Case Study: Small Scale Cooling Towers Bleed-Off (blow down) in Water Recycle System at a Local Hospital

Case Study: Small Scale Cooling Towers Bleed-Off (blow down) in Water Recycle System at a Local Hospital | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Case Study: Small Scale Cooling Towers Bleed-Off (blow down) in Water Recycle System at a Local Hospital Handojo Djati Utomo, Qishan Liu, Swee Lee Lim This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6587843/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Water in cooling tower gets concentrated due to evaporation of water for heat rejection. Apart from treatment against bacteria growth, every cooling tower condenser water system needs blow down to control the water quality to minimize scaling and corrosion. Approximately 7–10% of total water consumed by cooling tower will be bleed away with a typical cycle of concentration (COC) of 4–6 subject to make-up water quality. A nano recycle unit is installed at local Hospital in Singapore to recover 65–70% bleed-off water and recycle the permeate back to cooling tower as make-up water. The blow down recycle unit started to operate in mid Jan 2023 with the key performance indexes and observations of the recycle unit after 2 years of operation showed that the overflow constitutes more 21.4% blow down water loss, which was not recoverable; the recovery efficiency of the recycle unit is 65.3% at 1.61 kWhr per m 3 of permeate; COC improved from 12.7 to 34.6; during the first 6 months of monitoring, the minimal removing efficiency of the nano filter is above 90%, especially for hardness removal. For lowering conductivity and TDS, the nano filter performance is in-line with industry expectation (80–85%), at 81.7% and 82.7% respectively. Payback period after considering operation cost (S $ 0.76 per m 3 of permeate), maintenance cost (S $ 1,200 /year) and 6% discount rate is about 10 years. A net saving of S $ 1,100 p.a. can be expected after breakeven period at present prevailing water and electricity tariffs. cooling tower blow down cycle of concentration Figures Figure 1 Figure 2 Figure 3 Figure 4 1. Introduction Singapore, a city-state in Southeast Asia near the equator, experiences one of the world's highest levels of rainfall, averaging over 2,500 mm annually as recorded in 2023 (Meteorological Service Singapore, 2023). Due to the country's hot and humid climate, water-cooled chillers are commonly used to regulate indoor temperatures in buildings. Most air-conditioning (AC) systems rely on wet or evaporative cooling towers (with a smaller share using air-cooled towers) to expel heat by evaporating water. This process results in substantial energy loss through wasted water and heat. It is estimated that cooling towers in Singapore consume over 30 million gallons of water daily (PUB, 2017 ). Industrial and manufacturing facilities also use cooling towers to meet their cooling requirements. Although these towers are an efficient solution, factors such as drought, population growth, and increasing water demand are reducing the availability of freshwater. To address future shortages, industries must focus on minimizing their freshwater consumption and increasing recycling efforts. Many industries discharge large amounts of freshwater, treated or untreated, into natural water bodies (Wang et al., 2006 ). Historically, cooling tower blowdown water was released directly into surface water sources without being treated for reuse. This practice contributed to environmental pollution and increased wastewater volumes (Zhang et al., 2007). Concerns over water scarcity, excessive blowdown water, and rising water costs have driven recent research into treating and reusing blowdown water (Wang et al., 2014 ; Zhang et al., 2007; You et al., 1999 ). Sustainable economic growth requires reducing reliance on freshwater sources, particularly in Singapore, where surface water is the primary supply. Reusing water for alternative purposes is a crucial strategy in this effort (Obaideen et al., 2022 ). Large industrial operations such as power plants, fertilizer manufacturing, and chemical processing can lower their freshwater consumption by reusing cooling water, despite the costs associated with water use (Frick et al., 2014 ; Farahani et al., 2016; Soliman et al., 2022 ). Recent studies have explored methods to reuse blowdown water from cooling towers, leading to advancements in desalination efficiency (Arias et al., 2021 ). Various technologies are available for treating cooling tower blowdown water, including reverse osmosis (RO), electrodialysis (ED), nanofiltration (NF), electrocoagulation (EC), and membrane distillation (MD) (Soliman et al., 2022 ). These technologies have been applied in different settings, from laboratory experiments to industrial-scale implementation. While NF and RO are well-established processes, emerging methods such as advanced oxidation, MD, EC, and biomimetic desalination offer promising solutions for saline water treatment (Oviroh et al., 2018 ). Although different industries can reuse blowdown water through various technologies, Singapore’s Public Utilities Board has taken an integrated approach to reducing water consumption and energy use while minimizing atmospheric water loss by recycling water within the system for conservation purposes. Several water conservation methods have been developed to lower the volume of bleed-off or blowdown water, including optimizing the Cycles of Concentration (COC), enhancing bleed system controls, maintaining bleed valves, and calibrating conductivity sensors. This case study focuses on improving COC using a nano water filtration unit. The recommended blowdown percentage is 10% of the make-up water (EPA, 2017 ), but this figure can exceed 50%. Therefore, reclaiming and reusing blowdown water presents an opportunity to significantly reduce freshwater demand (Altman et al., 2012 ; Gartiser and Urich, 2002). Typically, 7–10% of the total water used in a cooling tower is lost as bleed-off, with COC values ranging from 4 to 6 depending on the quality of the make-up water (Edmonson, 2014 ). Cooling towers are categorized into three main types: open-circuit, closed-circuit, and hybrid cooling towers. Hybrid cooling towers are further classified into natural draft and mechanical draft types. The latter includes four subtypes: induced draft counterflow, induced draft crossflow, forced draft counterflow, and forced draft crossflow (US Department of Energy, 2011 ). In Singapore, the most commonly used type is the induced draft crossflow cooling tower, which requires approximately 0.5 to 4 m³/h/MW of water for cooling (PUB, 2017 ). This design is preferred because it minimizes air resistance and reduces the energy needed to operate the fans while meeting cooling demands (Wang and Wang, 2022 ). Figure 1 shows the schematic water balance of an induced draft cross flow tower involves all water inputs and outputs associated with the operation of the system. Water outputs include controlled losses such as evaporation (above the system), bleed-off/ blow down (below the system), and drift (inside, middle of the system) and water pump leakage and uncontrolled losses including leaks, splash out, overflows and windage (PUB, 2017 , Jan Taler et al ., 2021). All the water losses are replaced by makeup water from a water supply system, and in very small amount, by any rainwater that may enter the cooling tower from the air outlet opening. In general, water in cooling tower gets concentrated due to evaporation of water for heat rejection. Apart from treatment against bacteria growth, every cooling tower condenser water system needs blow down (bleed-off) to control the water quality to minimize scaling and corrosion (Farahani et al. , 2016; Rahmani, 2017 ; Saha et al., 2020 ). 2. Installation of Nano Water Filtration Unit in a Local Hospital For this study the blowdown water from KTP hospital will be conserved and reuse as a make- up water in the same hospital by improving COC. From common water treatment technologies such as reverse osmosis (RO), electrodialysis (ED), nanofiltration (NF), electrocoagulation (EC), and membrane distillation (MD) (Soliman et al., 2022 ), an ultra- nano was selected for an application in a hospital. Here at KTP hospital, an ultra- nano water filtration unit was installed to recover 65–70% bleed-off water and recycle the permeate back to cooling tower as the make-up water, which is much higher than recommended 10% of blowdown percentage with the feedwater can be from freshwater sources (EPA, 2017 ). In KTP hospital, the feedwater mainly sourced from NEWater. NEWater is high-grade recycled water produced from treated used water that is further purified using advanced membrane technologies and ultra-violet disinfection, making it ultra-clean with directly targeted for industrial usage (PUB, 2025 ). Due to a high quality of feedwater pre-treatment using ultra filtration was conducted instead of using conventional method of coagulation and flocculation (Farahani et al. , 2016). The project was commissioned with the objectives to optimize the cooling tower unit’s performance, evaluate the efficiency and cost effectiveness of the re-cycle unit. Details of recovery unit is present in Fig. 2 below. The nano water filtration unit was commissioned on 12th January 2023 and set to auto operation on 13th January 2023 with the performance monitoring commenced thereafter. The initial blow down and recycle water meter was recorded at 1,560.434 m 3 and 1.036 m 3 respectively after the power meter starting to record at 2.9 kWhr. Figure 3 shows the installation of nano water filtration unit and the meter readings of initial blow down, recycle water and power. 3. Base information, field data collection and computation 3.1 Bleed-Off (blow down) Water Recycle Unit: Although the system is commissioned on 12th Jan 2023, its operation only stabilized after end March 2023. This was because the cooling tower experienced overflow due to imbalance condenser water return to cooling towers. 3.2 Blow down rate and quantity The original predicted blow down rate was 7.3 m 3 /day. This was supported by the blow down quantity registered between 31st Mar 22 to 15th July 2022, where actual blow down is 7.44 m 3 /day. However, the average blow down reduced to 5.21 m 3 /day when longer period is considered (25st Mach 2022 to 11th Aug 2023). The reason for reducing blow down quantity was due to overflow of cooling tower and other water losses in months of July 2022, September 2022, December 2022, February 2023 and March 2023, where conductivity of tower water went below 800 µs/cm. Nonetheless, if we considered 31st Mar to 11th Aug 2023 period where no overflow happened to cooling towers, the average blow down rate was approximately 7.13 m 3 /day. This was close to the original prediction. However, cooling tower basin overflow problem persisted again within 17th Nov to 29th Dec 2023 and 22nd Nov to 27th Dec 2024. The average blow down rate for the period reduced to 5.74 m³/day. The water loss due to cooling tower overflow during this period account to almost 21.4% of the blow down water. 3.3. Cost of consumables - Anti-scalant and Anti-bacteria biocide used To avoid scales and biofilm developed on the nano filter, a small amount of chemical was injected to the blow down water before going through the nano filtration unit. Biocides are commercially available chemicals that can kill or inhibit the growth of small living organisms, such as bacteria, fungi, slimes ad molds. Typical biocides used in industrial cooling tower including chlorine gas, sodium hypochlorite, bromine, chlorine dioxide (ClO 2 ), and ozone (Wang and Wang, 2022 ). In this project, the estimated amounted of chemicals used were calculated as below: The anti-scalant used is Flocon 260, applied at 2 ppm, weekly consumption is about 150 mL. Purchase price of Flocon 260 is S $ 178.47 per 21.7 litres (or S $ 6.21 per litre). Estimated weekly dosing cost is about S $ 0.93 or S $ 0.029/m 3 of permeate (based on 4.57 m 3 water saved per day). The anti-bacteria biocide used is MIT-14, applied at 5 ppm, weekly consumption is about 300 mL. Purchase price of MIT-14 is S $ 7.0 per litre. Estimated weekly dosing cost is about S $ 2.1 or S $ 0.066/m 3 of permeate (based on 4.57 m 3 water saved per day) 3.4. Cost of consumables - Nano filter and Ultra filter media replacement (at 65% system efficiency) Ultra filter unit provides first line of defense to the nano filter unit. Base on the operation records (by observing the pressure built up at the pressure gauges at ultra filter), ultra filter media is replaced for every 120 m 3 of water (or about 4.6 weeks interval) in order to reduce the additional pump power. Filter replacement cost is S $ 10/set. This work out to be S $ 10/120 = 0.833/m 3 of permeate. Estimated life span of nano filter is about 2.5 year or after 4280 m 3 (base on 2.5 years @4.69 m 3 /day) of permeate. At S $ 950 per set, the replacement cost was calculated as S $ 950/4280 = S $ 0.222/m 3 of permeate. 3.5 NEWater & Electricity Tariffs As electricity tariff fluctuated during the period, average tariff rate was used in the analysis. The average electricity tariff during 2023 Quarter 3 (Q3) basing on average of peak (S $ 0.271/kWhr) and off-peak (S $ 0.164/kWhr) rates was S $ 0.218 / kWhr. The average tariff during 2022 Q1, Q2, Q3, Q4, 2023 Q1, and Q2 were recorded at S $ 0.194/kWhr, S $ 0.214/kWhr, S $ 0.236/kWhr, S $ 0.233/kWhr, S $ 0.227/kWhr, S $ 0.225/kWhr respectively. In addition, the average tariff for 2022 Q1 to 2023 Q3 is S $ 0.221/kWhr (or S $ 0.239/kWhr plus GST). NEWater was an ultrapure water which was developed in Singapore to be an alternative water source for semiconductor or high-tech industries. The NEWater rate applied was S $ 2.5/m 3 (exclude GST). 4. Analysis and Discussion 4.1 Sustainability of recovery efficiency for the nano filtration unit During the initial performance evaluation within 31 st March to 11 th August 2023, the total blow down quantity is 948.13 m³. The total recycled water (permeate) recorded was 618.56 m³. Therefore, the water recovery efficiency is 65.2%. This is within the original prediction of 65-70% recovery efficiency and greater than 50 %, therefore it is potential to reduce freshwater needs when blowdown water was partly reclaimed and reused (Altman et al ., 2012; Gartiser and Urich, 2002). The power consumption during this period is 1,000.7 kWhr. This work showed that 1.62 kWhr per m³ of permeate recovered. In a subsequent monitoring period (11 th Aug 2023 to 10 th Jan 2025), the total blow down quantity during this period was 2,756.94 m³. The total recycled water (permeate) recorded was 1,801.4 m³. The water recovery efficiency achieved was 65.3% and comparable to the earlier evaluation period from 31 st March to 11 th August 2023. The power consumption during this period is 2,895.5 kWhr. This work showed that 1.61 kWhr per m³ of permeate recovered over this period. The unit performance is thus sustainable. Figure 4 shows the relationship of recovery efficiency and energy consumption (kWh/m³) over the monitored period. While periodic adjustment of return water flow rate to nano filter keeps recovery efficiency at desirable range, energy consumption however continues to increase as fouling built up at the filter membrane. The average energy consumption reached above 3 kWh/m³ in August 2024. This condition was corrected after membrane undergo cleaning at the end of August 2024. 4.2. Overflow affecting blowdown 4.2.1 Blow down rate (31 st March to 11 th August 2023) The original predicted blow down rate was 7.3 m³/day. However, the average blow down reduced to 5.21 m³/day (31 st March to 11 th Aug 2023) due to overflow. The water loss due to cooling tower overflow during this period (31 st March to 11 th Aug 2023) account to almost 27% of the blow down water. 4.2.2 Blow down rate (11 th August 2023 to 10 th January 2025) Cooling tower basin overflow problem persisted again during 17 th November to 29 th December 2023 and 22 nd November to 27 th December 2024. The average blow down rate for this period reduce to 5.38 m³/day. If considered from 31 st Mar 2023 to 10 th January 2025, the overall blow down rate is 5.74 m³/day. This was substantially lower than original prediction. It meant that over the period, about 21.4% of blow down not being recovered. At 65% efficiency, this work out to be 654 m³ of permeate loss. This excludes the direct water loss from basin overflow. 4.3. Improvement of COC The following formula was used to calculate COC: Blow down = Evaporation / (COC - 1) [Eq. 1] B o = E o / COC o -1 [Eq. 2] COC o = Average CT conductivity / make-up water conductivity. [Eq. 3] COC o = 1163 (average value of 31 st Mar 23 to 10 th Jan 2025) / 91.85 (based on reference water index) =12.66 Where “ o “ is the initial value, “ 1 “ is the new value To determine the B o the following data was used: Average blow now during 31 st March 23 to 10 th Jan 2025 (645 days): Blow down meter reading on 31 st Mar 2023 = 1,676.39 m 3 Blow down meter reading on 10 th Jan 2025 = 5,382.15 m 3 Net blow down quantity = 3,705.76 m 3 over 645 days Blow down rate per day “ B o “ = 3,705.76 / 645 = 5.74 m 3 /day Average water saved during 31 st Mar 23 to 10 th Jan 2025 (645 days): Recycle water meter reading on 31 st Mar 2023 =: 67.7 m 3 Recycle water meter reading on 10 th Jan 2025 = 2,487.9 m 3 Net water quantity saved = 2,420.2 m 3 over 645 days Net water saving per day = 2,420.2 / 645 = 3.75 m 3 /day Recovery efficiency during this period is 2420.2 / 3705.76 = 65.3 % Compute E o : E o = B o * (12.66 - 1) = 5.74 * 11.66 = 66.93 m 3 /day Because we are recovering the blow down water, Evaporation (E o ) rate remains unchanged. E o = E 1 Compute new COC 1 : E 1 = B 1 *( COC 1 -1) B 1 = B o – water saved = 5.74 – 3.75 = 1.99 m 3 /day 66.93 = 1.99*(COC 1 -1) COC 1 = 34.6 This COC of 34.6 is achieved at recovery efficiency of 65.3%. As long as the recovery rate improved, COC will be better. In this case, the COC improved from 12.7 to 34.6, after implementing the nano filtration unit. The high COC was achieved due to possible higher quality of the make-up water which sourced from NEWater, in contrary to common COC of 4 to 6 that depended on make-up water quality (Edmonson, 2014). 4.4. Efficiency of Nano Filtration System Unit The efficiency of nano filter in removing the impurities in the blow down water were presented in Table 1. Table 1. Percentage of water impurities reduction using installed nano filtration unit at KTP Hospital Conductivity: 81.7% TDS: 82.7% Total Hardness: 98.9% Calcium Hardness: 99.5% Chloride: 84.7% M-Alk: 91.6% The results shown nano filter is very efficient in removing minimal and rejection ranges in-line with expectation (OSTI, 2010). 4.5. Review of Payback period The original estimate is that the pre-filter will be replaced at every 100 m³ of permeate produced (or about 3 weeks interval). Total of 20 times of pre-filter replacement were carried out within 31 st March 23 to 10 th January 2025, this work out to be 4.6 weeks interval (or 120.9 m³ of permeate). Operating cost for pre-filter is thus substantially lower than expectation. The original estimated life span of nano filter is about 2.5 year or after 4,280 m³of permeate recovery. As of 10 th January 2025, the nano filter has been operating for 2 years and recovered total of 2420.2 m³ of permeate. Only one time of chemical cleaning were carried out so far, we can thus expect the nano filter continue to perform within its life span expectancy. The operation cost consists of electricity consumed by the recycle pump and consumables as discussed in Section 3 above. Total operation cost for above equal to S$0.764 per m 3 of permeate. The maintenance cost (S$1,200 /year) and a 6% discount rate is being used for payback calculation. The calculation shows that the payback period is about 10 years. The payback is substantially affected by cooling tower overflow. A net saving of S$ 1,069 / year can be expected after breakeven period at present prevailing water and electricity tariffs. 5. Conclusion The blow down recycle unit started to operate in mid January 2023. In collaboration between Singapore Polytechnic’s Digital Building Innovation Centre [DBIC] and Medec Pte Ltd, the key performance indexes and observations of the recycle unit could be summarized as: Blow down rate at KTP Hospital was affected by cooling tower overflow. For the monitored period, the overflow constitutes nearly 21.4% blow down water loss, which is not recoverable. The recovery efficiency of the recycle unit is 65.3% at 1.61 kWhr per m 3 of recovered water (permeate), which is within the predication of 65–70%. Cycle of Concentration (COC) for cooling tower improved from 12.7 to 34.6. The minimal removing efficiency of the nano filter is above 90%, especially for hardness removal, which is above 98%. For lowering conductivity and TDS, the nano filter performance is in-line with industry expectation (80–85%), at 81.7% and 82.7% respectively. Payback period after considering operation cost (S $ 0.764 per m 3 of permeate), maintenance cost (S $ 1,200 p.a.) and 6% discount rate is about 10 years. A net saving of about S $ 1,100 p.a. can be expected after breakeven period at present prevailing water and electricity tariffs. Declarations Acknowledgements: we thank you Singapore Polytechnic’s management for facilitating a collaborative works Funding: Singapore Polytechnic, Ideafarm’s R725 for the design of the study and collection, analysis, and interpretation of data. Authors’ Contributions: all the 3 authors i.e. Handojo Djati Utomo, Qishan Liu and Swee Lee Lim were equally contributed to the project from conducting literature review, data collection, analysis and report writing. Ethical Approval: not applicable Consent to Participate: not applicable Consent to Publish: not applicable Competing Interest: the authors declare that they have no competing interests Data Availability Statement: the datasets used and/or analysed during the current study are available from the corresponding author on reasonable request. References Altman, S.J., Jensen, R.P., Cappelle, M.A., Sanchez, A.L., Everett, R.L., Anderson, H.L., McGrath, L.K., 2012. Membrane treatment of side-stream cooling tower water for reduction of water usage. Desalination 285, 177–183. https://doi.org/10.1016/j.desal.2011.09.052 Arias, B.G., N. Merayo, A. Mill´an, C. Negro, Sustainable recovery of wastewater to be reused in cooling towers: towards circular economy approach, J. Water Process Eng. 41 (2021), https://doi.org/10.1016/j.jwpe.2021.102064 Edmonson, Chad (2014), Cooling Tower and Condenser Water Design Part 9: Controlling Cycles of Concentration. (https://jmpcoblog.com/hvac-blog/cooling-tower-and-condenser-water-design-part-9-controlling-cycles-of-concentration) EPA, 2017. Water efficiency management guide: mechanical systems. EPA 832-F-17-016c. https://www.epa.gov/sites/default/files/2017-12/documents/ws-commercial buildings-waterscore-mechanical-systems-guide.pdf Farahani, M.H.D.A, Seyed Mehdi Borghei, Vahid Vatanpour (2016), Recovery of cooling tower blowdown water for reuse: The investigation of different types of pretreatment prior nanofiltration and reverse osmosis, Journal of Water Process Engineering 10, Page 188-199. Frick, J.M., L.A. F´eris, I.C. Tessaro, Evaluation of pretreatments for a blowdown stream to feed a filtration system with discarded reverse osmosis membranes, Desalination 341 (2014) 126–134, https://doi.org/10.1016/j.desal.2014.02.033 Gartiser, S., Urich, E., 2002. Environmentally compatible cooling water treatment chemicals. UBA, Germany, Berlin. Research Report 200 (24), 233. https://www.hydrotox.de/fileadmin/user_upload/pdfs/forschungen/projekte/UBA_cooling_water_text_05-11-02.pdf Jan Taler, Bartosz Jagieła, Magdalena Jaremkiewicz (2021), Improving efficiency and lowering operating costs of evaporative cooling, MATEC Web of Conferences 338, 01027. Meteorogical Meteorological Service Singapore (2023), Singapore Climate 2023: The Year in Numbers. https://www.weather.gov.sg/wp-content/uploads/2024/01/The_Year_in_Numbers_2023.pdf Obaideen, K., N. Shehata, E.T. Sayed, M.A. Abdelkareem, M.S. Mahmoud, A. G. Olabi, The role of wastewater treatment in achieving sustainable development goals (SDGs) and sustainability guideline, Energy Nexus 7 (2022), https://doi.org/10.1016/j.nexus.2022.100112 Oviroh, P.O., R. Akbarzadeh, T.C. Jen, Biomimetic membrane simulation for water desalination, ASME Int. Mech. Eng. Congr. Expo., Proc. (IMECE) (2018), https://doi.org/10.1115/IMECE201886664 OSTI (2010), Office of Scientific and Technical Information, Conference: Use of nanofiltration to reduce cooling tower water consumption. OSTI ID:1028304 Oct 2010. Altman, Susan Jeanne; Ciferno, Jared PUB (2017), Technical Reference for Water Conservation in Cooling Towers 1 st Edition: Nov 2017 PUB, 2025 NEWater Quality | PUB, Singapore’s National Water Agency Rahmani, Kh. (2017), Reducing water consumption by increasing the cycles of concentration and Considerations of corrosion and scaling in a cooling system, Applied Thermal Engineering 114, Pages 849-856. Saha, P., Thomas V. Wagner, Jiahao Ni, Alette A.M. Langenhoff, Harry Bruning, Huub H.M. Rijnaarts (2020), Cooling tower water treatment using a combination of electrochemical oxidation and constructed wetlands, Process Safety and Environmental Protection 144, Pages 42-5. Soliman, M., F. Eljack, M.K. Kazi, F. Almomani, E. Ahmed, Z. El Jack, Treatment technologies for cooling water blowdown: a critical review, Sustain. (Switz.) 14 (2022), https://doi.org/10.3390/su14010376 US Department of Energy (2011), Cooling Towers: Understanding Key Components of Cooling Towers and How to Improve Water Efficiency. Wang. Z, Z. Fan, L. Xie, S. Wang, Study of integrated membrane systems for the treatment of wastewater from cooling towers, Desalination 191 (2006) 117–124, https://doi.org/10.1016/j.desal.2005.04.125 Wang, MHS, and Wang, LK (2022). Cooling tower and boiler water treatment technologies. In: " Environmental Science, Technology, Engineering, and Mathematics (STEM)", Wang, LK, Wang, MHS, and Pankivskyi, YI (editors). Volume 2022, Number 3, March 2022; 88 pages. Lenox Institute Press, MA, USA https://doi.org/10.17613/jge9-ey13 Wang, F.H., H.T. Hao, R.F. Sun, S.Y. Li, R.M. Han, C. Papelis, Y. Zhang (2014), Bench-scale and pilot-scale evaluation of coagulation pre-treatment for wastewater reused by reverse osmosis in a petrochemical circulating cooling water system, Desalination 335 (1): 64–69. You, S.H. , D.H. Tseng, G.L. Guo, J.J. Yang (1999), The potential for the recovery and reuse of cooling water in Taiwan, Resour. Conserv. Recycl. 26 (1): 53–70. Zhang. J., L. Chen, H. Zeng, X. Yan, X. Song, H. Yang, C. Ye (2007), Pilot testing of outside-in MF and UF modules used for cooling tower blowdown pretreatment of power plants, Desalination 214 (1–3): 287–298. Additional Declarations No competing interests reported. Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-6587843","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":457094079,"identity":"a43d8616-f5a1-4331-a413-9ba8623f3a41","order_by":0,"name":"Handojo Djati Utomo","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAr0lEQVRIiWNgGAWjYBACAwYGxgdg1gEStDAbkKyFTYI0LebsZ49V/tzBIMd3I4H54xditFj25KXd5j3DYCx5I4FNWoYohx3IMbvN2MaQuAGohVmCKC3n35gV/mxjqAdqYf5MnJYbOWYMvG0MCQY3EhgkPxCn5Y2xNO8ZCcOZZx62SROjA+iwHMOPP3fYyPMdTz788QdRekCAsUECTDLzkKAFShNvyygYBaNgFIwkAAAvUzO11So+ZAAAAABJRU5ErkJggg==","orcid":"","institution":"Singapore Polytechnic","correspondingAuthor":true,"prefix":"","firstName":"Handojo","middleName":"Djati","lastName":"Utomo","suffix":""},{"id":457094080,"identity":"a0abfeb7-c1ab-442b-8daf-c84846586eaf","order_by":1,"name":"Qishan Liu","email":"","orcid":"","institution":"Singapore Polytechnic","correspondingAuthor":false,"prefix":"","firstName":"Qishan","middleName":"","lastName":"Liu","suffix":""},{"id":457094082,"identity":"ff9cbcd4-b557-41a6-92f9-ff16dece196f","order_by":2,"name":"Swee Lee Lim","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Swee","middleName":"Lee","lastName":"Lim","suffix":""}],"badges":[],"createdAt":"2025-05-04 10:08:13","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6587843/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6587843/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":83079185,"identity":"662e7e2d-e8bb-4405-ad84-5915fc8d658e","added_by":"auto","created_at":"2025-05-19 18:59:00","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":66408,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic of water balance in a cooling tower (PUB, 2017)\u003c/p\u003e","description":"","filename":"1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6587843/v1/043e902d70e849f3927be697.jpg"},{"id":83079192,"identity":"5046972e-dbdb-4867-a6b9-147a545ed4be","added_by":"auto","created_at":"2025-05-19 18:59:00","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":101992,"visible":true,"origin":"","legend":"\u003cp\u003eProcess flow diagram of the recovery system\u003c/p\u003e","description":"","filename":"Onlinefloatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-6587843/v1/b8b7ad9b076df9b2a0ddd0e2.png"},{"id":83079189,"identity":"8b069b6b-e17b-42fd-8212-27951eea4521","added_by":"auto","created_at":"2025-05-19 18:59:00","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":186360,"visible":true,"origin":"","legend":"\u003cp\u003eMeter readings of initial blow down and recycle water and power (left) and nano filtration unit (right).\u003c/p\u003e","description":"","filename":"3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6587843/v1/b3d62da584054657634d9482.jpg"},{"id":83079193,"identity":"7bfd34e6-684e-452c-9458-168585596255","added_by":"auto","created_at":"2025-05-19 18:59:00","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":115800,"visible":true,"origin":"","legend":"\u003cp\u003eRecovery efficiency \u0026amp; Energy consumption (kWh/m\u003csup\u003e3\u003c/sup\u003e)\u003c/p\u003e","description":"","filename":"4.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6587843/v1/f844baf87798795c8eb01ab7.jpg"},{"id":90507400,"identity":"a3c76f5c-8d81-468d-bb45-33679f4d7f73","added_by":"auto","created_at":"2025-09-03 12:54:21","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1066282,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6587843/v1/60ba221f-4b52-4e69-85fd-3fa41d020a37.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Case Study: Small Scale Cooling Towers Bleed-Off (blow down) in Water Recycle System at a Local Hospital","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eSingapore, a city-state in Southeast Asia near the equator, experiences one of the world\u0026apos;s highest levels of rainfall, averaging over 2,500 mm annually as recorded in 2023 (Meteorological Service Singapore, 2023). Due to the country\u0026apos;s hot and humid climate, water-cooled chillers are commonly used to regulate indoor temperatures in buildings. Most air-conditioning (AC) systems rely on wet or evaporative cooling towers (with a smaller share using air-cooled towers) to expel heat by evaporating water. This process results in substantial energy loss through wasted water and heat. It is estimated that cooling towers in Singapore consume over 30\u0026nbsp;million gallons of water daily (PUB, \u003cspan class=\"CitationRef\"\u003e2017\u003c/span\u003e).\u003c/p\u003e\n\u003cp\u003eIndustrial and manufacturing facilities also use cooling towers to meet their cooling requirements. Although these towers are an efficient solution, factors such as drought, population growth, and increasing water demand are reducing the availability of freshwater. To address future shortages, industries must focus on minimizing their freshwater consumption and increasing recycling efforts. Many industries discharge large amounts of freshwater, treated or untreated, into natural water bodies (Wang et al., \u003cspan class=\"CitationRef\"\u003e2006\u003c/span\u003e). Historically, cooling tower blowdown water was released directly into surface water sources without being treated for reuse. This practice contributed to environmental pollution and increased wastewater volumes (Zhang et al., 2007). Concerns over water scarcity, excessive blowdown water, and rising water costs have driven recent research into treating and reusing blowdown water (Wang et al., \u003cspan class=\"CitationRef\"\u003e2014\u003c/span\u003e; Zhang et al., 2007; You et al., \u003cspan class=\"CitationRef\"\u003e1999\u003c/span\u003e).\u003c/p\u003e\n\u003cp\u003eSustainable economic growth requires reducing reliance on freshwater sources, particularly in Singapore, where surface water is the primary supply. Reusing water for alternative purposes is a crucial strategy in this effort (Obaideen et al., \u003cspan class=\"CitationRef\"\u003e2022\u003c/span\u003e).\u003c/p\u003e\n\u003cp\u003eLarge industrial operations such as power plants, fertilizer manufacturing, and chemical processing can lower their freshwater consumption by reusing cooling water, despite the costs associated with water use (Frick et al., \u003cspan class=\"CitationRef\"\u003e2014\u003c/span\u003e; Farahani et al., 2016; Soliman et al., \u003cspan class=\"CitationRef\"\u003e2022\u003c/span\u003e). Recent studies have explored methods to reuse blowdown water from cooling towers, leading to advancements in desalination efficiency (Arias et al., \u003cspan class=\"CitationRef\"\u003e2021\u003c/span\u003e).\u003c/p\u003e\n\u003cp\u003eVarious technologies are available for treating cooling tower blowdown water, including reverse osmosis (RO), electrodialysis (ED), nanofiltration (NF), electrocoagulation (EC), and membrane distillation (MD) (Soliman et al., \u003cspan class=\"CitationRef\"\u003e2022\u003c/span\u003e). These technologies have been applied in different settings, from laboratory experiments to industrial-scale implementation. While NF and RO are well-established processes, emerging methods such as advanced oxidation, MD, EC, and biomimetic desalination offer promising solutions for saline water treatment (Oviroh et al., \u003cspan class=\"CitationRef\"\u003e2018\u003c/span\u003e).\u003c/p\u003e\n\u003cp\u003eAlthough different industries can reuse blowdown water through various technologies, Singapore\u0026rsquo;s Public Utilities Board has taken an integrated approach to reducing water consumption and energy use while minimizing atmospheric water loss by recycling water within the system for conservation purposes.\u003c/p\u003e\n\u003cp\u003eSeveral water conservation methods have been developed to lower the volume of bleed-off or blowdown water, including optimizing the Cycles of Concentration (COC), enhancing bleed system controls, maintaining bleed valves, and calibrating conductivity sensors. This case study focuses on improving COC using a nano water filtration unit.\u003c/p\u003e\n\u003cp\u003eThe recommended blowdown percentage is 10% of the make-up water (EPA, \u003cspan class=\"CitationRef\"\u003e2017\u003c/span\u003e), but this figure can exceed 50%. Therefore, reclaiming and reusing blowdown water presents an opportunity to significantly reduce freshwater demand (Altman et al., \u003cspan class=\"CitationRef\"\u003e2012\u003c/span\u003e; Gartiser and Urich, 2002). Typically, 7\u0026ndash;10% of the total water used in a cooling tower is lost as bleed-off, with COC values ranging from 4 to 6 depending on the quality of the make-up water (Edmonson, \u003cspan class=\"CitationRef\"\u003e2014\u003c/span\u003e).\u003c/p\u003e\n\u003cp\u003eCooling towers are categorized into three main types: open-circuit, closed-circuit, and hybrid cooling towers. Hybrid cooling towers are further classified into natural draft and mechanical draft types. The latter includes four subtypes: induced draft counterflow, induced draft crossflow, forced draft counterflow, and forced draft crossflow (US Department of Energy, \u003cspan class=\"CitationRef\"\u003e2011\u003c/span\u003e). In Singapore, the most commonly used type is the induced draft crossflow cooling tower, which requires approximately 0.5 to 4 m\u0026sup3;/h/MW of water for cooling (PUB, \u003cspan class=\"CitationRef\"\u003e2017\u003c/span\u003e). This design is preferred because it minimizes air resistance and reduces the energy needed to operate the fans while meeting cooling demands (Wang and Wang, \u003cspan class=\"CitationRef\"\u003e2022\u003c/span\u003e).\u003c/p\u003e\n\u003cp\u003eFigure 1 shows the schematic water balance of an induced draft cross flow tower involves all water inputs and outputs associated with the operation of the system. Water outputs include controlled losses such as evaporation (above the system), bleed-off/ blow down (below the system), and drift (inside, middle of the system) and water pump leakage and uncontrolled losses including leaks, splash out, overflows and windage (PUB, \u003cspan class=\"CitationRef\"\u003e2017\u003c/span\u003e, Jan Taler \u003cem\u003eet al\u003c/em\u003e., 2021). All the water losses are replaced by makeup water from a water supply system, and in very small amount, by any rainwater that may enter the cooling tower from the air outlet opening.\u003c/p\u003e\n\u003cp\u003eIn general, water in cooling tower gets concentrated due to evaporation of water for heat rejection. Apart from treatment against bacteria growth, every cooling tower condenser water system needs blow down (bleed-off) to control the water quality to minimize scaling and corrosion (Farahani \u003cem\u003eet al.\u003c/em\u003e, 2016; Rahmani, \u003cspan class=\"CitationRef\"\u003e2017\u003c/span\u003e; Saha et al., \u003cspan class=\"CitationRef\"\u003e2020\u003c/span\u003e).\u003c/p\u003e"},{"header":"2. Installation of Nano Water Filtration Unit in a Local Hospital","content":"\u003cp\u003eFor this study the blowdown water from KTP hospital will be conserved and reuse as a make- up water in the same hospital by improving COC.\u003c/p\u003e \u003cp\u003eFrom common water treatment technologies such as reverse osmosis (RO), electrodialysis (ED), nanofiltration (NF), electrocoagulation (EC), and membrane distillation (MD) (Soliman et al., \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2022\u003c/span\u003e), an ultra- nano was selected for an application in a hospital.\u003c/p\u003e \u003cp\u003eHere at KTP hospital, an ultra- nano water filtration unit was installed to recover 65\u0026ndash;70% bleed-off water and recycle the permeate back to cooling tower as the make-up water, which is much higher than recommended 10% of blowdown percentage with the feedwater can be from freshwater sources (EPA, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2017\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eIn KTP hospital, the feedwater mainly sourced from NEWater. NEWater is high-grade recycled water produced from treated used water that is further purified using advanced membrane technologies and ultra-violet disinfection, making it ultra-clean with directly targeted for industrial usage (PUB, \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). Due to a high quality of feedwater pre-treatment using ultra filtration was conducted instead of using conventional method of coagulation and flocculation (Farahani \u003cem\u003eet al.\u003c/em\u003e, 2016).\u003c/p\u003e \u003cp\u003eThe project was commissioned with the objectives to optimize the cooling tower unit\u0026rsquo;s performance, evaluate the efficiency and cost effectiveness of the re-cycle unit. Details of recovery unit is present in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e2\u003c/span\u003e below.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe nano water filtration unit was commissioned on 12th January 2023 and set to auto operation on 13th January 2023 with the performance monitoring commenced thereafter. The initial blow down and recycle water meter was recorded at 1,560.434 m\u003csup\u003e3\u003c/sup\u003e and 1.036 m\u003csup\u003e3\u003c/sup\u003e respectively after the power meter starting to record at 2.9 kWhr. Figure\u0026nbsp;3 shows the installation of nano water filtration unit and the meter readings of initial blow down, recycle water and power.\u003c/p\u003e"},{"header":"3. Base information, field data collection and computation","content":"\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e3.1 Bleed-Off (blow down) Water Recycle Unit:\u003c/h2\u003e \u003cp\u003eAlthough the system is commissioned on 12th Jan 2023, its operation only stabilized after end March 2023. This was because the cooling tower experienced overflow due to imbalance condenser water return to cooling towers.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e3.2 Blow down rate and quantity\u003c/h2\u003e \u003cp\u003eThe original predicted blow down rate was 7.3 m\u003csup\u003e3\u003c/sup\u003e/day. This was supported by the blow down quantity registered between 31st Mar 22 to 15th July 2022, where actual blow down is 7.44 m\u003csup\u003e3\u003c/sup\u003e/day. However, the average blow down reduced to 5.21 m\u003csup\u003e3\u003c/sup\u003e/day when longer period is considered (25st Mach 2022 to 11th Aug 2023). The reason for reducing blow down quantity was due to overflow of cooling tower and other water losses in months of July 2022, September 2022, December 2022, February 2023 and March 2023, where conductivity of tower water went below 800 \u0026micro;s/cm. Nonetheless, if we considered 31st Mar to 11th Aug 2023 period where no overflow happened to cooling towers, the average blow down rate was approximately 7.13 m\u003csup\u003e3\u003c/sup\u003e/day. This was close to the original prediction. However, cooling tower basin overflow problem persisted again within 17th Nov to 29th Dec 2023 and 22nd Nov to 27th Dec 2024. The average blow down rate for the period reduced to 5.74 m\u0026sup3;/day. The water loss due to cooling tower overflow during this period account to almost 21.4% of the blow down water.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e3.3. Cost of consumables - Anti-scalant and Anti-bacteria biocide used\u003c/h2\u003e \u003cp\u003eTo avoid scales and biofilm developed on the nano filter, a small amount of chemical was injected to the blow down water before going through the nano filtration unit. Biocides are commercially available chemicals that can kill or inhibit the growth of small living organisms, such as bacteria, fungi, slimes ad molds. Typical biocides used in industrial cooling tower including chlorine gas, sodium hypochlorite, bromine, chlorine dioxide (ClO\u003csub\u003e2\u003c/sub\u003e), and ozone (Wang and Wang, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2022\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eIn this project, the estimated amounted of chemicals used were calculated as below:\u003c/p\u003e \u003cp\u003eThe anti-scalant used is Flocon 260, applied at 2 ppm, weekly consumption is about 150 mL. Purchase price of Flocon 260 is S\u003cspan\u003e$\u003c/span\u003e178.47 per 21.7 litres (or S\u003cspan\u003e$\u003c/span\u003e 6.21 per litre). Estimated weekly dosing cost is about S\u003cspan\u003e$\u003c/span\u003e0.93 or S\u003cspan\u003e$\u003c/span\u003e0.029/m\u003csup\u003e3\u003c/sup\u003e of permeate (based on 4.57 m\u003csup\u003e3\u003c/sup\u003e water saved per day).\u003c/p\u003e \u003cp\u003eThe anti-bacteria biocide used is MIT-14, applied at 5 ppm, weekly consumption is about 300 mL. Purchase price of MIT-14 is S\u003cspan\u003e$\u003c/span\u003e7.0 per litre. Estimated weekly dosing cost is about S\u003cspan\u003e$\u003c/span\u003e2.1 or S\u003cspan\u003e$\u003c/span\u003e0.066/m\u003csup\u003e3\u003c/sup\u003e of permeate (based on 4.57 m\u003csup\u003e3\u003c/sup\u003e water saved per day)\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e3.4. Cost of consumables - Nano filter and Ultra filter media replacement (at 65% system efficiency)\u003c/h2\u003e \u003cp\u003eUltra filter unit provides first line of defense to the nano filter unit. Base on the operation records (by observing the pressure built up at the pressure gauges at ultra filter), ultra filter media is replaced for every 120 m\u003csup\u003e3\u003c/sup\u003e of water (or about 4.6 weeks interval) in order to reduce the additional pump power. Filter replacement cost is S\u003cspan\u003e$\u003c/span\u003e10/set. This work out to be S\u003cspan\u003e$\u003c/span\u003e 10/120\u0026thinsp;=\u0026thinsp;0.833/m\u003csup\u003e3\u003c/sup\u003e of permeate. Estimated life span of nano filter is about 2.5 year or after 4280 m\u003csup\u003e3\u003c/sup\u003e (base on 2.5 years @4.69 m\u003csup\u003e3\u003c/sup\u003e/day) of permeate. At S\u003cspan\u003e$\u003c/span\u003e950 per set, the replacement cost was calculated as S\u003cspan\u003e$\u003c/span\u003e950/4280\u0026thinsp;=\u0026thinsp;S\u003cspan\u003e$\u003c/span\u003e 0.222/m\u003csup\u003e3\u003c/sup\u003e of permeate.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e3.5 NEWater \u0026amp; Electricity Tariffs\u003c/h2\u003e \u003cp\u003eAs electricity tariff fluctuated during the period, average tariff rate was used in the analysis. The average electricity tariff during 2023 Quarter 3 (Q3) basing on average of peak (S\u003cspan\u003e$\u003c/span\u003e0.271/kWhr) and off-peak (S\u003cspan\u003e$\u003c/span\u003e0.164/kWhr) rates was S\u003cspan\u003e$\u003c/span\u003e 0.218 / kWhr.\u003c/p\u003e \u003cp\u003eThe average tariff during 2022 Q1, Q2, Q3, Q4, 2023 Q1, and Q2 were recorded at S\u003cspan\u003e$\u003c/span\u003e 0.194/kWhr, S\u003cspan\u003e$\u003c/span\u003e 0.214/kWhr, S\u003cspan\u003e$\u003c/span\u003e 0.236/kWhr, S\u003cspan\u003e$\u003c/span\u003e 0.233/kWhr, S\u003cspan\u003e$\u003c/span\u003e 0.227/kWhr, S\u003cspan\u003e$\u003c/span\u003e 0.225/kWhr respectively. In addition, the average tariff for 2022 Q1 to 2023 Q3 is S\u003cspan\u003e$\u003c/span\u003e 0.221/kWhr (or S\u003cspan\u003e$\u003c/span\u003e 0.239/kWhr plus GST).\u003c/p\u003e \u003cp\u003eNEWater was an ultrapure water which was developed in Singapore to be an alternative water source for semiconductor or high-tech industries. The NEWater rate applied was S\u003cspan\u003e$\u003c/span\u003e 2.5/m\u003csup\u003e3\u003c/sup\u003e (exclude GST).\u003c/p\u003e \u003c/div\u003e"},{"header":"4. Analysis and Discussion","content":"\u003cp\u003e4.1 Sustainability of recovery efficiency for the nano filtration unit\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eDuring the initial performance evaluation within 31\u003csup\u003est\u003c/sup\u003e March to 11\u003csup\u003eth\u0026nbsp;\u003c/sup\u003eAugust 2023, the total blow down quantity is 948.13 m\u0026sup3;. The total recycled water (permeate) recorded was 618.56 m\u0026sup3;. Therefore, the water recovery efficiency is 65.2%. This is within the original prediction of 65-70% recovery efficiency\u0026nbsp;and greater than 50 %, therefore it is potential to reduce freshwater needs when blowdown water was partly reclaimed and reused (Altman \u003cem\u003eet al\u003c/em\u003e., 2012; Gartiser and Urich, 2002). The power consumption during this period is 1,000.7 kWhr. This work showed that 1.62 kWhr per m\u0026sup3; of permeate recovered.\u003c/p\u003e\n\u003cp\u003eIn a subsequent monitoring period (11\u003csup\u003eth\u0026nbsp;\u003c/sup\u003eAug 2023 to 10\u003csup\u003eth\u003c/sup\u003e Jan 2025), the\u0026nbsp;total blow down quantity during this period was 2,756.94 m\u0026sup3;. The total recycled water (permeate) recorded was 1,801.4 m\u0026sup3;. The water recovery efficiency achieved was 65.3% and comparable to the earlier evaluation period\u0026nbsp;from 31\u003csup\u003est\u003c/sup\u003e March to 11\u003csup\u003eth\u0026nbsp;\u003c/sup\u003eAugust 2023. The power consumption during this period is 2,895.5 kWhr. This work showed that 1.61 kWhr per m\u0026sup3; of permeate recovered over this period. The unit performance is thus sustainable.\u003c/p\u003e\n\u003cp\u003eFigure 4 shows the relationship of recovery efficiency and energy consumption (kWh/m\u0026sup3;) over the monitored period. While periodic adjustment of return water flow rate to nano filter keeps recovery efficiency at desirable range, energy consumption however continues to increase as fouling built up at the filter membrane. The average energy consumption reached above 3 kWh/m\u0026sup3; in August 2024. This condition was corrected after membrane undergo cleaning at the end of August 2024.\u003c/p\u003e\n\u003cp\u003e4.2. Overflow affecting blowdown\u003c/p\u003e\n\u003cp\u003e4.2.1\u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp;Blow down rate (31\u003csup\u003est\u003c/sup\u003e March to 11\u003csup\u003eth\u003c/sup\u003e August 2023)\u003c/p\u003e\n\u003cp\u003eThe original predicted blow down rate was 7.3 m\u0026sup3;/day. However, the average blow down reduced to 5.21 m\u0026sup3;/day (31\u003csup\u003est\u003c/sup\u003e March to 11\u003csup\u003eth\u003c/sup\u003e Aug 2023) due to overflow. The water loss due to cooling tower overflow during this period (31\u003csup\u003est\u003c/sup\u003e March to 11\u003csup\u003eth\u003c/sup\u003e Aug 2023) account to almost 27% of the blow down water.\u003c/p\u003e\n\u003cp\u003e4.2.2\u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp;Blow down rate (11\u003csup\u003eth\u003c/sup\u003e August 2023 to 10\u003csup\u003eth\u0026nbsp;\u003c/sup\u003eJanuary 2025)\u003c/p\u003e\n\u003cp\u003eCooling tower basin overflow problem persisted again during 17\u003csup\u003eth\u003c/sup\u003e November to 29\u003csup\u003eth\u003c/sup\u003e December 2023 and 22\u003csup\u003end\u003c/sup\u003e November to 27\u003csup\u003eth\u003c/sup\u003e December 2024. The average blow down rate for this period reduce to 5.38 m\u0026sup3;/day.\u003c/p\u003e\n\u003cp\u003eIf considered from 31\u003csup\u003est\u0026nbsp;\u003c/sup\u003eMar 2023 to 10\u003csup\u003eth\u003c/sup\u003e January 2025, the overall blow down rate is 5.74 m\u0026sup3;/day. This was substantially lower than original prediction. It meant that over the period, about 21.4% of blow down not being recovered. At 65% efficiency, this work out to be 654 m\u0026sup3; of permeate loss. This excludes the direct water loss from basin overflow.\u003c/p\u003e\n\u003cp\u003e4.3. Improvement of COC\u003c/p\u003e\n\u003cp\u003eThe following formula was used to calculate COC:\u003c/p\u003e\n\u003cp\u003eBlow down = Evaporation / (COC - 1) \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp;[Eq. 1]\u003c/p\u003e\n\u003cp\u003eB\u003csub\u003eo\u003c/sub\u003e = E\u003csub\u003eo\u003c/sub\u003e / COC\u003csub\u003eo\u003c/sub\u003e -1\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp;[Eq. 2]\u003c/p\u003e\n\u003cp\u003eCOC\u003csub\u003eo\u003c/sub\u003e = Average CT conductivity / make-up water conductivity. \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;[Eq. 3]\u003c/p\u003e\n\u003cp\u003eCOC\u003csub\u003eo\u0026nbsp;\u003c/sub\u003e= 1163 (average value of 31\u003csup\u003est\u0026nbsp;\u003c/sup\u003eMar 23 to 10\u003csup\u003eth\u003c/sup\u003e Jan 2025) / 91.85\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;(based on reference water index) =12.66\u003c/p\u003e\n\u003cp\u003eWhere \u0026ldquo;\u003csub\u003eo\u003c/sub\u003e \u0026ldquo; is the initial value, \u0026ldquo; 1 \u0026ldquo; is the new value\u003c/p\u003e\n\u003cp\u003eTo determine the B\u003csub\u003eo\u003c/sub\u003e the following data was used:\u003c/p\u003e\n\u003cp\u003eAverage blow now during 31\u003csup\u003est\u0026nbsp;\u003c/sup\u003eMarch 23 to 10\u003csup\u003eth\u003c/sup\u003e Jan 2025 (645 days):\u003c/p\u003e\n\u003cp\u003eBlow down meter reading on 31\u003csup\u003est\u003c/sup\u003e Mar 2023 = 1,676.39 m\u003csup\u003e3\u003c/sup\u003e\u003c/p\u003e\n\u003cp\u003eBlow down meter reading on 10\u003csup\u003eth\u003c/sup\u003e Jan 2025 = 5,382.15 m\u003csup\u003e3\u003c/sup\u003e\u003c/p\u003e\n\u003cp\u003eNet blow down quantity = 3,705.76 m\u003csup\u003e3\u003c/sup\u003e over 645 days\u003c/p\u003e\n\u003cp\u003eBlow down rate per day \u0026ldquo; B\u003csub\u003eo\u003c/sub\u003e \u0026ldquo; = 3,705.76 / 645 = 5.74 m\u003csup\u003e3\u003c/sup\u003e/day\u003c/p\u003e\n\u003cp\u003eAverage water saved during 31\u003csup\u003est\u0026nbsp;\u003c/sup\u003eMar 23 to 10\u003csup\u003eth\u003c/sup\u003e Jan 2025 (645 days):\u003c/p\u003e\n\u003cp\u003eRecycle water meter reading on 31\u003csup\u003est\u003c/sup\u003e Mar 2023 =: 67.7 m\u003csup\u003e3\u003c/sup\u003e\u003c/p\u003e\n\u003cp\u003eRecycle water meter reading on 10\u003csup\u003eth\u003c/sup\u003e Jan 2025 = 2,487.9 m\u003csup\u003e3\u003c/sup\u003e\u003c/p\u003e\n\u003cp\u003eNet water quantity saved = 2,420.2 m\u003csup\u003e3\u003c/sup\u003e over 645 days\u003c/p\u003e\n\u003cp\u003eNet water saving per day = 2,420.2 / 645 = 3.75 m\u003csup\u003e3\u003c/sup\u003e/day\u003c/p\u003e\n\u003cp\u003eRecovery efficiency during this period is 2420.2 / 3705.76 = 65.3 %\u003c/p\u003e\n\u003cp\u003eCompute E\u003csub\u003eo\u003c/sub\u003e :\u003c/p\u003e\n\u003cp\u003eE\u003csub\u003eo\u0026nbsp;\u003c/sub\u003e= B\u003csub\u003eo\u003c/sub\u003e * (12.66 - 1) = 5.74 * 11.66 = 66.93 m\u003csup\u003e3\u003c/sup\u003e/day\u003c/p\u003e\n\u003cp\u003eBecause we are recovering the blow down water, Evaporation (E\u003csub\u003eo\u003c/sub\u003e) rate remains unchanged. E\u003csub\u003eo\u0026nbsp;\u003c/sub\u003e= E\u003csub\u003e1\u003c/sub\u003e\u003c/p\u003e\n\u003cp\u003eCompute new COC\u003csub\u003e1\u003c/sub\u003e :\u003c/p\u003e\n\u003cp\u003eE\u003csub\u003e1\u003c/sub\u003e = B\u003csub\u003e1\u003c/sub\u003e *( COC\u003csub\u003e1\u003c/sub\u003e -1)\u003c/p\u003e\n\u003cp\u003eB\u003csub\u003e1\u003c/sub\u003e = B\u003csub\u003eo\u003c/sub\u003e \u0026ndash; water saved = 5.74 \u0026ndash; 3.75 = 1.99 m\u003csup\u003e3\u003c/sup\u003e/day\u003c/p\u003e\n\u003cp\u003e66.93 = 1.99*(COC\u003csub\u003e1\u003c/sub\u003e -1)\u003c/p\u003e\n\u003cp\u003eCOC\u003csub\u003e1\u0026nbsp;\u003c/sub\u003e= 34.6\u003c/p\u003e\n\u003cp\u003eThis COC of 34.6 is achieved at recovery efficiency of 65.3%. As long as the recovery rate improved, COC will be better. In this case, the COC improved from 12.7 to 34.6, after implementing the nano filtration unit.\u0026nbsp;The high COC was achieved due to possible higher quality of the make-up water which sourced from NEWater, in contrary to common COC of 4 to 6 that depended on make-up water quality (Edmonson, 2014).\u003c/p\u003e\n\u003cp\u003e4.4. Efficiency of Nano Filtration System Unit\u003c/p\u003e\n\u003cp\u003eThe efficiency of nano filter in removing the impurities in the blow down water were presented in Table 1.\u003c/p\u003e\n\u003cp\u003eTable 1. Percentage of water impurities reduction using installed nano filtration unit at KTP Hospital\u003c/p\u003e\n\u003cdiv\u003e\n \u003ctable border=\"1\" cellspacing=\"0\" cellpadding=\"0\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\"\u003e\n \u003cp\u003eConductivity: 81.7%\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\"\u003e\n \u003cp\u003eTDS: 82.7%\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\"\u003e\n \u003cp\u003eTotal Hardness: 98.9%\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\"\u003e\n \u003cp\u003eCalcium Hardness: 99.5%\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\"\u003e\n \u003cp\u003eChloride: 84.7%\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\"\u003e\n \u003cp\u003eM-Alk: 91.6%\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n\u003c/div\u003e\n\u003cp\u003eThe results shown nano filter is very efficient in removing minimal and rejection ranges in-line with expectation (OSTI, 2010).\u003c/p\u003e\n\u003cp\u003e4.5.\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp;Review of Payback period\u003c/p\u003e\n\u003cp\u003eThe original estimate is that the pre-filter will be replaced at every 100 m\u0026sup3; of permeate produced (or about 3 weeks interval). Total of 20 times of pre-filter replacement were carried out within 31\u003csup\u003est\u003c/sup\u003e March 23 to 10\u003csup\u003eth\u003c/sup\u003e January 2025, this work out to be 4.6 weeks interval (or 120.9 m\u0026sup3; of permeate). Operating cost for pre-filter is thus substantially lower than expectation.\u003c/p\u003e\n\u003cp\u003eThe original estimated life span of nano filter is about 2.5 year or after 4,280 m\u0026sup3;of permeate recovery. As of 10\u003csup\u003eth\u003c/sup\u003e January 2025, the nano filter has been operating for 2 years and recovered total of 2420.2 m\u0026sup3; of permeate. Only\u0026nbsp;one time of chemical cleaning were carried out so far, we can thus expect the nano filter continue to perform within its life span expectancy.\u003c/p\u003e\n\u003cp\u003eThe operation cost consists of electricity consumed by the recycle pump and consumables as discussed in Section 3 above. Total operation cost for above equal to S$0.764 per m\u003csup\u003e3\u003c/sup\u003e of permeate. The maintenance cost (S$1,200 /year) and a 6% discount rate is being used for payback calculation. The calculation shows that the payback period is about 10 years. The payback is substantially affected by cooling tower overflow.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eA net saving of S$ 1,069 / year can be expected after breakeven period at present prevailing water and electricity tariffs.\u0026nbsp;\u003c/p\u003e"},{"header":"5. Conclusion","content":"\u003cp\u003eThe blow down recycle unit started to operate in mid January 2023. In collaboration between Singapore Polytechnic\u0026rsquo;s Digital Building Innovation Centre [DBIC] and Medec Pte Ltd, the key performance indexes and observations of the recycle unit could be summarized as:\u003c/p\u003e \u003cp\u003eBlow down rate at KTP Hospital was affected by cooling tower overflow. For the monitored period, the overflow constitutes nearly 21.4% blow down water loss, which is not recoverable. The recovery efficiency of the recycle unit is 65.3% at 1.61 kWhr per m\u003csup\u003e3\u003c/sup\u003e of recovered water (permeate), which is within the predication of 65\u0026ndash;70%. Cycle of Concentration (COC) for cooling tower improved from 12.7 to 34.6. The minimal removing efficiency of the nano filter is above 90%, especially for hardness removal, which is above 98%. For lowering conductivity and TDS, the nano filter performance is in-line with industry expectation (80\u0026ndash;85%), at 81.7% and 82.7% respectively. Payback period after considering operation cost (S\u003cspan\u003e$\u003c/span\u003e 0.764 per m\u003csup\u003e3\u003c/sup\u003e of permeate), maintenance cost (S\u003cspan\u003e$\u003c/span\u003e1,200 p.a.) and 6% discount rate is about 10 years. A net saving of about S\u003cspan\u003e$\u003c/span\u003e1,100 p.a. can be expected after breakeven period at present prevailing water and electricity tariffs.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003eAcknowledgements: we thank you Singapore Polytechnic\u0026rsquo;s management for facilitating a collaborative works\u003c/p\u003e\n\u003cp\u003eFunding: Singapore Polytechnic, Ideafarm\u0026rsquo;s R725 for the design of the study and collection, analysis, and interpretation of data.\u003c/p\u003e\n\u003cp\u003eAuthors\u0026rsquo; Contributions: all the 3 authors i.e. Handojo Djati Utomo, Qishan Liu and Swee Lee Lim were equally contributed to the project from conducting literature review, data collection, analysis and report writing.\u003c/p\u003e\n\u003cp\u003eEthical Approval:\u0026nbsp;not applicable\u003c/p\u003e\n\u003cp\u003eConsent to Participate: not applicable\u003c/p\u003e\n\u003cp\u003eConsent to Publish: not applicable\u003c/p\u003e\n\u003cp\u003eCompeting Interest: the authors declare that they have no competing interests\u003c/p\u003e\n\u003cp\u003eData Availability Statement: the datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eAltman, S.J., Jensen, R.P., Cappelle, M.A., Sanchez, A.L., Everett, R.L., Anderson, H.L., McGrath, L.K., 2012. Membrane treatment of side-stream cooling tower water for reduction of water usage. Desalination 285, 177\u0026ndash;183. https://doi.org/10.1016/j.desal.2011.09.052\u003c/li\u003e\n\u003cli\u003eArias, B.G., N. Merayo, A. Mill\u0026acute;an, C. Negro, Sustainable recovery of wastewater to be reused in cooling towers: towards circular economy approach, J. Water Process Eng. 41 (2021), https://doi.org/10.1016/j.jwpe.2021.102064\u003c/li\u003e\n\u003cli\u003eEdmonson, Chad (2014), Cooling Tower and Condenser Water Design Part 9: Controlling Cycles of Concentration. (https://jmpcoblog.com/hvac-blog/cooling-tower-and-condenser-water-design-part-9-controlling-cycles-of-concentration)\u003c/li\u003e\n\u003cli\u003eEPA, 2017. Water efficiency management guide: mechanical systems. EPA 832-F-17-016c. https://www.epa.gov/sites/default/files/2017-12/documents/ws-commercial buildings-waterscore-mechanical-systems-guide.pdf\u003c/li\u003e\n\u003cli\u003eFarahani, M.H.D.A, Seyed Mehdi Borghei, Vahid Vatanpour (2016), Recovery of cooling tower blowdown water for reuse: The investigation of different types of pretreatment prior nanofiltration and reverse osmosis, Journal of Water Process Engineering 10, Page 188-199.\u003c/li\u003e\n\u003cli\u003eFrick, J.M., L.A. F\u0026acute;eris, I.C. Tessaro, Evaluation of pretreatments for a blowdown stream to feed a filtration system with discarded reverse osmosis membranes, Desalination 341 (2014) 126\u0026ndash;134, https://doi.org/10.1016/j.desal.2014.02.033\u003c/li\u003e\n\u003cli\u003eGartiser, S., Urich, E., 2002. Environmentally compatible cooling water treatment chemicals. UBA, Germany, Berlin. Research Report 200 (24), 233. https://www.hydrotox.de/fileadmin/user_upload/pdfs/forschungen/projekte/UBA_cooling_water_text_05-11-02.pdf\u003c/li\u003e\n\u003cli\u003eJan Taler, Bartosz Jagieła, Magdalena Jaremkiewicz (2021), Improving efficiency and lowering operating costs of evaporative cooling, MATEC Web of Conferences 338, 01027.\u003c/li\u003e\n\u003cli\u003eMeteorogical Meteorological Service Singapore (2023), Singapore Climate 2023: The Year in Numbers. https://www.weather.gov.sg/wp-content/uploads/2024/01/The_Year_in_Numbers_2023.pdf\u003c/li\u003e\n\u003cli\u003eObaideen, K., N. Shehata, E.T. Sayed, M.A. Abdelkareem, M.S. Mahmoud, A. G. Olabi, The role of wastewater treatment in achieving sustainable development goals (SDGs) and sustainability guideline, Energy Nexus 7 (2022), https://doi.org/10.1016/j.nexus.2022.100112\u003c/li\u003e\n\u003cli\u003eOviroh, P.O., R. Akbarzadeh, T.C. Jen, Biomimetic membrane simulation for water desalination, ASME Int. Mech. Eng. Congr. Expo., Proc. (IMECE) (2018), https://doi.org/10.1115/IMECE201886664\u003c/li\u003e\n\u003cli\u003eOSTI (2010), Office of Scientific and Technical Information, Conference: Use of nanofiltration to reduce cooling tower water consumption. OSTI ID:1028304 Oct 2010. Altman, Susan Jeanne; Ciferno, Jared\u003c/li\u003e\n\u003cli\u003ePUB (2017), Technical Reference for Water Conservation in Cooling Towers 1\u003csup\u003est\u003c/sup\u003e Edition: Nov 2017\u003c/li\u003e\n\u003cli\u003ePUB, 2025 NEWater Quality | PUB, Singapore\u0026rsquo;s National Water Agency\u003c/li\u003e\n\u003cli\u003eRahmani, Kh. (2017), Reducing water consumption by increasing the cycles of concentration and Considerations of corrosion and scaling in a cooling system, Applied Thermal Engineering 114, Pages 849-856.\u003c/li\u003e\n\u003cli\u003eSaha, P., Thomas V. Wagner, Jiahao Ni, Alette A.M. Langenhoff, Harry Bruning, Huub H.M. Rijnaarts (2020), Cooling tower water treatment using a combination of electrochemical oxidation and constructed wetlands, Process Safety and Environmental Protection 144, Pages 42-5.\u003c/li\u003e\n\u003cli\u003eSoliman, M., F. Eljack, M.K. Kazi, F. Almomani, E. Ahmed, Z. El Jack, Treatment technologies for cooling water blowdown: a critical review, Sustain. (Switz.) 14 (2022), https://doi.org/10.3390/su14010376\u003c/li\u003e\n\u003cli\u003eUS Department of Energy (2011), Cooling Towers: Understanding Key Components of Cooling Towers and How to Improve Water Efficiency.\u003c/li\u003e\n\u003cli\u003eWang. Z, Z. Fan, L. Xie, S. Wang, Study of integrated membrane systems for the treatment of wastewater from cooling towers, Desalination 191 (2006) 117\u0026ndash;124, https://doi.org/10.1016/j.desal.2005.04.125\u003c/li\u003e\n\u003cli\u003eWang, MHS, and Wang, LK (2022). Cooling tower and boiler water treatment technologies. In: \"\u003cem\u003eEnvironmental Science, Technology, Engineering, and Mathematics (STEM)\", \u003c/em\u003eWang, LK, Wang, MHS, and Pankivskyi, YI (editors). Volume 2022, Number 3, March 2022; 88 pages. Lenox Institute Press, MA, USA https://doi.org/10.17613/jge9-ey13\u003c/li\u003e\n\u003cli\u003eWang, F.H., H.T. Hao, R.F. Sun, S.Y. Li, R.M. Han, C. Papelis, Y. Zhang (2014), Bench-scale and pilot-scale evaluation of coagulation pre-treatment for wastewater reused by reverse osmosis in a petrochemical circulating cooling water system, Desalination 335 (1): 64\u0026ndash;69.\u003c/li\u003e\n\u003cli\u003eYou, S.H. , D.H. Tseng, G.L. Guo, J.J. Yang (1999), The potential for the recovery and reuse of cooling water in Taiwan, Resour. Conserv. Recycl. 26 (1): 53\u0026ndash;70.\u003c/li\u003e\n\u003cli\u003eZhang. J., L. Chen, H. Zeng, X. Yan, X. Song, H. Yang, C. Ye (2007), Pilot testing of outside-in MF and UF modules used for cooling tower blowdown pretreatment of power plants, Desalination 214 (1\u0026ndash;3): 287\u0026ndash;298.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"cooling tower, blow down, cycle of concentration","lastPublishedDoi":"10.21203/rs.3.rs-6587843/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6587843/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eWater in cooling tower gets concentrated due to evaporation of water for heat rejection. Apart from treatment against bacteria growth, every cooling tower condenser water system needs blow down to control the water quality to minimize scaling and corrosion. Approximately 7\u0026ndash;10% of total water consumed by cooling tower will be bleed away with a typical cycle of concentration (COC) of 4\u0026ndash;6 subject to make-up water quality. A nano recycle unit is installed at local Hospital in Singapore to recover 65\u0026ndash;70% bleed-off water and recycle the permeate back to cooling tower as make-up water. The blow down recycle unit started to operate in mid Jan 2023 with the key performance indexes and observations of the recycle unit after 2 years of operation showed that the overflow constitutes more 21.4% blow down water loss, which was not recoverable; the recovery efficiency of the recycle unit is 65.3% at 1.61 kWhr per m\u003csup\u003e3\u003c/sup\u003e of permeate; COC improved from 12.7 to 34.6; during the first 6 months of monitoring, the minimal removing efficiency of the nano filter is above 90%, especially for hardness removal. For lowering conductivity and TDS, the nano filter performance is in-line with industry expectation (80\u0026ndash;85%), at 81.7% and 82.7% respectively. Payback period after considering operation cost (S\u003cspan\u003e$\u003c/span\u003e 0.76 per m\u003csup\u003e3\u003c/sup\u003e of permeate), maintenance cost (S\u003cspan\u003e$\u003c/span\u003e1,200 /year) and 6% discount rate is about 10 years. A net saving of S\u003cspan\u003e$\u003c/span\u003e 1,100 p.a. can be expected after breakeven period at present prevailing water and electricity tariffs.\u003c/p\u003e","manuscriptTitle":"Case Study: Small Scale Cooling Towers Bleed-Off (blow down) in Water Recycle System at a Local Hospital","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-05-19 18:58:56","doi":"10.21203/rs.3.rs-6587843/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"7e9ff179-b6b5-4aed-9bdc-49c6559b56f7","owner":[],"postedDate":"May 19th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2025-09-03T12:54:04+00:00","versionOfRecord":[],"versionCreatedAt":"2025-05-19 18:58:56","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-6587843","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6587843","identity":"rs-6587843","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}