Department Sanitation, Water and Solid Waste for Development

Fecal sludge dewatering and supernatant treatment

MEWS researches fundamental mechanisms governing dewatering and settling in faecal sludge to gain insights for improved FSM
MEWS researches fundamental mechanisms governing dewatering and settling in faecal sludge to gain insights for improved FSM.

This research focuses on understanding how the properties of fecal sludge received at treatment plants impacts dewatering performance, and subsequent treatment of supernatant. Our earlier research has confirmed that particle size, extracellular polymeric substances (EPS), lignin, cellulose, hemicellulose and lipids impact dewatering performance. The level of stabilization also plays a role, however, conventional stabilization metrics such as VS/TS and C/N ratios are not relevant for comparison and stabilization metrics that are indicative of the potential for further biological degradation need to be established. Attached growth biofilm reactors are being evaluated for supernatant treatment, along with COD discharge standards for effluents of fecal sludge treatment plants.

Context

The sanitation needs of nearly half (46%) the global population are met through non-sewered sanitation. When containments for onsite storage are full, the fecal sludge that accumulates needs to be emptied and transported to treatment to ensure protection of public and environmental health. Currently, the bulk of the emptied fecal sludge is dumped directly in the environment due to lack of existing treatment units, long haulage distances and high cost of service among other reasons. To maximize the amount of sludge being treated at treatment plants, there is need to improve the understanding of the properties of fecal sludge arriving at these plants for improved dewatering and downstream biological treatment of supernatant after dewatering.

Goal

Developing safely managed solutions through an understanding of faecal sludge that accumulates with microbial degradation and stabilization during storage in containment, in order to develop reliable and safe management of faecal sludge arriving at treatment.

 

 

 

Collaborator

  • Prof. Eberhard Morgenroth, Eawag and ETH Zurich, Zurich, Switzerland
  • Dr. James Tembo and Prof. Joel Kabika, University of Zambia, Lusaka, Zambia
  • Prof. Charles Niwagaba, Makerere University, Kampala, Uganda
  • Naomi Korir, Sanivation, Naivasha, Kenya

Funding

  • Swiss National Science Foundation (SNF)
  • Swiss Agency for Development and Cooperation (SDC)
  • Eawag discretionary funding

Publications

Changes in organic fractions, cations, and stabilization from feces to fecal sludge: implications for dewatering performance and management solutions

Reliable dewatering performance remains a key challenge in fecal sludge management, and the controlling factors or mechanisms are not well understood. There remain limited studies on constituents in feces and fecal sludge and how they affect the dewaterability of fecal sludge. This study aimed at evaluating a range of constituents in feces, and to gain empirical knowledge toward a mechanistic understanding of how they influence dewaterability. In this study, cellulose reduced capillary suction time, decreased supernatant turbidity, and increased cake solids. While hemicellulose decreased supernatant turbidity, lignin increased supernatant turbidity, capillary suction time, and cake solids. Extracellular polymeric substances (EPS) increased both capillary suction time and supernatant turbidity and decreased cake solids, whereas lipids increased turbidity. Cations had no significant effect on dewatering properties. Overall, fecal sludge stored in containments had better dewatering performance than ‘fresh’ fecal sludge, which was attributed to stabilization. Field fecal sludge had a higher relative abundance of Pseudomonas, which is associated with better aggregation, and fewer small particles (<10 μm) that clog filters to reduce dewatering performance. Further understanding of stabilization and developing an agreed-upon metrics of stabilization are essential for predicting fecal sludge dewatering performance, and developing smaller footprint dewatering treatment technologies.
Sam, S. B.; Morgenroth, E.; Strande, L. (2023) Changes in organic fractions, cations, and stabilization from feces to fecal sludge: implications for dewatering performance and management solutions, Journal of Water Sanitation and Hygiene for Development, 13(9), 699-710, doi:10.2166/washdev.2023.086, Institutional Repository

Particle size as a driver of dewatering performance and its relationship to stabilization in fecal sludge

Poor and unpredictable dewatering performance of fecal sludge is a major barrier to sanitation provision in urban areas not served by sewers. Fecal sludge comprises everything that accumulates in onsite containments, and its characteristics are distinct from wastewater sludges and from feces. There is little fundamental understanding of what causes poor dewatering in fecal sludge. For the first time, we demonstrate that particle size distribution is a driver of dewatering performance in fecal sludge, and is associated with level of stabilization. Higher concentrations of small particles (<10 μm) and smaller median aggregate size (D50) corresponded to poor dewatering performance (measured by capillary suction time (CST) and supernatant turbidity) in field samples from Kenya and Uganda and in controlled laboratory anaerobic storage experiments. More stabilized fecal sludge (higher C/N, lower VSS/TSS) had better dewatering performance, corresponding to lower concentrations of small particles. Samples with the largest aggregates (D50 > 90 μm) had higher abundance of Gammaproteobacteria Pseudomonas, and samples with the smallest aggregates (D50 ≤ 50 μm) were characterized by higher abundance of Bacteroidetes Vadin HA17 and Rikenellaceae. Contrary to common perceptions, stabilization, particle size distribution, and dewatering performance were not dependent on time intervals between emptying of onsite containments or on time in controlled anaerobic storage experiments. Our results suggest that the stabilization process in onsite containments, and hence the dewaterability of sludge arriving at treatment facilities, is not dependent on time in containment but is more likely associated with specific microbial populations and the in-situ environmental conditions which promote or discourage their growth.
Ward, B. J.; Nguyen, M. T.; Sam, S. B.; Korir, N.; Niwagaba, C. B.; Morgenroth, E.; Strande, L. (2023) Particle size as a driver of dewatering performance and its relationship to stabilization in fecal sludge, Journal of Environmental Management, 326, 116801 (12 pp.), doi:10.1016/j.jenvman.2022.116801, Institutional Repository

Elucidating the role of extracellular polymeric substances (EPS) in dewaterability of fecal sludge from onsite sanitation systems, and changes during anaerobic storage.

As the importance of fecal sludge management (FSM) is increasingly being realized, the need for adequately designed and functioning fecal sludge (FS) treatment plants is also increasing. Research to fill this gap is only emerging and dewatering is a key challenge for developing sustainable treatment solutions. This study evaluated the effect of extracellular polymeric substances (EPS) on dewaterability of FS, and how EPS and dewaterability change during anaerobic storage (as a proxy for time in onsite containment). EPS was extracted from FS and activated sludge using Na2CO3 and sonication and added to sludge samples to determine the effect on dewaterability. The results confirmed that an increase in EPS had a direct impact of decreasing FS dewaterability (as capillary suction time). In this context, we evaluated FS degradation during anaerobic storage, the effect of anaerobic storage time on EPS, EPS fractions and particle size distribution, and the effect of variations in these factors on FS dewaterability. Variations in EPS, EPS fraction and particle size distribution during anaerobic storage were less than expected and average VS reduction of 20% was recorded over 7 weeks. Although anaerobic digestion was verified (biogas production), the results indicate that kinetics of degradation of FS is different from wastewater sludges. Comparatively, EPS fractions in FS were 70 – 75% lower and with higher fractions of humic-like substances than wastewater sludges. Although EPS significantly affects FS dewaterability, anaerobic storage time is not a predictor of dewaterability.
Sam, S. B.; Ward, B. J.; Niederdorfer, R.; Morgenroth, E.; Strande, L. (2022) Elucidating the role of extracellular polymeric substances (EPS) in dewaterability of fecal sludge from onsite sanitation systems, and changes during anaerobic storage., Water Research, 222, 118915 (12 pp.), doi:10.1016/j.watres.2022.118915, Institutional Repository

Predictive models using "cheap and easy" field measurements: can they fill a gap in planning, monitoring, and implementing fecal sludge management solutions?

The characteristics of fecal sludge delivered to treatment plants are highly variable. Adapting treatment process operations accordingly is challenging due to a lack of analytical capacity for characterization and monitoring at many treatment plants. Cost-efficient and simple field measurements such as photographs and probe readings could be proxies for process control parameters that normally require laboratory analysis. To investigate this, we evaluated questionnaire data, expert assessments, and simple analytical measurements for fecal sludge collected from 421 onsite containments. This data served as inputs to models of varying complexity. Random forest and linear regression models were able to predict physical-chemical characteristics including total solids (TS) and ammonium (NH4+-N) concentrations, and solid-liquid separation performance including settling efficiency and filtration time (R2 from 0.51-0.66) based on image analysis of photographs (sludge color, supernatant color, and texture) and probe readings (conductivity (EC) and pH). Supernatant color was the best predictor of settling efficiency and filtration time, EC was the best predictor of NH4+-N, and texture was the best predictor of TS. Predictive models have the potential to be applied for real-time monitoring and process control if a database of measurements is developed and models are validated in other cities. Simple decision tree models based on the single classifier of containment type can also be used to make predictions about citywide planning, where a lower degree of accuracy is required.
Ward, B. J.; Andriessen, N.; Tembo, J. M.; Kabika, J.; Grau, M.; Scheidegger, A.; Morgenroth, E.; Strande, L. (2021) Predictive models using "cheap and easy" field measurements: can they fill a gap in planning, monitoring, and implementing fecal sludge management solutions?, Water Research, 196, 116997 (12 pp.), doi:10.1016/j.watres.2021.116997, Institutional Repository

Evaluation of conceptual model and predictors of faecal sludge dewatering performance in Senegal and Tanzania

Unpredictable dewatering performance is a barrier to the effective management and treatment of faecal sludge. While mechanisms of dewatering in sludges from wastewater treatment are well understood, it is not clear how dewatering of faecal sludge fits into the framework of existing knowledge. We evaluate physical-chemical parameters, including EPS and cations, and demographic (source), environmental (microbial community), and technical factors (residence time) as possible predictors of dewatering performance in faecal sludge, and make comparisons to the existing conceptual model for wastewater sludge. Faecal sludge from public toilets took longer to dewater than sludge from other sources, and had turbid supernatant after settling. Slow dewatering and turbid supernatant corresponded to high EPS and monovalent cation concentrations, conductivity, and pH, but cake solids after dewatering was not correlated with EPS or other factors. Faecal sludges with higher EPS appeared less stabilised than those with lower EPS, potentially a result of inhibition of biological degradation due to high urine concentrations. However, distinct microbial community compositions were also observed in samples with higher and lower EPS concentrations. Higher EPS faecal sludge was comparable in dewatering behaviour and EPS content to anaerobically digested and primary wastewater sludges. However lower EPS faecal sludges had different dewatering behaviour than wastewater sludges and may be governed by different mechanisms.
Ward, B. J.; Traber, J.; Gueye, A.; Diop, B.; Morgenroth, E.; Strande, L. (2019) Evaluation of conceptual model and predictors of faecal sludge dewatering performance in Senegal and Tanzania, Water Research, 167, 115101 (13 pp.), doi:10.1016/j.watres.2019.115101, Institutional Repository

Use of chitosan and Moringa oleifera as conditioners for improved dewatering of faecal sludge

A pilot-scale dewatering research facility was built in Dar es Salaam, Tanzania, and was used to test chitosan and Moringa oleifera as conditioners to improve the dewatering of faecal sludge. Laboratory-scale jar tests were first conducted to determine optimal dosages for the conditioners in faecal sludge samples with varying total solids concentrations. The results for chitosan were 0.5-0.6 mL/gTS, and for Moringa oleifera 5-15mL/gTS. Based on these results, pilot-scale tests were conducted with chitosan, but the use of Moringa was ruled out as it was too resource intensive. Three loading cycles were conducted, and an average of 15.3% reduction in dewatering time was achieved. Based on the laboratory and pilot-scale tests, chitosan is recommended as a conditioner for improved FS dewatering performance. It could be employed at full-scale, but still requires jar tests to determine optimal dosing.
Moto, N.; Esanju, M.; Andriessen, N.; Kimwaga, R.; Strande, L. (2018) Use of chitosan and Moringa oleifera as conditioners for improved dewatering of faecal sludge, (6 pp.), Institutional Repository

Cross-country analysis of faecal sludge dewatering

Dewatering of faecal sludge (FS) is indispensable for adequate FS management. However, comprehensive knowledge is lacking on FS dewatering performance. This study compared the dewatering performance of FS from different countries and onsite sanitation technologies, to assess influential characteristics on dewatering, and to compare dewatering performance of FS with wastewater sludge. We collected 73 FS samples from septic tanks, lined pit latrines, unlined pit latrines and johkasou tanks in Uganda, Vietnam and Japan, and 18 samples of wastewater sludge in Switzerland. Capillary suction time (CST) and total solids (TS) of centrifuged sludge (%TSfinal) were determined as metrics of dewatering rate and dewaterability, respectively, together with relevant sludge characteristics. Data were analysed by bootstrapping comparison of median results of each sample category and by bootstrapping multiple regression analysis to quantify the relative importance of sludge characteristics on dewatering performance. Results showed that the dewatering rate was significantly different between FS from different technologies, whereas dewaterability was significantly different within the same technology. FS had a significantly lower dewatering rate than wastewater sludge. In contrast, FS dewaterability was greater than wastewater sludge. However, this could be attributed to higher concentrations of sand in FS. Electrochemical properties such as NH4-N and surface charge had the strongest correlation to dewatering rate, and solid properties such as sand content and total volatile solids to dewaterability. The results identify potential characteristics that could explain and predict the high variability of FS dewatering performance that is observed in the field.
Gold, M.; Harada, H.; Therrien, J.-D.; Nishida, T.; Cunningham, M.; Semiyaga, S.; Fujii, S.; Dorea, C.; Nguyen, V.-A.; Strande, L. (2018) Cross-country analysis of faecal sludge dewatering, Environmental Technology, 39(23), 3077-3087, doi:10.1080/09593330.2017.1374472, Institutional Repository

Locally produced natural conditioners for dewatering of faecal sludge

In urban areas of low-income countries, treatment of faecal sludge (FS) is insufficient or non-existent. This results in large amounts of FS being dumped into the environment. Existing treatment technologies for FS, such as settling-thickening tanks and drying beds, are land intensive which is limiting in urban areas. Enhanced settling and dewatering by conditioning was evaluated in order to reduce the treatment footprint (or increase treatment capacity). Conventional wastewater conditioners, such as commercially available lime and polymers, are expensive, and commonly rely on complex supply chains for use in low-income countries. Therefore, the treatment performance of five conditioners which could be produced locally was evaluated: Moringa oleifera seeds and press cake, Jatropha curcas seeds, Jatropha Calotropis leaves and chitosan. M. oleifera seeds and press cake, and chitosan improved settling and dewatering and had a similar performance compared to lime and polymers. Optimal dosages were 400–500 kg M. oleifera/t TS, 300–800 kg lime/t TS and 25–50 kg polymer solution/t TS. In comparison, chitosan required 1.5–3.75 kg/t TS. These dosages are comparable to those recommended for wastewater (sludge). The results indicate that conditioning of FS can reduce total suspended solids (TSS) in the effluent of settling-thickening tanks by 22–81% and reduce dewatering time with drying beds by 59–97%. This means that the area of drying beds could be reduced by 59–97% with end-use as soil conditioner, or 9–26% as solid fuel. Least expensive options and availability will depend on the local context. In Dakar, Senegal, chitosan produced from shrimp waste appears to be most promising.
Gold, M.; Dayer, P.; Faye, M. C. A. S.; Clair, G.; Seck, A.; Niang, S.; Morgenroth, E.; Strande, L. (2016) Locally produced natural conditioners for dewatering of faecal sludge, Environmental Technology, 37(21), 1-13, doi:10.1080/09593330.2016.1165293, Institutional Repository

Research priorities and progress in faecal sludge dewatering

Difficulties in dewatering of faecal sludge are a crucial barrier to implementation of effective faecal sludge management. This article focuses on progress with locally available conditioners and the next steps toward implementing high-throughput, low-footprint dewatering technologies.
Ward, B. J.; Sam, S.; Andriessen, N.; Morgenroth, E.; Strande, L. (2018) Research priorities and progress in faecal sludge dewatering, Sandec News, 19, 10-11, Institutional Repository
  • The trials of developing a biomethane potential method for faecal sludge Sandec News No 21, Sam, S., Kapanda, K., Ward, B.J., Tembo, J., Morgenroth, E., Strande, L.
  • Research strategy for overcoming the faecal sludge dewatering bottleneck Sandec News No. 21, Ward, B.J., Sam, S., Kapanda, K., Tembo J.M., Morgenroth, E., Strande, L.
  • Predicting faecal sludge dewatering performance for improved treatment Sandec News No. 20, Ward, B.J., Sam, S., Gueye, A., Diop, B., Morgenroth, E., Strande, L.

Contact

Ednah Kemboi Tel. +41 58 765 6860 Send Mail
Dr. Linda Strande Tel. +41 58 765 5553 Send Mail