobic conditions.\u003c/p\u003e \u003cp\u003eFrom a fecal sludge management (FSM) perspective, the combined interpretation of k, L\u003csub\u003eu\u003c/sub\u003e and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\text{\u0026tau;}\\)\u003c/span\u003e\u003c/span\u003e is critical for treatment system design and environmental risk assessment. Fractions with high k, moderate L\u003csub\u003eu\u003c/sub\u003e and lower \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\text{\u0026tau;}\\)\u003c/span\u003e\u003c/span\u003e values present immediate oxygen demand risks and require rapid stabilization, while fractions with low k but high \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\text{\u0026tau;}\\)\u003c/span\u003e\u003c/span\u003e and L\u003csub\u003eu\u003c/sub\u003e necessitate longer treatment times or post-stabilization processes to prevent delayed pollution impacts. Ignoring L\u003csub\u003eu\u003c/sub\u003e in FSM planning may underestimate the long-term oxygen demand and environmental footprint of disposed sludge, particularly under conditions that enhance re-aeration or microbial reactivation. Therefore, incorporating ultimate BOD alongside degradation kinetics provides a more comprehensive basis for selecting appropriate treatment technologies, retention times, and disposal strategies for septic tank fecal sludge (Strande et al., \u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e2014\u003c/span\u003e).\u003c/p\u003e \u003c/div\u003e"},{"header":"4.0 CONCLUSION","content":"\u003cp\u003eHeavy metals are critical parameters in fecal sludge characterization due to their persistence, bioaccumulation, and well-documented links to chronic human health conditions such as cancer, cardiovascular, kidney, and liver diseases. Their presence also directly affects septic tank performance by inhibiting microbial activity and reducing biodegradation efficiency. Previous studies have shown that metals such as iron, lead, chromium, copper, zinc, and cadmium can interfere with anaerobic digestion processes, lower substrate degradation rates, and reduce biogas production, with concentrations varying widely across septic tank layers depending on location and input sources. These variations highlight the necessity of routinely assessing heavy metal concentrations prior to sludge disposal or reuse to ensure compliance with environmental regulations and to minimize ecological and public health risks.\u003c/p\u003e \u003cp\u003eIn this study, heavy metals were predominantly concentrated in the sludge layers, with iron showing the highest mean concentration (1271.38 mg/L in bottom sludge) and manganese the lowest (0.2544 mg/L in top liquid), following the general order Fe\u0026thinsp;\u0026gt;\u0026thinsp;Pb\u0026thinsp;\u0026gt;\u0026thinsp;Cr\u0026thinsp;\u0026gt;\u0026thinsp;Cu\u0026thinsp;\u0026gt;\u0026thinsp;Ni\u0026thinsp;\u0026gt;\u0026thinsp;Cd\u0026thinsp;\u0026gt;\u0026thinsp;Zn\u0026thinsp;\u0026gt;\u0026thinsp;Mn. Most metals, particularly lead, copper, cadmium, nickel, and chromium, exceeded national (FEPA, DWAF) and international (US-EPA/NPDES) permissible limits for wastewater discharge, indicating significant pollution potential. The high standard deviations observed reflect strong variability in household inputs such as detergents, cleaning agents, plumbing materials, and surface runoff. These elevated concentrations pose serious environmental concerns, as improper fecal sludge management can facilitate heavy metal accumulation in soils, crops, aquatic systems, and ultimately the food chain. Overall, the findings emphasize the need for stricter control of sludge disposal practices, targeted treatment strategies, and regulatory enforcement to mitigate the environmental and health risks associated with heavy metals in fecal sludge.\u003c/p\u003e \u003cp\u003eThe pollution and ecological risk assessment using NCPI, PLI, and Igeo clearly demonstrates that the fecal sludge across all fractions is severely to extremely polluted with heavy metals. NCPI values (55.9\u0026ndash;209.6), PLI values (6.37\u0026ndash;28.22), and Igeo classifications (moderately to extremely polluted) far exceed accepted safety thresholds, indicating strong anthropogenic enrichment and high contamination potential. These elevated indices imply that disposal of untreated fecal sludge poses a substantial threat to both groundwater and surface water systems through leaching, runoff, and sediment accumulation, with risks of long-term water quality degradation, metal bioaccumulation in aquatic ecosystems, and subsequent human health impacts. The results confirm that soils and sediments in contact with such sludge may no longer function as effective contaminant sinks, increasing the likelihood of pollutant mobility and persistence in the environment.\u003c/p\u003e \u003cp\u003eBeyond hydrological impacts, the high pollution indices also indicate acute toxicological risks to soil microbial communities, which are essential for nutrient cycling and organic matter decomposition. Extremely high NCPI, PLI, and Igeo values are associated with strong inhibition of microbial biomass, enzyme activities, and metabolic functions, leading to reduced soil fertility and impaired ecosystem services. The observed significant correlations among heavy metals, particularly Cu\u0026ndash;Ni and Cu\u0026ndash;Cr, suggest common anthropogenic sources and co-mobilization mechanisms influenced by pH and organic matter, further amplifying ecological risks when sludge is land-applied. Overall, the combined pollution indices and metal correlations emphasize that septic tank fecal sludge in the study area represents a critical environmental hazard, underscoring the urgent need for source control, targeted treatment, and strict regulatory enforcement before disposal or reuse to prevent irreversible damage to soil and water ecosystems. This study demonstrated that septic tank fecal sludge in Nsukka exhibits high organic strength and strong spatial heterogeneity across liquid and sludge fractions. Mean BOD concentrations increased both with incubation time and depth, following the order TL\u0026thinsp;\u0026lt;\u0026thinsp;BL\u0026thinsp;\u0026lt;\u0026thinsp;BS\u0026thinsp;\u0026lt;\u0026thinsp;TS, indicating effective settling of organic solids and progressive stabilization with depth. The higher BOD values in sludge layers, particularly at the top of sludge, reflect the presence of partially degraded organic matter, while lower BOD at the bottom of sludge confirms more advanced digestion and maturity. Although BOD concentrations observed were lower than those reported in some SANDEC studies, they remain far above recommended discharge limits, highlighting the significant risk posed by untreated or poorly treated fecal sludge to surface water and groundwater through oxygen depletion, nutrient loading, and pathogen transport.\u003c/p\u003e \u003cp\u003eThe BOD degradation rate constants (k) further revealed marked fraction-specific biodegradation behavior within septic tanks. Higher k values in liquid fractions, especially at the bottom of the liquid layer, indicate greater biodegradability, higher microbial activity, and rapid oxygen demand, whereas very low or near-zero k values in bottom sludge signify highly stabilized, mature material with limited remaining biodegradable carbon. These kinetic differences confirm that septic tanks function as partial anaerobic digesters, with stabilization increasing with depth and age. Importantly, the results show that fecal sludge management strategies must account for fraction-specific kinetics, as high-k fractions pose immediate environmental risks upon disposal, while low-k fractions require longer retention times and more intensive treatment. Overall, the findings emphasize that ignoring these kinetic variations can lead to treatment system under-performance or overload, underscoring the need for tailored design, appropriate treatment selection, and controlled disposal to protect soil and water resources.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e \u003ch2\u003eEthical Approval:\u003c/h2\u003e \u003cp\u003e\u0026bull; The research work followed all ethical compliance. All participants involved during the sample collection from onsite sanitation systems (septic tanks) were pre-informed before the collection and the samples were collected ensuring that the environment was not polluted.\u003c/p\u003e \u003c/p\u003e \u003cp\u003e \u003cstrong\u003eConsent to Participate:\u003c/strong\u003e \u003cp\u003e\u0026bull; All participants that were involved during the collection of preliminary data were properly informed.\u003c/p\u003e \u003c/p\u003e \u003cp\u003e \u003cstrong\u003eConsent to Publish:\u003c/strong\u003e \u003cp\u003e\u0026bull; All contributing authors has read the manuscript and proper approval for publication was given\u003c/p\u003e \u003c/p\u003e \u003cp\u003e \u003cstrong\u003eCompeting Interests:\u003c/strong\u003e \u003cp\u003e\u0026bull; The authors have no relevant financial or non-financial interests to disclose.\u003c/p\u003e \u003c/p\u003e\u003ch2\u003eFunding:\u003c/h2\u003e \u003cp\u003eThe authors declare that no funds, grants, or other support were received during the preparation of this manuscript.\u003c/p\u003e\u003ch2\u003eAuthor Contributions:\u003c/h2\u003e \u003cp\u003eAll authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by \u003cb\u003eOzota, Chidera Emmanuel\u003c/b\u003e and under the supervision of \u003cb\u003eNnaji, Chidozie Charles\u003c/b\u003e. The first draft of the manuscript was written by \u003cb\u003eOzota, Chidera Emmanuel\u003c/b\u003e and \u003cb\u003eNnaji, Chidozie Charles\u003c/b\u003e commented and made all necessary corrections on the manuscript. All authors read and approved the final manuscript for submission.\u003c/p\u003e\u003ch2\u003eACKNOWLEDGMENTS\u003c/h2\u003e \u003cp\u003eThe authors sincerely appreciate the support and contributions that made this research possible. We extend our gratitude to the \u003cb\u003eDepartment of Civil Engineering, University of Nigeria Nsukka, Enugu, Nigeria and Faculty of Engineering and the Built Environment, University of Johannesburg, Johannesburg, South Africa\u003c/b\u003e, for providing the necessary facilities and resources for this study.\u003c/p\u003e \u003cp\u003eWe are also grateful to all individuals and institutions that assisted in data collection and fieldwork, including septic tank owners who granted access to their facilities. Special thanks go to colleagues and research assistants for their valuable input and technical support during the study.\u003c/p\u003e \u003cp\u003eFinally, we acknowledge the reviewers and editorial team of \u003cem\u003eEnvironmental Science and Pollution Research\u003c/em\u003e for their insightful feedback and guidance in improving this manuscript.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eAbd-Elhalim BT, Gideon M, Anton K, Boyi MO (2025) Impact of dumpsite compost on heavy metal accumulation in some cultivated plants. 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Environ Sci Pollut Res 27(16):20215\u0026ndash;20226. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1007/s11356-020-08538-1\u003c/span\u003e\u003cspan address=\"10.1007/s11356-020-08538-1\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"environmental-science-and-pollution-research","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"espr","sideBox":"Learn more about [Environmental Science and Pollution Research](https://www.springer.com/journal/11356)","snPcode":"11356","submissionUrl":"https://submission.nature.com/new-submission/11356/3","title":"Environmental Science and Pollution Research","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Fecal sludge, heavy metal, contamination, kinetics, pollution indices, Environmental impact","lastPublishedDoi":"10.21203/rs.3.rs-8824149/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8824149/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eFecal sludge from septic tanks represents a major environmental and public health concern in rapidly urbanizing regions due to its high organic load and accumulation of toxic heavy metals. This study assessed the physico-chemical characteristics, pollution potential, and biodegradation kinetics of septic tank fecal sludge in Nsukka, Nigeria. Thirty-two samples were collected from eight septic tanks, stratified into four fractions: top of liquid (TL), bottom of liquid (BL), top of sludge (TS), and bottom of sludge (BS). Heavy metal concentrations (Fe, Pb, Cr, Cu, Ni, Cd, Zn, and Mn), Biological Oxygen Demand (BOD₁\u0026ndash;BOD₉), and BOD degradation rate constants (k) were analyzed using standard methods. Pollution and ecological risks were evaluated using the Nemero Comprehensive Pollution Index (NCPI), Pollution Load Index (PLI), and Geo-accumulation Index (Igeo). Results showed pronounced fraction-specific variability. Heavy metals were generally enriched in sludge layers, with iron exhibiting the highest mean concentration (up to 1271.38 mg/L in BS), while manganese showed the lowest levels. Most metals, particularly Pb, Cd, Cu, Cr, and Ni, exceeded Federal Environmental Protection Agency (FEPA), South African (DWAF), and US-EPA permissible limits. NCPI values ranged from 55.9 to 209.6, classifying the sludge as heavily polluted, while PLI (6.37\u0026ndash;28.22) and Igeo (2.09\u0026ndash;4.23) indicated strong to extreme contamination, posing severe risks to soil, groundwater, and surface water systems. Correlation analysis revealed significant positive associations among Cu\u0026ndash;Ni, Cu\u0026ndash;Cr, and Cd\u0026ndash;Pb, suggesting common anthropogenic sources and co-mobilization behavior. BOD consumed increased with incubation time and depth, following the order TL\u0026thinsp;\u0026lt;\u0026thinsp;BL\u0026thinsp;\u0026lt;\u0026thinsp;BS\u0026thinsp;\u0026lt;\u0026thinsp;TS, confirming effective solids settling and partial stabilization within septic tanks. BOD degradation rate constants varied widely from 0.0004\u0026ndash;0.0673 d⁻\u0026sup1;, with higher k values in liquid fractions indicating greater biodegradability and immediate environmental risk, while low to near-zero k values in bottom sludge reflected highly stabilized, mature material. These findings demonstrate that septic tanks function as partial anaerobic digesters with increasing stabilization with depth and age. Overall, the study highlights the critical need for fraction-specific characterization in fecal sludge management.\u003c/p\u003e","manuscriptTitle":"Heavy Metal Contamination and Bod Kinetics of Septic Tank Fecal Sludge: Implications for Environmental Risk","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-04-13 11:27:42","doi":"10.21203/rs.3.rs-8824149/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"reviewerAgreed","content":"","date":"2026-04-21T14:22:48+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2026-04-04T20:53:06+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"Environmental Science and Pollution Research","date":"2026-02-23T13:44:08+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2026-02-18T04:33:43+00:00","index":"","fulltext":""},{"type":"submitted","content":"Environmental Science and Pollution Research","date":"2026-02-15T14:09:17+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"environmental-science-and-pollution-research","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"espr","sideBox":"Learn more about [Environmental Science and Pollution Research](https://www.springer.com/journal/11356)","snPcode":"11356","submissionUrl":"https://submission.nature.com/new-submission/11356/3","title":"Environmental Science and Pollution Research","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"28062c46-9202-4591-8375-ec211b8fe690","owner":[],"postedDate":"April 13th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2026-04-13T11:27:43+00:00","versionOfRecord":[],"versionCreatedAt":"2026-04-13 11:27:42","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8824149","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8824149","identity":"rs-8824149","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}