Simulation of  oxygen concentration in the matrix pores at different phases of composting process based on two-region model

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However, it is unclear how the change of air-immobile regions affects temporal variation of O2 in the pores during the whole composting process and after turning. In this study, we first used a tracer-inverse calculation protocol to obtain feature parameters (proportional coefficient of gas in the air-immobile region, φ; the first-order mass transfer coefficient, α) of the air-immobile regions in the matrix pores before and after turning during whole composting process, and then predicted the temporal variation of O2 in the pores using two-region model with these measured parameters. The φ values in compost piles for initial-material, temperature-increasing, thermophilic, and curing phases were 0.38/0.40, 0.42/0.40, 0.46/0.46, and 0.41/0.45 before/after turning, respectively, while the corresponding α values were 0.002/0.001, 0.001/0, 0.004/0, and 0.005/0.001 min-1, respectively. The proportion of air-immobile regions was higher in the temperature-increasing and thermophilic phases than in the curing phase. The air-immobile regions caused difference of predicted O2 concentrations between air-mobile and air-immobile regions, and the difference was enhanced during the composting mainly by the rate of organic-matter biodegradation. Turning piles slightly decreased φ in the temperature-increasing phase and had little change in thermophilic phase, while it caused slight increases in φ during other phases. The value of α declined throughout composting process after turning. These findings provide support for reducing the production of harmful off-gases in composting. Composting Two-region model Air-immobile regions Proportional coefficient of air-mobile regions O2 concentration Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 1. Introduction A large quantity of organic solid waste is being produced, which mainly includes livestock and poultry manure, crop straw, sewage sludge, and municipal solid waste, etc. (Guo et al., 2021 ; Yishui et al., 2021 ; Wu et al., 2020 ). Among the various methods available for managing organic solid waste, aerobic compositing has several outstanding advantages such as recycling of nutrients, stabilization, etc. However, there is still a challenge, reducing the emission of harmful off-gases which include greenhouse gases (CO 2 , CH 4 , N 2 O, etc.) and malodorous gases (NH 3 , H 2 S, smelly volatile organic compounds). When released from the composting pile, these harmful gases will cause adverse environmental impacts (Gutiérrez et al., 2017 ), which hinder the application of composting. For example, hundreds of composting facilities were closed down because of the complaint about odor emissions from the neighboring public in U. S. (Andersson et al., 2009 ; Han et al., 2019 ; Andraskar et al., 2021 ). Those harmful gases are found to be mainly generated in the anaerobic zones of the compost particles (Jiang et al., 2015 ; Li et al., 2021 ). These anaerobic zones are considered to be the core of compost particles described with a dual-layer (aerobic/anaerobic) structure, which structure was proposed firstly by Hamelers ( 2004 ) and developed by Ge et al. ( 2014 ). The formation of the anaerobic zones is regulated by the microbial consumption of dissolved O 2 combined with the O 2 transfer into the internal part of compost particles from pile’s pores in the vicinity (Wang and Ai, 2016; Rafiee et al., 2017 ; Abbas et al., 2019; Martinez-Hernandez et al., 2022). Thus, comprehending temporal-spatial distribution of O 2 in the pile’s pores is a key premise to regulate anaerobic zones during composting process. Nevertheless, it is very difficult to measure the O 2 in the pores in the compost matrix. Numerical simulation is considered an ideal solution, which can predict a large amount of the O 2 concentrations in the pores at different layers in the matrix (Yamada and Kawase, 2006 ; Ge et al., 2016 b). As a basic model, the one-dimensional convection-dispersion model has been widely used to simulate O 2 concentrations during composting process (Zeng et al., 2016 ; Martalò et al., 2020 ; He et al. 2020 ). In this model, the homogeneity assumption is applied, which assumes that the O 2 concentration in the pores is uniformly distributed in the whole matrix disregarding the spatial variation of O 2 (Ge et al., 2016 ; He et al. 2020 ). However, this model does not consider the heterogeneity of pore connectivity which occurs ubiquitously in porous media such as compost pile, landfill, and soil (Konstantaki et al., 2015 ; Pérez-Reche et al., 2012 ; Ruggieri et al., 2008 ). Consequently, this simple model cannot describe the effect of pore connectivity on the O 2 concentration in the pores of porous media, which could overestimate or underestimate O 2 in these pores. There are kinds of pores in the compost piles, including open, semi-closed, and closed pores, and it is very difficult to numerically solve the mathematical model if these pores were classified into more than two types (≥ 3). To simplify the model and numerical simulation, sorting these pores into two categories is optimal. The effect of pore connectivity can be characterized by a two-region model (TRM) developed by Chen et al. ( 2022 ). In this model, the pores are simplified to two types of open pores and closed/semi-closed pores, which are called air-mobile (air flowing) regions and air-immobile (air stagnant) regions in TRM, respectively (Chen et al. 2022 ; Deng et al., 2008; Field and Pinsky, 2000 ; Poulsen et al., 2006 ). The temporal-spatial distribution of air-immobile regions reflects the pore connectivity, is characterized by four parameters (proportional coefficient of gas in the air-mobile region, β ; mass exchange coefficient, which expresses mass exchange rates between the mobile and immobile regions, ω ; proportional coefficient of gas in the air-immobile region, φ ; the first-order mass transfer coefficient, α ) (Chen et al. 2022 ). In the previous report, Chen et al. ( 2022 ) has built a tracer-inverse calculation protocol based on (TRM) to measure these parameters. With this novel method, they explored the values of these parameters and predicted the O 2 concentration in compost pores during temperature-increasing and thermophilic phases of sewage-sludge composting. However, it is not unclear that how the change of air-immobile regions affects temporal variation of O 2 concentration in the matrix pores during the entire composting process and effects of turning, even though understanding the effect is crucial for developing the technology of reducing the production of harmful off-gases in composting. Thus, the aims of this study were to: 1) explore the effects of air-immobile regions on the O 2 concentration in the matrix pores during the whole composting process; 2) explore whether the turning changes the effects or not. 2. Material and methods 2.1 Raw materials The raw materials for composting were sewage sludge and rice bran, which were acquired from a local municipal wastewater treatment plant and a farmer’s market in Guilin, China, respectively. The basic physicochemical properties of the raw materials and composts were analyzed by the following methods. Moisture content (MC) was determined conducting by loss on drying at 105°C for 24 h in an oven, and volatile solid (VS) content was determined by measuring loss of weight on ignition at 550 ± 50°C for 2 h in a muffle furnace. The C/N ratio of samples was calculated after determining the C and N contents using an elemental analyzer (CHNS/O 2400, PerkinElmer, USA). Fresh samples were extracted with ultrapure water at a ratio of 1:10 (weight: volume) to determine the pH value of the solid samples using a pH meter. The basic physicochemical properties of the raw materials are shown in Table S1 (Supplementary materials). 2.2 Composting experiments and gas tracer tests Sewage sludge (107.65 kg, wet weight) and rice bran (52.64 kg, wet weight) were mixed to obtain the mixture with MC of approximately 60% for 30-day pilot aerobic composting. The composting experiment was carried out in a series of 150-L bioreactors made of polyvinyl chloride (PVC), which were cone frustum-shaped (height, 71.0 cm; top diameter, 66.0 cm; bottom diameter, 55.0 cm) (Fig. 1 A). During composting process, the matrix in each bioreactor was continuously ventilated at a rate of 1250 mL/min with an air pump outside the bioreactor through a perforated pipe placed at the bottom of the bioreactor. To avoid blocking holes on the perforated pipe, the bottom was filled with two layers of hollow balls (diameter: 5.0 cm) and a sheet of gauze above these balls (Fig. 1 A). During the composting process, the matrix was turned every 3 days. Temperatures in each pile and the ambient environment were monitored with portable thermometers (TP-101, Haoyuansen, China), and the O 2 concentration in the headspace above the pile was determined with O 2 meters (JSA5-O 2 -A, Jidaan, China). For each pile, temperatures were measured at three locations (8, 25, and 45 cm depth under the matrix surface along the central axis) and averaged the values to represent the matrix temperature. The temperature and the O 2 concentration were measured twice a day (9:00 a.m. and 21:00 p. m.) to obtain the daily values. On days 0, 2, 7, and 30, approximately 1.5 kg of composting piles were carefully transferred from the matrix center into the tracer chamber (Fig. 1 B) to determine the O 2 consumption curve and then run the gas tracer test of 200 minutes. During this test, the carrier gas (N 2 , 99.9%) was continuously introduced into the chamber from the bottom, and mixed off-gas samples out of the chamber were collected every two minutes. (Chen et al. 2022 ). To investigate the effect of turning on air-immobile regions and the O 2 concentration in the matrix pores, a parallel 1.5-kg compost was collected from the composting pile, mixed to homogeneity, and transferred into a 2.3-L tracer chamber (height, 30 cm; diameter, 10 cm) to carry out the O 2 consumption curve analysis and gas tracer test as above. The concentration of O 2 in the emitted gas was determined per minute by conducting the emitted gas from an air outlet at the top of the chamber throughout the sensor. The whole test lasted for 60 minutes in triplicate. After transferring the compost materials, 250 g of compost was collected evenly from three location at the center of the top, central and bottom layers of the matrix, for the measurement of physicochemical properties of composts at different phases. Figure 1 . 2.3 Inverse calculation of parameters In the semi-open composting cell, gases transmission in the piles are similar the liquid transmission in porous media, which regarded as incompressible fluid (Yazdani et al., 2010 ; Chen et al., 2022 ). TRM can effectively simulate the parameters ( φ , α , β , and ω ) describing distribution characteristics of the air-immobile regions. This model is a non-equilibrium model based on the convection–dispersion-rection equations (CDE) of gases in air-mobile and air-immobile regions. The equations are shown in Section 1 of Supplementary materials. Firstly, the parameters β and ω were inversely calculated by the software STANMOD CXTFIT based on the breakthrough curves (BTCs) of the tracer gas (Helium, simplified as He gas). He gas was used as the tracer gas in this study, owing to a close D (dispersion coefficient) to O 2 , high recovery, short time for peak arrival, and weak degree of tailing in the BTC (Chen et al. 2022 ). In the software, the following key options were selected: inverse problem, deterministic nonequilibrium CDE, Dirac Delta input and resident concentration (third-type inlet). To complete the inversion, the parameters below need to be pre-set: L (characteristic length, equal to the height of the pile), v (average velocity of the carrier gas in the free air space), D (diffusion coefficients of gas), µ 1 (first-order decay coefficients of tracer gas in air-mobile regions), µ 2 (first-order decay coefficients of tracer gas in air-immobile regions), β , ω , and R (the retardation factor) (Toride et al., 1995). In this study, v , D He and D O2 (He gas and O 2 modified diffusion coefficients, respectively) were obtained by applying the formula Equations S5–S10, while L was set at 30 cm equaling to the length of the gas-tracer chamber. As for µ 1 and µ 2 , both were assumed to be zero, due to the chemical resistance of He gas in the matrix environment. Furthermore, β , ω , φ , and α were inversely calculated in STANMOD with normalized BTCs as the input data, which protocol was developed by Chen et al. ( 2022 ). In this study, the initial values of β and ω were set at 0.5, 0.5 to run the inverse calculation, and the maximum number of iteration was set at 20, which initial values were set at the same as those in the previous report (Chen et al., 2022 ). Through the inversion protocol, the simulated values of β and ω were obtained for the transferred compost. Then, the φ and α values can be calculated using the Table S2, which two parameters are used to characterize the distribution characteristics of the air-immobile regions. 2.4 O 2 Simulation of O 2 concentration in pores With the calculated parameters ( β and ω ), we simulated the temporal variation of O 2 concentrations in the matrix pores using the STANMOD, (Supplementary materials, equation S3–S4). Compared with the inverse calculation, there were three different key options for inverse calculation in the simulation procedure: the direct problem, zero initial concentration, and pulse input (ambient air, O 2 concentration, 20.9%) at application time. The other options are the same as inverse calculation. These options reflected the replenishment of O 2 in the compost pores starting from a completely anaerobic status. As for the parameters, besides β and ω , only µ 1 , µ 2 had different values from that in the inverse calculation protocol. The µ 1 , µ 2 were calculated based on the O 2 consumption curve (OCC) with the method in Equations S11–S12 and Table S2 (Supplementary materials). As for L , v , and D , keep consistent with inverse calculation accordingly. By setting the number of gas output positions, the O 2 in the cross section of different output positions can be obtained. In this study, the number was set to be 1, and we predicted the O 2 concentration in the off-gas and pores at the central location (a depth of 15 cm from the top surface) of the pile. All pulse input lasted for 60 minutes, which predicted the temporal O 2 concentrations during 60-min aeration process. 3. Results and discussion 3.1 Physicochemical properties of piles In this study, there were typical changes in the physicochemical properties of the matrix during composting, which including temperature, O 2 concentration in the off-gas, pH, VS, MC, and C/N (Fig. 2 ). The temperature of the piles rose rapidly and then dropped during the 30-d period of composting (Fig. 2 A). The high temperature of piles was attributed to the decomposition of easily biodegradable organic matter, such as proteins, starch, and simple sugars in aerobic environment, which process released a large amount of heat. These heat emission were less than those released and then accumulated, causing an increase in temperature of the piles (Said-Pullicino et al., 2007 ). On the third day of composting, which continued for 16 days, the compost transitioned from the temperature-increasing phase (temperatures up to approximately 50 ℃) to the thermophilic phase (temperatures from 50 up to 70 ℃). The organic substrate was insufficient for decomposing during the composting process, resulting in the gradual weakening of microbial catabolism in the final phase of composting. In addition, the O 2 concentration in the off-gas rapidly decreased initially and then increased gradually (Fig. 2 B). As for the other physicochemical properties, the pH values of the composting piles increased gradually from 6.37 to 8.34 during the composting process (Fig. 2 C). The increase in pH is caused by the rapid formation of NH 3 from the biodegradation of protein, especially in temperature-increasing and thermophilic phases (Sundberg et al., 2004 ; Chen et al., 2015 ; Wu et al., 2020 ). During composting, VS and MC initially rose a little and then came down (Fig. 2 D, E, G, and H). Additionally, the C/N ratios gradually decreased during the composting process (Fig. 2 F). Free air space (FAS) and porosity of the piles declined from the initial 71.08% and 32.14–70.64% and 20.34% during the curing, respectively (Table S4, Supplementary materials). Figure 2 . 3.2 Variation of air-immobile regions (1) Breakthrough curve of He gas During curing phase, the peak-arrival time of BTCs was shorter than that in the other phases during composting process (Fig. 3 ). However, there were no evident changes in the peak-arrival time in the initial-material, temperature-increasing, and thermophilic phases. This was due to the gradual reduction of FAS and porosity of the piles during composting, which may have been caused by compaction during the process. The peak-arrival time of BTCs was shorter than that after turning during the temperature-increasing, thermophilic, and curing phases, while it exhibited little variation after turning in the initial material. This phenomenon is possibly because turning alleviates the compaction of the piles (Scheutz et al., 2017 ). The FAS and porosity had little change after turning in the initial-material, while they rose in the other phases (Table S4, Supplementary materials). Figure 3 . Figure 3 . He gas concentration content‒time curve of the piles before and after turning during composting: A‒D are the initial-material, temperature-increasing, thermophilic, and curing of the piles before tuning, respectively; E‒H are the initial-material, temperature-increasing, thermophilic, and curing of the piles after turning, respectively. The relative root-mean-square error (RRMSE) of the measured and simulated BTCs data of the gas tracer was extremely low (Table S5). The simulation results indicated that the inverse calculation method accurately estimated the values of φ and α . There were lower RRMSE between the measured and simulated BTCs after turning piles in composting process, except during the curing phase. This may be due to the two-region becoming more obvious in the piles after turning in these phases. The He gas simulated values and measured values before and after turning in different phases are shown in Fig. 4 (Figure S1 presents a complete figure). The results showed that the fitting curve and measured value during composting process were consistent. Figure 4 . (2) Variation of the air-immobile regions During composting process, the air-immobile regions within the piles increased first and then decreased gradually, while the first-order mass transfer coefficient of gas showed an opposite trend to the air-immobile regions (Fig. 5 ). The variations of φ and α affected the mass transfer of O 2 in the composting piles and the spatial distribution of the anaerobic regions in the composting particles. These value changes showed that there are more air-immobile regions in the temperature-increasing and thermophilic phases and less gas exchange between two regions in initial-material and temperature-increasing phase. Inadequate replenishment worsens the shortage of O 2 in air-immobile regions and expands the spatial distribution of the anaerobic part, further exacerbating the generation and emission of harmful off-gases (He et al., 2020 ). The α value reached its minimum during the temperature-increasing phase of the entire composting period. This was because of relatively low porosity and high moisture content (Rafiee et al., 2017 ). The φ value was the highest during the thermophilic phase in the whole composting process. The porous structure of the piles became more stable during curing as organic matter decomposed and the MC of the piles reduced (Castaldi et al., 2005 ; Ge et al., 2015 b; Ibrahim and Horton, 2021). Therefore, more pores progressively became available for air as moisture decreased during the curing phase, resulting in the largest α value in the composting process. From this study, we should reduce air-immobile regiones through increasing aeration rate or O 2 concentration mainly in temperature-increasing and thermophilic phases. Turning piles reduced air-immobile regions in temperature-increasing and thermophilic phases. As for mass transfer of gas, it was poor between the air-immobile and air-mobile regions of the piles after turning in the whole composting progress (Fig. 5 ). Turning increased FAS and porosity of piles, which compacted during composting (Zeng et al., 2016 ). In the initial-material, the φ and α values were similar before and after turning, which may be attributed to the similar porous structure on day 0 before and after turning. During the temperature-increasing and thermophilic phases, there were fewer air-immobile regions and gas exchange between the air-immobile and air-mobile regions after turning. Amounts of O 2 was consumed in these two phases. Anaerobic regions were caused to develop by the insufficient O 2 (Chen et al., 2011 ; Vergara and Silver, 2019 ; Zhang et al., 2021 ). Turning decreased the air-immobile regions during curing phase, which may be because turning results in better pore connectivity. Overall, the φ values did not change much after turning in each phase of composting, indicating that it was more important to select raw materials of the compost to increase the air-mobile regions. Additionally, the α values changed little in the initial-material before and after turning. Comparatively, in the other phases of composting, the change in α values were smaller after turning, which meant that the gas exchange between the air-immobile and air-mobile regions of the matrix pores slowed down. Figure 5 . 3.3 Simulation of O 2 concentration in the matrix pores based on the TRM The occurrence of air-immobile regions caused marked difference in predicted O 2 concentrations between air-mobile and air-immobile regions, and the difference was enhanced mainly by the reaction rate of organic-matter biodegradation during composting (Fig. 6 ). The predicted O 2 concentration underwent little change during composting in both air-mobile and air-immobile regions based on the assumption that µ 1 and µ 2 were same in the whole composting process. This phenomenon suggests that only changing φ and α resulted in negligible changes on O 2 concentration in the air-immobile and air-mobile regions. With the variation of µ 1 and µ 2 , there was higher predicted O 2 concentration in the air-mobile and air-immobile regions during curing than that observed in other phases of composting process. The variation of the µ 1 and µ 2 during composting process initially increased and then decreased (Table S6, Supplementary materials). During the thermophilic phase, the predicted O 2 concentration in the air-mobile regions was higher than that in the initial-material and temperature-increasing phase, which may be caused by the greater O 2 consumption. The predicted O 2 concentrations were close to zero in the air-immobile regions during the temperature-increasing and thermophilic phases of composting, and remained low in the initial-material and curing phase, which illustrated that the air-immobile regions obviously clogged the transfer of O 2 in the piles. In the curing phase, the predicted O 2 concentration in the air-immobile regions was higher than that in other phases. These findings showed that the variation in predicted O 2 concentration within the matrix pores was affected by air-immobile regions in combination with first order reaction constant of O 2 . Generally, turning increased the predicted O 2 concentration in the air-mobile/air-immobile regions in the whole composting process (Fig. 7 ). The variation of O 2 concentration in the matrix pores reflects the composting progress (Zeng et al., 2018 ). There were higher predicted O 2 concentrations in the air-mobile and air-immobile regions in the initial-material after turning. This was due to the low O 2 consumption, high FAS, and high porosity in the initial-material. During the temperature-increasing and thermophilic phases, the predicted O 2 concentrations increased in the air-mobile regions but there were minimal changes in the air-immobile regions after turning. This may be due to more FAS and porosity after turning, resulting in a lower gas flow velocity, even when the O 2 supply remains unchanged in the temperature-increasing and thermophilic phases. Oxygen uptake rate (OUR) was greater during the temperature-increasing and thermophilic phases than that in the other phases during composting (Figures S2 and S3). During composting, organic matter is mineralized with O 2 to form small-molecule inorganic matter through the action of aerobic microorganisms (Zhang et al., 2021 ). Thus, in composting, OUR can reflect the intensity of aerobic microbial activity (Barrena et al., 2011 ; Ge et al., 2015 b). In the initial-material, the aerobic microorganisms were in the lag phase, resulting in low O 2 consumption (Getahun et al., 2012 ). During the temperature-increasing phase, aerobic microorganisms grew rapidly, resulting in large amounts of organic matter and O 2 consumption (Tiquia, 2005 ). The OUR during the thermophilic phase was only lower than that in the temperature-increasing phase in the whole composting process. This was because the high temperature inhibited microbial activity (Van Gestel et al., 2003 ). The OUR was relatively low and remained almost stable in the curing phase. Thus, OUR can be used to assess compost stability in the final phase of composting. Zheng et al. (2004) further verified the feasibility of using OUR as a measure of compost maturity by correlating it with the germination index. Figure 6 . Figure 7 . 3.4 Effects of air-immobile regions on O 2 concentration in matrix pores The piles compact during the composting process, resulting in an increase in air-immobile regions that inhibit the transfer of O 2 between the two regions (Fig. 8 ). The O 2 transfer process between FAS and particles in the piles is reflected by the distribution of the air-immobile regions. There were fewer air-immobile regions in the piles in the initial-material. This was due to the piles compacted slightly in the initial-material, and the aerobic microorganisms were inactive. The interplay of the change in FAS, MC, and O 2 concentration resulted in high φ , µ 1 and µ 2 values but a low α value during the temperature-increasing phase (Ge et al., 2016 ; Yue et al., 2008 ; Wu et al., 2019 ). The compost piles were most severely compacted during the thermophilic phase, in which the φ value was the highest. The φ value was smaller during curing than that in the temperature-increasing and thermophilic phases, and the α value was the opposite. There were fewer air-immobile regions in the piles at this phase, while the exchange rate of O 2 between the air-mobile and air-immobile regions was faster. The variation of the air-immobile regions in the piles mainly was influenced by the changes of porosity, FAS, and MC. The MC of the piles decreased generally during composting process because of the continuous ventilation (Ahn et al., 2008 ; Shi et al., 1999 ). The microbial activity diminishes during the last phase of composting due to the loss of moisture (Liang et al., 2003 ). Furthermore, the pore structure became more stable during the last phase of composting, possibly because the organic matter was consumed, which opened some of the originally closed or semi-closed pores inside the piles to form channels for gas flow (Chen et al., 2022 ). Simultaneously, porosity and FAS in the piles gradually decreased during composting. However, there are still some knowledge gaps that need to be filled. These gaps have not been discussed in this study. For example, the change in characteristics of the microbial community in the air-immobile regions and O 2 concentration in the particle were not investigated. Further studies are expected to clarify the mechanism of the coupled relationship between O 2 concentration in matrix pores and the microbial community in the air-immobile regions. Figure 8 . 4. Conclusions This study identified how the change of air-immobile regions drives temporal variation of O 2 in the pores during the whole composting process by simulating the temporal changes in O 2 concentration in the matrix pores and inverting the parameters of the air-immobile regions based on TRM. The main conclusions are as follows: (1) During composting process, the φ values for the initial-material, temperature-increasing phase, thermophilic phase, and curing phase were 0.38/0.40, 0.42/0.40, 0.46/0.46, and 0.41/0.45 before/after turning, respectively, while the α values were 0.002/0.001, 0.001/0, 0.004/0, and 0.005/0.001 min − 1 , respectively. The air-immobile regions caused the difference of predicted O 2 concentrations between two regions, and the difference was enhanced during the composting primarily due to the reaction rate of organic-matter biodegradation. (2) Turning piles slightly decreased φ in the temperature-increasing phase and had little change in thermophilic phase, while it increases in initial-material and curing phase. The α values declined after turning throughout the composting process. Declarations Acknowledgements This work was financially supported by the National Natural Science Foundation of China (51868011), and Special Fund for Guangxi Distinguished Experts. Funding This work was financially supported by the National Natural Science Foundation of China (51868011), and Special Fund for Guangxi Distinguished Experts. Competing Interests The authors have no relevant financial or non-financial interests to disclose. Author Contributions All authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by Haiguang Qin, Hongtao Liu, Yulan Lu, Jun Zhang. The first draft of the manuscript was written by Haiguang Qin and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript. Data availability All data generated and analyzed during the current study are available from the corresponding author on reasonable request. 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Effects of aeration rates on the structural changes in humic substance during co-composting of digestates and chicken manure. Science of the Total Environment 658, 510–520. https://doi.org/10.1016/j.scitotenv.2018.12.198 Yamada, Y., Kawase, Y., 2006. Aerobic composting of waste activated sludge: Kinetic analysis for microbiological reaction and oxygen consumption. Waste Management 26, 49–61. https://doi.org/10.1016/j.wasman.2005.03.012 Yazdani, R., Mostafid, M.E., Han, B., Imhoff, P.T., Chiu, P., Augenstein, D., Kayhanian, M., Tchobanoglous, G., 2010. Quantifying factors limiting aerobic degradation during aerobic bioreactor landfilling. Environmental Science and Technology 44, 6215–6220. https://doi.org/10.1021/es1022398 Yishui, T., Ming, S., Geng, K., Linwei, M., Si, S., 2021. Development Strategy of Biomass Economy in China. Chinese Journal of Engineering Science 23, 133. https://doi.org/10.15302/j-sscae-2021.01.004 Yue, B., Chen, T. Bin, Gao, D., Zheng, G. Di, Liu, B., Lee, D.J., 2008. Pile settlement and volume reduction measurement during forced-aeration static composting. Bioresource Technology 99, 7450–7457. https://doi.org/10.1016/j.biortech.2008.02.029 Zeng, J., Shen, X., Han, L., Huang, G., 2016. Dynamics of oxygen supply and consumption during mainstream large-scale composting in China. Bioresource Technology 220, 104–109. https://doi.org/10.1016/j.biortech.2016.08.070 Zeng, J., Shen, X., Sun, X., Liu, N., Han, L., Huang, G., 2018. Spatial and temporal distribution of pore gas concentrations during mainstream large-scale trough composting in China. Waste Management 75, 297–304. https://doi.org/10.1016/j.wasman.2018.01.044 Zhang, S., Wang, J., Chen, X., Gui, J., Sun, Y., Wu, D., 2021. Industrial-scale food waste composting: Effects of aeration frequencies on oxygen consumption, enzymatic activities and bacterial community succession. Bioresource Technology 320, 124357. https://doi.org/10.1016/j.biortech.2020.124357 ZHENG Yuqi, CHEN Tongbin, KONG Jiansong, GAO Ding, HUANG Qifei, L.W., 2004. Maturity assessment by oxygen consumption rate in aerobic composting. ACTA SCIENTIAE CIRCUMSTANTIAE 24, 1–11. https://doi.org/10.13671/j.hjkxxb.2004.05.029 Supplementary Files graphicalabstract.pdf supplementarymaterial.docx Cite Share Download PDF Status: Published Journal Publication published 04 Oct, 2024 Read the published version in Waste and Biomass Valorization → Version 1 posted Reviewers agreed at journal 13 May, 2024 Reviewers invited by journal 09 May, 2024 Editor invited by journal 29 Apr, 2024 Editor assigned by journal 08 Apr, 2024 First submitted to journal 07 Apr, 2024 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. 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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-4233312","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":300477486,"identity":"4a7043e0-4fba-4922-9f51-c7a3fdc0da3d","order_by":0,"name":"Haiguang Qin","email":"","orcid":"","institution":"Guangxi Key Laboratory of Environmental Pollution Control Theory and Technology","correspondingAuthor":false,"prefix":"","firstName":"Haiguang","middleName":"","lastName":"Qin","suffix":""},{"id":300477487,"identity":"cf6422c0-557f-483f-b401-a21b455c4c0e","order_by":1,"name":"Hongtao Liu","email":"","orcid":"","institution":"Institute of Geographic Sciences and Natural Resources Research Chinese Academy of Sciences","correspondingAuthor":false,"prefix":"","firstName":"Hongtao","middleName":"","lastName":"Liu","suffix":""},{"id":300477488,"identity":"e1cb2add-c569-4460-a419-c1ae57f2104c","order_by":2,"name":"Yulan Lu","email":"","orcid":"","institution":"China Energy Engineering Group Guangxi Electric Power Design Institute CO., LTD","correspondingAuthor":false,"prefix":"","firstName":"Yulan","middleName":"","lastName":"Lu","suffix":""},{"id":300477489,"identity":"67cb4d35-a547-4761-b11a-44036ca4837b","order_by":3,"name":"Jun Zhang","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAwUlEQVRIiWNgGAWjYFCCAwwMHxgswEwJorUwzgAq5iFBCwMDMw9JWuQbz5g9tvklkbifgfngbR4GuzyCWhgbzpgb5/ZJJPYwsCVb8zAkFxN2FMMZM+ncHpAWHjNpHoYDiQ2EtLCBtFiCtfB/I04LD0gLww+wLWzEaZFgOFYm2dsgYdxzmM3Yco5BMmEt8jMOb5P48cdGtr29+eGNNxV2hLUwSBwABlsbAygggMCAoHog4AeZ+ocYlaNgFIyCUTBiAQBzvzRfyY17SAAAAABJRU5ErkJggg==","orcid":"https://orcid.org/0000-0002-3647-4414","institution":"Guilin University of Technology","correspondingAuthor":true,"prefix":"","firstName":"Jun","middleName":"","lastName":"Zhang","suffix":""}],"badges":[],"createdAt":"2024-04-08 02:08:36","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4233312/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4233312/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s12649-024-02752-5","type":"published","date":"2024-10-04T15:57:59+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":56752886,"identity":"4a4bdb82-ad9d-493f-84b5-6c1fdd71a90b","added_by":"auto","created_at":"2024-05-20 04:14:16","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":148626,"visible":true,"origin":"","legend":"\u003cp\u003eComposting facility diagram (A) and tracer chamber (B).\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-4233312/v1/48fc7fb7f4e4c25ae32b1b70.png"},{"id":56752888,"identity":"1664dc8a-3156-402d-931d-ab7bc22bef34","added_by":"auto","created_at":"2024-05-20 04:14:16","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":71124,"visible":true,"origin":"","legend":"\u003cp\u003eTemporal changes of physicochemical properties of compost piles during composting: temperature (A), O\u003csub\u003e2\u003c/sub\u003e (B), pH (C), C/N ratio (D), MC (E), VS (F), loss of MC (G), and loss of VS (H) in the composting process. AT, AO refer to the temperature and O\u003csub\u003e2\u003c/sub\u003e concentration in the ambient environment and ambient O\u003csub\u003e2\u003c/sub\u003e concentration, respectively. RT means the temperature concentration in the piles and RO means the O\u003csub\u003e2\u003c/sub\u003e in the off-gas.\u003cbr\u003e\n\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-4233312/v1/12f056cb4f9b33a51756db93.png"},{"id":56752884,"identity":"d66ace05-a139-4a28-8279-f2ef946b2bac","added_by":"auto","created_at":"2024-05-20 04:14:16","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":266761,"visible":true,"origin":"","legend":"\u003cp\u003eHe gas concentration content‒time curve of the piles before and after turning during composting: A‒D are the initial-material, temperature-increasing, thermophilic, and curing of the piles before tuning, respectively; E‒H are the initial-material, temperature-increasing, thermophilic, and curing of the piles after turning, respectively.\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-4233312/v1/a7f19df7728de829fb1b906e.png"},{"id":56752885,"identity":"cdaadc73-5e93-45f7-bfda-ed66699bd3ef","added_by":"auto","created_at":"2024-05-20 04:14:16","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":94310,"visible":true,"origin":"","legend":"\u003cp\u003eConcentration-time curves of He gas simulated values and measured values in different phases of composting piles before and turning: A‒D are the initial-material, temperature-increasing, thermophilic, and curing of the piles before turning, respectively; E‒H are the initial-material, temperature-increasing, thermophilic, and curing of the turning piles, respectively.\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-4233312/v1/70b3d0b92e587a9fc3897ea8.png"},{"id":56752894,"identity":"c0d2fba0-9e3d-4bca-afc8-0d7aba2e3f91","added_by":"auto","created_at":"2024-05-20 04:14:17","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":64830,"visible":true,"origin":"","legend":"\u003cp\u003eChanges in proportional coefficient of air-immobile regions (\u003cem\u003eφ\u003c/em\u003e) (A) and first-order mass transfer coefficient (\u003cem\u003eα\u003c/em\u003e) (B) before and after turning in different phases of composting.\u003c/p\u003e","description":"","filename":"floatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-4233312/v1/4a4a8a89724369809d7864e8.png"},{"id":56753209,"identity":"be914a0c-4a8c-44d3-9820-19aa6bf6266b","added_by":"auto","created_at":"2024-05-20 04:22:16","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":143903,"visible":true,"origin":"","legend":"\u003cp\u003ePrediction of temporal variation curves of O\u003csub\u003e2\u003c/sub\u003e concentration in the pores during composting : A‒D are the O\u003csub\u003e2\u003c/sub\u003e in the air-mobile regions of initial-material, temperature-increasing, thermophilic, and curing of origin materials respectively; E‒H are the O\u003csub\u003e2\u003c/sub\u003e in the air-immobile regions of initial-material, temperature-increasing, thermophilic, and curing of origin materials respectively; I‒L are the O\u003csub\u003e2\u003c/sub\u003e in the air-mobile regions of initial-material, temperature-increasing, thermophilic, and curing of origin materials respectively; M‒P are the O\u003csub\u003e2\u003c/sub\u003e in the air-immobile of initial-material, temperature-increasing, thermophilic, and curing of origin materials respectively.\u003c/p\u003e\n\u003cp\u003eIn Figs. 6A–6H, the unchanged values of \u003cem\u003eμ\u003c/em\u003e\u003csub\u003e\u003cem\u003e1\u003c/em\u003e\u003c/sub\u003e, \u003cem\u003eμ\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e in the whole composting were used, while in Figs. 6I–6P the temperature-adjusted values of \u003cem\u003eμ\u003c/em\u003e\u003csub\u003e\u003cem\u003e1\u003c/em\u003e\u003c/sub\u003e, \u003cem\u003eμ\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e were used considering the effect of the matrix temperature.\u003c/p\u003e","description":"","filename":"floatimage7.png","url":"https://assets-eu.researchsquare.com/files/rs-4233312/v1/86ee3281e6a39a8990549a80.png"},{"id":56752889,"identity":"8f951e58-09b5-49d6-9efe-386b46c95a28","added_by":"auto","created_at":"2024-05-20 04:14:16","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":154018,"visible":true,"origin":"","legend":"\u003cp\u003ePrediction of temporal variation curves of O\u003csub\u003e2\u003c/sub\u003e in in the air-mobile regions and air-immobile regions at different phases before and after turning: A‒D are O\u003csub\u003e2\u003c/sub\u003e in the air-mobile regions of the initial-material, temperature-increasing, thermophilic, and curing phases before turning piles respectively; E‒H are O\u003csub\u003e2\u003c/sub\u003e in the air-immobile regions of the initial-material, temperature-increasing, thermophilic, and curing phases before turning respectively; I‒L are O\u003csub\u003e2\u003c/sub\u003e in the air-mobile regions of the initial-material, temperature-increasing, thermophilic, and curing phases after turning piles respectively; M‒P are O\u003csub\u003e2\u003c/sub\u003e in the air-immobile regions of the initial-material, temperature-increasing, thermophilic, and curing phases after turning piles respectively.\u003c/p\u003e","description":"","filename":"floatimage8.png","url":"https://assets-eu.researchsquare.com/files/rs-4233312/v1/2f55299acd039312e1c155bc.png"},{"id":56752883,"identity":"d699787b-1f9b-4a09-8286-daa875f34cd1","added_by":"auto","created_at":"2024-05-20 04:14:16","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":112288,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic diagram of the effect of the gas-immobile regions on the O\u003csub\u003e2\u003c/sub\u003e of the matrix pores.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e","description":"","filename":"floatimage9.png","url":"https://assets-eu.researchsquare.com/files/rs-4233312/v1/f4e5c8cac8fd761428ef1f31.png"},{"id":66097477,"identity":"7f6ef1eb-c775-4995-a341-a48e07b9e92a","added_by":"auto","created_at":"2024-10-07 16:14:19","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1581974,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4233312/v1/5d6a98a0-88db-4ad7-9438-ce8d88716bb7.pdf"},{"id":56752892,"identity":"0647d4f9-56f4-4e0a-8ae8-469c0577d146","added_by":"auto","created_at":"2024-05-20 04:14:17","extension":"pdf","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":111340,"visible":true,"origin":"","legend":"","description":"","filename":"graphicalabstract.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4233312/v1/d9fb8e226751c0f24a61daf1.pdf"},{"id":56752891,"identity":"554d7015-07e2-4d8e-9f42-613ce572e535","added_by":"auto","created_at":"2024-05-20 04:14:17","extension":"docx","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":525276,"visible":true,"origin":"","legend":"","description":"","filename":"supplementarymaterial.docx","url":"https://assets-eu.researchsquare.com/files/rs-4233312/v1/1727ea9add97e74c57758896.docx"}],"financialInterests":"","formattedTitle":"Simulation of oxygen concentration in the matrix pores at different phases of composting process based on two-region model","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eA large quantity of organic solid waste is being produced, which mainly includes livestock and poultry manure, crop straw, sewage sludge, and municipal solid waste, etc. (Guo et al., \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Yishui et al., \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Wu et al., \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Among the various methods available for managing organic solid waste, aerobic compositing has several outstanding advantages such as recycling of nutrients, stabilization, etc. However, there is still a challenge, reducing the emission of harmful off-gases which include greenhouse gases (CO\u003csub\u003e2\u003c/sub\u003e, CH\u003csub\u003e4\u003c/sub\u003e, N\u003csub\u003e2\u003c/sub\u003eO, etc.) and malodorous gases (NH\u003csub\u003e3\u003c/sub\u003e, H\u003csub\u003e2\u003c/sub\u003eS, smelly volatile organic compounds). When released from the composting pile, these harmful gases will cause adverse environmental impacts (Guti\u0026eacute;rrez et al., \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2017\u003c/span\u003e), which hinder the application of composting. For example, hundreds of composting facilities were closed down because of the complaint about odor emissions from the neighboring public in U. S. (Andersson et al., \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2009\u003c/span\u003e; Han et al., \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2019\u003c/span\u003e; Andraskar et al., \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). Those harmful gases are found to be mainly generated in the anaerobic zones of the compost particles (Jiang et al., \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Li et al., \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). These anaerobic zones are considered to be the core of compost particles described with a dual-layer (aerobic/anaerobic) structure, which structure was proposed firstly by Hamelers (\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2004\u003c/span\u003e) and developed by Ge et al. (\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). The formation of the anaerobic zones is regulated by the microbial consumption of dissolved O\u003csub\u003e2\u003c/sub\u003e combined with the O\u003csub\u003e2\u003c/sub\u003e transfer into the internal part of compost particles from pile\u0026rsquo;s pores in the vicinity (Wang and Ai, 2016; Rafiee et al., \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2017\u003c/span\u003e; Abbas et al., 2019; Martinez-Hernandez et al., 2022). Thus, comprehending temporal-spatial distribution of O\u003csub\u003e2\u003c/sub\u003e in the pile\u0026rsquo;s pores is a key premise to regulate anaerobic zones during composting process.\u003c/p\u003e \u003cp\u003eNevertheless, it is very difficult to measure the O\u003csub\u003e2\u003c/sub\u003e in the pores in the compost matrix. Numerical simulation is considered an ideal solution, which can predict a large amount of the O\u003csub\u003e2\u003c/sub\u003e concentrations in the pores at different layers in the matrix (Yamada and Kawase, \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2006\u003c/span\u003e; Ge et al., \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2016\u003c/span\u003eb). As a basic model, the one-dimensional convection-dispersion model has been widely used to simulate O\u003csub\u003e2\u003c/sub\u003e concentrations during composting process (Zeng et al., \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Martal\u0026ograve; et al., \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; He et al. \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). In this model, the homogeneity assumption is applied, which assumes that the O\u003csub\u003e2\u003c/sub\u003e concentration in the pores is uniformly distributed in the whole matrix disregarding the spatial variation of O\u003csub\u003e2\u003c/sub\u003e (Ge et al., \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; He et al. \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). However, this model does not consider the heterogeneity of pore connectivity which occurs ubiquitously in porous media such as compost pile, landfill, and soil (Konstantaki et al., \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; P\u0026eacute;rez-Reche et al., \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2012\u003c/span\u003e; Ruggieri et al., \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2008\u003c/span\u003e). Consequently, this simple model cannot describe the effect of pore connectivity on the O\u003csub\u003e2\u003c/sub\u003e concentration in the pores of porous media, which could overestimate or underestimate O\u003csub\u003e2\u003c/sub\u003e in these pores.\u003c/p\u003e \u003cp\u003eThere are kinds of pores in the compost piles, including open, semi-closed, and closed pores, and it is very difficult to numerically solve the mathematical model if these pores were classified into more than two types (\u0026ge;\u0026thinsp;3). To simplify the model and numerical simulation, sorting these pores into two categories is optimal. The effect of pore connectivity can be characterized by a two-region model (TRM) developed by Chen et al. (\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). In this model, the pores are simplified to two types of open pores and closed/semi-closed pores, which are called air-mobile (air flowing) regions and air-immobile (air stagnant) regions in TRM, respectively (Chen et al. \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Deng et al., 2008; Field and Pinsky, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2000\u003c/span\u003e; Poulsen et al., \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2006\u003c/span\u003e). The temporal-spatial distribution of air-immobile regions reflects the pore connectivity, is characterized by four parameters (proportional coefficient of gas in the air-mobile region, \u003cem\u003eβ\u003c/em\u003e; mass exchange coefficient, which expresses mass exchange rates between the mobile and immobile regions, \u003cem\u003eω\u003c/em\u003e; proportional coefficient of gas in the air-immobile region, \u003cem\u003eφ\u003c/em\u003e; the first-order mass transfer coefficient, \u003cem\u003eα\u003c/em\u003e) (Chen et al. \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). In the previous report, Chen et al. (\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2022\u003c/span\u003e) has built a tracer-inverse calculation protocol based on (TRM) to measure these parameters. With this novel method, they explored the values of these parameters and predicted the O\u003csub\u003e2\u003c/sub\u003e concentration in compost pores during temperature-increasing and thermophilic phases of sewage-sludge composting. However, it is not unclear that how the change of air-immobile regions affects temporal variation of O\u003csub\u003e2\u003c/sub\u003e concentration in the matrix pores during the entire composting process and effects of turning, even though understanding the effect is crucial for developing the technology of reducing the production of harmful off-gases in composting.\u003c/p\u003e \u003cp\u003eThus, the aims of this study were to: 1) explore the effects of air-immobile regions on the O\u003csub\u003e2\u003c/sub\u003e concentration in the matrix pores during the whole composting process; 2) explore whether the turning changes the effects or not.\u003c/p\u003e"},{"header":"2. Material and methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Raw materials\u003c/h2\u003e \u003cp\u003eThe raw materials for composting were sewage sludge and rice bran, which were acquired from a local municipal wastewater treatment plant and a farmer\u0026rsquo;s market in Guilin, China, respectively. The basic physicochemical properties of the raw materials and composts were analyzed by the following methods. Moisture content (MC) was determined conducting by loss on drying at 105\u0026deg;C for 24 h in an oven, and volatile solid (VS) content was determined by measuring loss of weight on ignition at 550\u0026thinsp;\u0026plusmn;\u0026thinsp;50\u0026deg;C for 2 h in a muffle furnace. The C/N ratio of samples was calculated after determining the C and N contents using an elemental analyzer (CHNS/O 2400, PerkinElmer, USA). Fresh samples were extracted with ultrapure water at a ratio of 1:10 (weight: volume) to determine the pH value of the solid samples using a pH meter. The basic physicochemical properties of the raw materials are shown in Table S1 (Supplementary materials).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Composting experiments and gas tracer tests\u003c/h2\u003e \u003cp\u003eSewage sludge (107.65 kg, wet weight) and rice bran (52.64 kg, wet weight) were mixed to obtain the mixture with MC of approximately 60% for 30-day pilot aerobic composting. The composting experiment was carried out in a series of 150-L bioreactors made of polyvinyl chloride (PVC), which were cone frustum-shaped (height, 71.0 cm; top diameter, 66.0 cm; bottom diameter, 55.0 cm) (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eA). During composting process, the matrix in each bioreactor was continuously ventilated at a rate of 1250 mL/min with an air pump outside the bioreactor through a perforated pipe placed at the bottom of the bioreactor. To avoid blocking holes on the perforated pipe, the bottom was filled with two layers of hollow balls (diameter: 5.0 cm) and a sheet of gauze above these balls (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eA). During the composting process, the matrix was turned every 3 days.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTemperatures in each pile and the ambient environment were monitored with portable thermometers (TP-101, Haoyuansen, China), and the O\u003csub\u003e2\u003c/sub\u003e concentration in the headspace above the pile was determined with O\u003csub\u003e2\u003c/sub\u003e meters (JSA5-O\u003csub\u003e2\u003c/sub\u003e-A, Jidaan, China). For each pile, temperatures were measured at three locations (8, 25, and 45 cm depth under the matrix surface along the central axis) and averaged the values to represent the matrix temperature. The temperature and the O\u003csub\u003e2\u003c/sub\u003e concentration were measured twice a day (9:00 a.m. and 21:00 p. m.) to obtain the daily values.\u003c/p\u003e \u003cp\u003eOn days 0, 2, 7, and 30, approximately 1.5 kg of composting piles were carefully transferred from the matrix center into the tracer chamber (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eB) to determine the O\u003csub\u003e2\u003c/sub\u003e consumption curve and then run the gas tracer test of 200 minutes. During this test, the carrier gas (N\u003csub\u003e2\u003c/sub\u003e, 99.9%) was continuously introduced into the chamber from the bottom, and mixed off-gas samples out of the chamber were collected every two minutes. (Chen et al. \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). To investigate the effect of turning on air-immobile regions and the O\u003csub\u003e2\u003c/sub\u003e concentration in the matrix pores, a parallel 1.5-kg compost was collected from the composting pile, mixed to homogeneity, and transferred into a 2.3-L tracer chamber (height, 30 cm; diameter, 10 cm) to carry out the O\u003csub\u003e2\u003c/sub\u003e consumption curve analysis and gas tracer test as above. The concentration of O\u003csub\u003e2\u003c/sub\u003e in the emitted gas was determined per minute by conducting the emitted gas from an air outlet at the top of the chamber throughout the sensor. The whole test lasted for 60 minutes in triplicate. After transferring the compost materials, 250 g of compost was collected evenly from three location at the center of the top, central and bottom layers of the matrix, for the measurement of physicochemical properties of composts at different phases.\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3 Inverse calculation of parameters\u003c/h2\u003e \u003cp\u003eIn the semi-open composting cell, gases transmission in the piles are similar the liquid transmission in porous media, which regarded as incompressible fluid (Yazdani et al., \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Chen et al., \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). TRM can effectively simulate the parameters (\u003cem\u003eφ\u003c/em\u003e, \u003cem\u003eα\u003c/em\u003e, \u003cem\u003eβ\u003c/em\u003e, and \u003cem\u003eω\u003c/em\u003e) describing distribution characteristics of the air-immobile regions. This model is a non-equilibrium model based on the convection\u0026ndash;dispersion-rection equations (CDE) of gases in air-mobile and air-immobile regions. The equations are shown in Section \u003cspan refid=\"Sec1\" class=\"InternalRef\"\u003e1\u003c/span\u003e of Supplementary materials.\u003c/p\u003e \u003cp\u003eFirstly, the parameters \u003cem\u003eβ\u003c/em\u003e and \u003cem\u003eω\u003c/em\u003e were inversely calculated by the software STANMOD CXTFIT based on the breakthrough curves (BTCs) of the tracer gas (Helium, simplified as He gas). He gas was used as the tracer gas in this study, owing to a close \u003cem\u003eD\u003c/em\u003e (dispersion coefficient) to O\u003csub\u003e2\u003c/sub\u003e, high recovery, short time for peak arrival, and weak degree of tailing in the BTC (Chen et al. \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). In the software, the following key options were selected: inverse problem, deterministic nonequilibrium CDE, Dirac Delta input and resident concentration (third-type inlet). To complete the inversion, the parameters below need to be pre-set: \u003cem\u003eL\u003c/em\u003e (characteristic length, equal to the height of the pile), \u003cem\u003ev\u003c/em\u003e (average velocity of the carrier gas in the free air space), \u003cem\u003eD\u003c/em\u003e (diffusion coefficients of gas), \u003cem\u003e\u0026micro;\u003c/em\u003e\u003csub\u003e\u003cem\u003e1\u003c/em\u003e\u003c/sub\u003e (first-order decay coefficients of tracer gas in air-mobile regions), \u003cem\u003e\u0026micro;\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e (first-order decay coefficients of tracer gas in air-immobile regions), \u003cem\u003eβ\u003c/em\u003e, \u003cem\u003eω\u003c/em\u003e, and \u003cem\u003eR\u003c/em\u003e (the retardation factor) (Toride et al., 1995). In this study, \u003cem\u003ev\u003c/em\u003e, \u003cem\u003eD\u003c/em\u003e\u003csub\u003e\u003cem\u003eHe\u003c/em\u003e\u003c/sub\u003e and \u003cem\u003eD\u003c/em\u003e\u003csub\u003e\u003cem\u003eO2\u003c/em\u003e\u003c/sub\u003e (He gas and O\u003csub\u003e2\u003c/sub\u003e modified diffusion coefficients, respectively) were obtained by applying the formula Equations S5\u0026ndash;S10, while \u003cem\u003eL\u003c/em\u003e was set at 30 cm equaling to the length of the gas-tracer chamber. As for \u003cem\u003e\u0026micro;\u003c/em\u003e\u003csub\u003e\u003cem\u003e1\u003c/em\u003e\u003c/sub\u003e and \u003cem\u003e\u0026micro;\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e, both were assumed to be zero, due to the chemical resistance of He gas in the matrix environment. Furthermore, \u003cem\u003eβ\u003c/em\u003e, \u003cem\u003eω\u003c/em\u003e, \u003cem\u003eφ\u003c/em\u003e, and \u003cem\u003eα\u003c/em\u003e were inversely calculated in STANMOD with normalized BTCs as the input data, which protocol was developed by Chen et al. (\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). In this study, the initial values of \u003cem\u003eβ\u003c/em\u003e and \u003cem\u003eω\u003c/em\u003e were set at 0.5, 0.5 to run the inverse calculation, and the maximum number of iteration was set at 20, which initial values were set at the same as those in the previous report (Chen et al., \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). Through the inversion protocol, the simulated values of \u003cem\u003eβ\u003c/em\u003e and \u003cem\u003eω\u003c/em\u003e were obtained for the transferred compost. Then, the \u003cem\u003eφ\u003c/em\u003e and \u003cem\u003eα\u003c/em\u003e values can be calculated using the Table S2, which two parameters are used to characterize the distribution characteristics of the air-immobile regions.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e2.4 O\u003csub\u003e2\u003c/sub\u003e Simulation of O\u003csub\u003e2\u003c/sub\u003e concentration in pores\u003c/h2\u003e \u003cp\u003eWith the calculated parameters (\u003cem\u003eβ\u003c/em\u003e and \u003cem\u003eω\u003c/em\u003e), we simulated the temporal variation of O\u003csub\u003e2\u003c/sub\u003e concentrations in the matrix pores using the STANMOD, (Supplementary materials, equation S3\u0026ndash;S4). Compared with the inverse calculation, there were three different key options for inverse calculation in the simulation procedure: the direct problem, zero initial concentration, and pulse input (ambient air, O\u003csub\u003e2\u003c/sub\u003e concentration, 20.9%) at application time. The other options are the same as inverse calculation. These options reflected the replenishment of O\u003csub\u003e2\u003c/sub\u003e in the compost pores starting from a completely anaerobic status. As for the parameters, besides \u003cem\u003eβ\u003c/em\u003e and \u003cem\u003eω\u003c/em\u003e, only \u003cem\u003e\u0026micro;\u003c/em\u003e\u003csub\u003e\u003cem\u003e1\u003c/em\u003e\u003c/sub\u003e, \u003cem\u003e\u0026micro;\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e had different values from that in the inverse calculation protocol. The \u003cem\u003e\u0026micro;\u003c/em\u003e\u003csub\u003e\u003cem\u003e1\u003c/em\u003e\u003c/sub\u003e, \u003cem\u003e\u0026micro;\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e were calculated based on the O\u003csub\u003e2\u003c/sub\u003e consumption curve (OCC) with the method in Equations S11\u0026ndash;S12 and Table S2 (Supplementary materials). As for \u003cem\u003eL\u003c/em\u003e, \u003cem\u003ev\u003c/em\u003e, and \u003cem\u003eD\u003c/em\u003e, keep consistent with inverse calculation accordingly.\u003c/p\u003e \u003cp\u003eBy setting the number of gas output positions, the O\u003csub\u003e2\u003c/sub\u003e in the cross section of different output positions can be obtained. In this study, the number was set to be 1, and we predicted the O\u003csub\u003e2\u003c/sub\u003e concentration in the off-gas and pores at the central location (a depth of 15 cm from the top surface) of the pile. All pulse input lasted for 60 minutes, which predicted the temporal O\u003csub\u003e2\u003c/sub\u003e concentrations during 60-min aeration process.\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results and discussion","content":"\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e3.1 Physicochemical properties of piles\u003c/h2\u003e \u003cp\u003eIn this study, there were typical changes in the physicochemical properties of the matrix during composting, which including temperature, O\u003csub\u003e2\u003c/sub\u003e concentration in the off-gas, pH, VS, MC, and C/N (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). The temperature of the piles rose rapidly and then dropped during the 30-d period of composting (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eA). The high temperature of piles was attributed to the decomposition of easily biodegradable organic matter, such as proteins, starch, and simple sugars in aerobic environment, which process released a large amount of heat. These heat emission were less than those released and then accumulated, causing an increase in temperature of the piles (Said-Pullicino et al., \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2007\u003c/span\u003e). On the third day of composting, which continued for 16 days, the compost transitioned from the temperature-increasing phase (temperatures up to approximately 50 ℃) to the thermophilic phase (temperatures from 50 up to 70 ℃). The organic substrate was insufficient for decomposing during the composting process, resulting in the gradual weakening of microbial catabolism in the final phase of composting. In addition, the O\u003csub\u003e2\u003c/sub\u003e concentration in the off-gas rapidly decreased initially and then increased gradually (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eB). As for the other physicochemical properties, the pH values of the composting piles increased gradually from 6.37 to 8.34 during the composting process (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eC). The increase in pH is caused by the rapid formation of NH\u003csub\u003e3\u003c/sub\u003e from the biodegradation of protein, especially in temperature-increasing and thermophilic phases (Sundberg et al., \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2004\u003c/span\u003e; Chen et al., \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Wu et al., \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). During composting, VS and MC initially rose a little and then came down (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eD, E, G, and H). Additionally, the C/N ratios gradually decreased during the composting process (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eF). Free air space (FAS) and porosity of the piles declined from the initial 71.08% and 32.14\u0026ndash;70.64% and 20.34% during the curing, respectively (Table S4, Supplementary materials).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e3.2 Variation of air-immobile regions\u003c/h2\u003e \u003cp\u003e \u003cb\u003e(1) Breakthrough curve of He gas\u003c/b\u003e \u003c/p\u003e \u003cp\u003eDuring curing phase, the peak-arrival time of BTCs was shorter than that in the other phases during composting process (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). However, there were no evident changes in the peak-arrival time in the initial-material, temperature-increasing, and thermophilic phases. This was due to the gradual reduction of FAS and porosity of the piles during composting, which may have been caused by compaction during the process. The peak-arrival time of BTCs was shorter than that after turning during the temperature-increasing, thermophilic, and curing phases, while it exhibited little variation after turning in the initial material. This phenomenon is possibly because turning alleviates the compaction of the piles (Scheutz et al., \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). The FAS and porosity had little change after turning in the initial-material, while they rose in the other phases (Table S4, Supplementary materials).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. He gas concentration content‒time curve of the piles before and after turning during composting: A‒D are the initial-material, temperature-increasing, thermophilic, and curing of the piles before tuning, respectively; E‒H are the initial-material, temperature-increasing, thermophilic, and curing of the piles after turning, respectively.\u003c/p\u003e \u003cp\u003eThe relative root-mean-square error (RRMSE) of the measured and simulated BTCs data of the gas tracer was extremely low (Table S5). The simulation results indicated that the inverse calculation method accurately estimated the values of \u003cem\u003eφ\u003c/em\u003e and \u003cem\u003eα\u003c/em\u003e. There were lower RRMSE between the measured and simulated BTCs after turning piles in composting process, except during the curing phase. This may be due to the two-region becoming more obvious in the piles after turning in these phases. The He gas simulated values and measured values before and after turning in different phases are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e (Figure S1 presents a complete figure). The results showed that the fitting curve and measured value during composting process were consistent.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cb\u003e(2) Variation of the air-immobile regions\u003c/b\u003e \u003c/p\u003e \u003cp\u003eDuring composting process, the air-immobile regions within the piles increased first and then decreased gradually, while the first-order mass transfer coefficient of gas showed an opposite trend to the air-immobile regions (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). The variations of \u003cem\u003eφ\u003c/em\u003e and \u003cem\u003eα\u003c/em\u003e affected the mass transfer of O\u003csub\u003e2\u003c/sub\u003e in the composting piles and the spatial distribution of the anaerobic regions in the composting particles. These value changes showed that there are more air-immobile regions in the temperature-increasing and thermophilic phases and less gas exchange between two regions in initial-material and temperature-increasing phase. Inadequate replenishment worsens the shortage of O\u003csub\u003e2\u003c/sub\u003e in air-immobile regions and expands the spatial distribution of the anaerobic part, further exacerbating the generation and emission of harmful off-gases (He et al., \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). The \u003cem\u003eα\u003c/em\u003e value reached its minimum during the temperature-increasing phase of the entire composting period. This was because of relatively low porosity and high moisture content (Rafiee et al., \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). The \u003cem\u003eφ\u003c/em\u003e value was the highest during the thermophilic phase in the whole composting process. The porous structure of the piles became more stable during curing as organic matter decomposed and the MC of the piles reduced (Castaldi et al., \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2005\u003c/span\u003e; Ge et al., \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2015\u003c/span\u003eb; Ibrahim and Horton, 2021). Therefore, more pores progressively became available for air as moisture decreased during the curing phase, resulting in the largest \u003cem\u003eα\u003c/em\u003e value in the composting process. From this study, we should reduce air-immobile regiones through increasing aeration rate or O\u003csub\u003e2\u003c/sub\u003e concentration mainly in temperature-increasing and thermophilic phases.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTurning piles reduced air-immobile regions in temperature-increasing and thermophilic phases. As for mass transfer of gas, it was poor between the air-immobile and air-mobile regions of the piles after turning in the whole composting progress (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). Turning increased FAS and porosity of piles, which compacted during composting (Zeng et al., \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). In the initial-material, the \u003cem\u003eφ\u003c/em\u003e and \u003cem\u003eα\u003c/em\u003e values were similar before and after turning, which may be attributed to the similar porous structure on day 0 before and after turning. During the temperature-increasing and thermophilic phases, there were fewer air-immobile regions and gas exchange between the air-immobile and air-mobile regions after turning. Amounts of O\u003csub\u003e2\u003c/sub\u003e was consumed in these two phases. Anaerobic regions were caused to develop by the insufficient O\u003csub\u003e2\u003c/sub\u003e (Chen et al., \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Vergara and Silver, \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2019\u003c/span\u003e; Zhang et al., \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). Turning decreased the air-immobile regions during curing phase, which may be because turning results in better pore connectivity. Overall, the \u003cem\u003eφ\u003c/em\u003e values did not change much after turning in each phase of composting, indicating that it was more important to select raw materials of the compost to increase the air-mobile regions. Additionally, the \u003cem\u003eα\u003c/em\u003e values changed little in the initial-material before and after turning. Comparatively, in the other phases of composting, the change in \u003cem\u003eα\u003c/em\u003e values were smaller after turning, which meant that the gas exchange between the air-immobile and air-mobile regions of the matrix pores slowed down.\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003e\u003cb\u003e3.3 Simulation of O\u003c/b\u003e\u003csub\u003e\u003cb\u003e2\u003c/b\u003e\u003c/sub\u003e \u003cb\u003econcentration in the matrix pores based on the TRM\u003c/b\u003e\u003c/h2\u003e \u003cp\u003eThe occurrence of air-immobile regions caused marked difference in predicted O\u003csub\u003e2\u003c/sub\u003e concentrations between air-mobile and air-immobile regions, and the difference was enhanced mainly by the reaction rate of organic-matter biodegradation during composting (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e). The predicted O\u003csub\u003e2\u003c/sub\u003e concentration underwent little change during composting in both air-mobile and air-immobile regions based on the assumption that \u003cem\u003e\u0026micro;\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003e and \u003cem\u003e\u0026micro;\u003c/em\u003e\u003csub\u003e2\u003c/sub\u003e were same in the whole composting process. This phenomenon suggests that only changing \u003cem\u003eφ\u003c/em\u003e and \u003cem\u003eα\u003c/em\u003e resulted in negligible changes on O\u003csub\u003e2\u003c/sub\u003e concentration in the air-immobile and air-mobile regions. With the variation of \u003cem\u003e\u0026micro;\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003e and \u003cem\u003e\u0026micro;\u003c/em\u003e\u003csub\u003e2\u003c/sub\u003e, there was higher predicted O\u003csub\u003e2\u003c/sub\u003e concentration in the air-mobile and air-immobile regions during curing than that observed in other phases of composting process. The variation of the \u003cem\u003e\u0026micro;\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003e and \u003cem\u003e\u0026micro;\u003c/em\u003e\u003csub\u003e2\u003c/sub\u003e during composting process initially increased and then decreased (Table S6, Supplementary materials). During the thermophilic phase, the predicted O\u003csub\u003e2\u003c/sub\u003e concentration in the air-mobile regions was higher than that in the initial-material and temperature-increasing phase, which may be caused by the greater O\u003csub\u003e2\u003c/sub\u003e consumption. The predicted O\u003csub\u003e2\u003c/sub\u003e concentrations were close to zero in the air-immobile regions during the temperature-increasing and thermophilic phases of composting, and remained low in the initial-material and curing phase, which illustrated that the air-immobile regions obviously clogged the transfer of O\u003csub\u003e2\u003c/sub\u003e in the piles. In the curing phase, the predicted O\u003csub\u003e2\u003c/sub\u003e concentration in the air-immobile regions was higher than that in other phases. These findings showed that the variation in predicted O\u003csub\u003e2\u003c/sub\u003e concentration within the matrix pores was affected by air-immobile regions in combination with first order reaction constant of O\u003csub\u003e2\u003c/sub\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eGenerally, turning increased the predicted O\u003csub\u003e2\u003c/sub\u003e concentration in the air-mobile/air-immobile regions in the whole composting process (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e). The variation of O\u003csub\u003e2\u003c/sub\u003e concentration in the matrix pores reflects the composting progress (Zeng et al., \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). There were higher predicted O\u003csub\u003e2\u003c/sub\u003e concentrations in the air-mobile and air-immobile regions in the initial-material after turning. This was due to the low O\u003csub\u003e2\u003c/sub\u003e consumption, high FAS, and high porosity in the initial-material. During the temperature-increasing and thermophilic phases, the predicted O\u003csub\u003e2\u003c/sub\u003e concentrations increased in the air-mobile regions but there were minimal changes in the air-immobile regions after turning. This may be due to more FAS and porosity after turning, resulting in a lower gas flow velocity, even when the O\u003csub\u003e2\u003c/sub\u003e supply remains unchanged in the temperature-increasing and thermophilic phases.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eOxygen uptake rate (OUR) was greater during the temperature-increasing and thermophilic phases than that in the other phases during composting (Figures S2 and S3). During composting, organic matter is mineralized with O\u003csub\u003e2\u003c/sub\u003e to form small-molecule inorganic matter through the action of aerobic microorganisms (Zhang et al., \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). Thus, in composting, OUR can reflect the intensity of aerobic microbial activity (Barrena et al., \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Ge et al., \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2015\u003c/span\u003eb). In the initial-material, the aerobic microorganisms were in the lag phase, resulting in low O\u003csub\u003e2\u003c/sub\u003e consumption (Getahun et al., \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2012\u003c/span\u003e). During the temperature-increasing phase, aerobic microorganisms grew rapidly, resulting in large amounts of organic matter and O\u003csub\u003e2\u003c/sub\u003e consumption (Tiquia, \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2005\u003c/span\u003e). The OUR during the thermophilic phase was only lower than that in the temperature-increasing phase in the whole composting process. This was because the high temperature inhibited microbial activity (Van Gestel et al., \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2003\u003c/span\u003e). The OUR was relatively low and remained almost stable in the curing phase. Thus, OUR can be used to assess compost stability in the final phase of composting. Zheng et al. (2004) further verified the feasibility of using OUR as a measure of compost maturity by correlating it with the germination index.\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003e3.4 Effects of air-immobile regions on O\u003csub\u003e2\u003c/sub\u003e concentration in matrix pores\u003c/h2\u003e \u003cp\u003eThe piles compact during the composting process, resulting in an increase in air-immobile regions that inhibit the transfer of O\u003csub\u003e2\u003c/sub\u003e between the two regions (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e). The O\u003csub\u003e2\u003c/sub\u003e transfer process between FAS and particles in the piles is reflected by the distribution of the air-immobile regions. There were fewer air-immobile regions in the piles in the initial-material. This was due to the piles compacted slightly in the initial-material, and the aerobic microorganisms were inactive. The interplay of the change in FAS, MC, and O\u003csub\u003e2\u003c/sub\u003e concentration resulted in high \u003cem\u003eφ\u003c/em\u003e, \u003cem\u003e\u0026micro;\u003c/em\u003e\u003csub\u003e\u003cem\u003e1\u003c/em\u003e\u003c/sub\u003e and \u003cem\u003e\u0026micro;\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e values but a low \u003cem\u003eα\u003c/em\u003e value during the temperature-increasing phase (Ge et al., \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Yue et al., \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e2008\u003c/span\u003e; Wu et al., \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). The compost piles were most severely compacted during the thermophilic phase, in which the \u003cem\u003eφ\u003c/em\u003e value was the highest. The \u003cem\u003eφ\u003c/em\u003e value was smaller during curing than that in the temperature-increasing and thermophilic phases, and the α value was the opposite. There were fewer air-immobile regions in the piles at this phase, while the exchange rate of O\u003csub\u003e2\u003c/sub\u003e between the air-mobile and air-immobile regions was faster.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe variation of the air-immobile regions in the piles mainly was influenced by the changes of porosity, FAS, and MC. The MC of the piles decreased generally during composting process because of the continuous ventilation (Ahn et al., \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2008\u003c/span\u003e; Shi et al., \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e1999\u003c/span\u003e). The microbial activity diminishes during the last phase of composting due to the loss of moisture (Liang et al., \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2003\u003c/span\u003e). Furthermore, the pore structure became more stable during the last phase of composting, possibly because the organic matter was consumed, which opened some of the originally closed or semi-closed pores inside the piles to form channels for gas flow (Chen et al., \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). Simultaneously, porosity and FAS in the piles gradually decreased during composting. However, there are still some knowledge gaps that need to be filled. These gaps have not been discussed in this study. For example, the change in characteristics of the microbial community in the air-immobile regions and O\u003csub\u003e2\u003c/sub\u003e concentration in the particle were not investigated. Further studies are expected to clarify the mechanism of the coupled relationship between O\u003csub\u003e2\u003c/sub\u003e concentration in matrix pores and the microbial community in the air-immobile regions.\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"4. Conclusions","content":"\u003cp\u003eThis study identified how the change of air-immobile regions drives temporal variation of O\u003csub\u003e2\u003c/sub\u003e in the pores during the whole composting process by simulating the temporal changes in O\u003csub\u003e2\u003c/sub\u003e concentration in the matrix pores and inverting the parameters of the air-immobile regions based on TRM. The main conclusions are as follows:\u003c/p\u003e \u003cp\u003e(1) During composting process, the \u003cem\u003eφ\u003c/em\u003e values for the initial-material, temperature-increasing phase, thermophilic phase, and curing phase were 0.38/0.40, 0.42/0.40, 0.46/0.46, and 0.41/0.45 before/after turning, respectively, while the \u003cem\u003eα\u003c/em\u003e values were 0.002/0.001, 0.001/0, 0.004/0, and 0.005/0.001 min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, respectively. The air-immobile regions caused the difference of predicted O\u003csub\u003e2\u003c/sub\u003e concentrations between two regions, and the difference was enhanced during the composting primarily due to the reaction rate of organic-matter biodegradation.\u003c/p\u003e \u003cp\u003e(2) Turning piles slightly decreased \u003cem\u003eφ\u003c/em\u003e in the temperature-increasing phase and had little change in thermophilic phase, while it increases in initial-material and curing phase. The \u003cem\u003eα\u003c/em\u003e values declined after turning throughout the composting process.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis work was financially supported by the National Natural Science Foundation of China (51868011), and Special Fund for Guangxi Distinguished Experts.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis work was financially supported by the National Natural Science Foundation of China (51868011), and Special Fund for Guangxi Distinguished Experts.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting Interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors have no relevant financial or non-financial interests to disclose.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor Contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by Haiguang Qin, Hongtao Liu, Yulan Lu, Jun Zhang. The first draft of the manuscript was written by Haiguang Qin and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll data generated and analyzed during the current study are available from the corresponding author on reasonable request.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eDeclaration of interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e☒\u0026nbsp;The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.\u003c/p\u003e\n\u003cp\u003e☐ The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eAhn, H.K., Richard, T.L., Glanville, T.D., 2008. Laboratory determination of compost physical parameters for modeling of airflow characteristics. Waste Management 28, 660\u0026ndash;670. https://doi.org/10.1016/j.wasman.2007.04.008\u003c/li\u003e\n \u003cli\u003eAndersson, M.J.E., Andersson, L., Bende, M., Millqvist, E., Nordin, S., 2009. The idiopathic environmental intolerance symptom inventory: Development, evaluation, and application. Journal of Occupational and Environmental Medicine 51, 838\u0026ndash;847. https://doi.org/10.1097/JOM.0b013e3181a7f021\u003c/li\u003e\n \u003cli\u003eAndraskar, J., Yadav, S., Kapley, A., 2021. Challenges and Control Strategies of Odor Emission from Composting Operation. Applied Biochemistry and Biotechnology 193, 2331\u0026ndash;2356. https://doi.org/10.1007/s12010-021-03490-3\u003c/li\u003e\n \u003cli\u003eBarrena, R., Turet, J., Busquets, A., Farr\u0026eacute;s, M., Font, X., S\u0026aacute;nchez, A., 2011. Respirometric screening of several types of manure and mixtures intended for composting. 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ACTA SCIENTIAE CIRCUMSTANTIAE 24, 1\u0026ndash;11. https://doi.org/10.13671/j.hjkxxb.2004.05.029\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":true,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"waste-and-biomass-valorization","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"wave","sideBox":"Learn more about [Waste and Biomass Valorization](http://link.springer.com/journal/12649)","snPcode":"12649","submissionUrl":"https://submission.nature.com/new-submission/12649/3","title":"Waste and Biomass Valorization","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Composting, Two-region model, Air-immobile regions, Proportional coefficient of air-mobile regions, O2 concentration","lastPublishedDoi":"10.21203/rs.3.rs-4233312/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4233312/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"Insufficient O2 concentration in the matrix pores, which is adjusted by air-immobile regions in compost piles, is a main factor in forming anaerobic cores in compost particles and then generating harmful off-gases during composting. However, it is unclear how the change of air-immobile regions affects temporal variation of O2 in the pores during the whole composting process and after turning. In this study, we first used a tracer-inverse calculation protocol to obtain feature parameters (proportional coefficient of gas in the air-immobile region, φ; the first-order mass transfer coefficient, α) of the air-immobile regions in the matrix pores before and after turning during whole composting process, and then predicted the temporal variation of O2 in the pores using two-region model with these measured parameters. The φ values in compost piles for initial-material, temperature-increasing, thermophilic, and curing phases were 0.38/0.40, 0.42/0.40, 0.46/0.46, and 0.41/0.45 before/after turning, respectively, while the corresponding α values were 0.002/0.001, 0.001/0, 0.004/0, and 0.005/0.001 min-1, respectively. The proportion of air-immobile regions was higher in the temperature-increasing and thermophilic phases than in the curing phase. The air-immobile regions caused difference of predicted O2 concentrations between air-mobile and air-immobile regions, and the difference was enhanced during the composting mainly by the rate of organic-matter biodegradation. Turning piles slightly decreased φ in the temperature-increasing phase and had little change in thermophilic phase, while it caused slight increases in φ during other phases. The value of α declined throughout composting process after turning. These findings provide support for reducing the production of harmful off-gases in composting.","manuscriptTitle":"Simulation of oxygen concentration in the matrix pores at different phases of composting process based on two-region model","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-05-20 04:14:11","doi":"10.21203/rs.3.rs-4233312/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"reviewerAgreed","content":"","date":"2024-05-13T11:54:56+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2024-05-09T06:34:03+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"Waste and Biomass Valorization","date":"2024-04-30T01:21:36+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2024-04-08T16:00:06+00:00","index":"","fulltext":""},{"type":"submitted","content":"Waste and Biomass Valorization","date":"2024-04-07T22:08:27+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"waste-and-biomass-valorization","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"wave","sideBox":"Learn more about [Waste and Biomass Valorization](http://link.springer.com/journal/12649)","snPcode":"12649","submissionUrl":"https://submission.nature.com/new-submission/12649/3","title":"Waste and Biomass Valorization","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"cbbc0151-f261-4010-84ed-a56537bff01e","owner":[],"postedDate":"May 20th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2024-10-07T16:08:34+00:00","versionOfRecord":{"articleIdentity":"rs-4233312","link":"https://doi.org/10.1007/s12649-024-02752-5","journal":{"identity":"waste-and-biomass-valorization","isVorOnly":false,"title":"Waste and Biomass Valorization"},"publishedOn":"2024-10-04 15:57:59","publishedOnDateReadable":"October 4th, 2024"},"versionCreatedAt":"2024-05-20 04:14:11","video":"","vorDoi":"10.1007/s12649-024-02752-5","vorDoiUrl":"https://doi.org/10.1007/s12649-024-02752-5","workflowStages":[]},"version":"v1","identity":"rs-4233312","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4233312","identity":"rs-4233312","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}