Solid-Liquid Separation of Fecal Sludge: Understanding the Governing Mechanisms for Improved Global Sanitation

Abstract

Non-sewered sanitation relies on at-source containment and storage of fecal sludge, which is then collected and transported to treatment. This is a relatively new field that has started to gain attention. Contrary to the initial perception that non-sewered sanitation only serves rural areas, it plays a crucial role in fulfilling sanitation needs for over one-third of the world's population. In urban areas of sub-Saharan Africa, onsite sanitation caters for 65-100 % of the sanitation needs. Despite the progress made in enhancing access to sanitation facilities over the years, there is a persistent discharge of untreated fecal sludge into the environment, which is due to the absence of adequate fecal sludge treatment facilities. The consequences of this is the increase in diarrhoea diseases especially in low and middle-income countries. Treatment of fecal sludge is thus essential for public and environmental health protection. Since fecal sludge consist of more than 95% water, dewatering is indispensable for adequate fecal sludge management. However, a key challenge in fecal sludge treatment is the unpredictable dewatering performance, which stems from the high variability in fecal sludge characteristics, and a lack of knowledge on the components of fecal sludge that could influence dewatering performance. Developing consistent treatment solutions for fecal sludge necessitates a deeper understanding of feces and fecal sludge composition, as well as the factors and mechanisms that control dewatering performance. In addition, changes that occur in feces or fecal sludge during anaerobic storage provides information on the characteristics of fecal sludge is arriving at the treatment. The main objective of this thesis was to elucidate the composition of feces and fecal sludge, and to understand the governing mechanisms of solid-liquid separation or dewatering of fecal sludge. To achieve this objective, we investigated in this thesis: (i) the general composition of feces and fecal sludge with emphasis on the concentrations of extracellular polymeric substances (EPS), proteins, polysaccharides, lipids, fibres and cations, to gain a mechanistic understanding of how these constituents affect the dewatering properties of fecal sludge, (ii) the role of EPS in the dewatering of fecal sludge under anaerobic storage, focusing on how EPS changes with time under anaerobic storage and the effect on dewatering behavior, (iii) the role of stabilization, or level of biochemical degradation, on dewatering performance of fecal sludge, and (iv) the association between fecal sludge microbiome to fecal sludge characteristics and factors that affect fecal sludge qualities and quantities (demographic, environmental, and technical data). Summary ii This study showed that feces and fecal sludge contains EPS, proteins, polysaccharides, fibres, lipids and cations that are known to influence fecal sludge dewatering performance. However, the concentrations of EPS are lower compared to what is found in activated sludges. Cellulose was found to increase dewatering performance by decreasing supernatant turbidity and capillary suction time whiles increasing the sludge cake solids. On the contrary, EPS was detrimental to dewatering performance by increasing the capillary suction time and supernatant turbidity. Similarly, lipids had a negative effect on dewatering performance through an increase in supernatant turbidity. Interestingly, cations did not have a significant effect on fecal sludge dewatering performance. In addition to the role of components on dewatering performance, what was compelling in this thesis was that fecal sludge that had been stored in containments had better dewatering performance than fresh fecal sludge that had not undergone storage, which is likely due to the role of stabilization. By using anaerobic batch reactors as a proxy for storage in containment and stabilization, the roles of EPS and particle size distribution on dewatering performance were evaluated in this thesis. While the addition of extracted EPS (from fecal sludge and activated sludge) generally decreased dewatering performance, the effect of anaerobic storage on EPS, EPS fractions and particle size distribution were not substantial, owing to the minimal reduction of the total volatile solids concentration. The kinetics of degradation in fecal sludge appeared to be different from that of wastewater sludges, and anaerobic storage was neither analogous to process controlled anaerobic digestion nor a predictor of stabilization. What was clear in this study was that fecal sludge had a lower concentration of EPS and a higher EPS fraction of humic-like substances compared to wastewater sludges. The different degradation kinetics in fecal sludge compared to wastewater, coupled with variation in levels of stabilization pointed to probable differences in microbial communities in fecal sludge. This thesis observed similarities in the microbial composition among fecal sludge from eight countries but with different proportional abundances, which helps to explain the possible contribution of microbial communities to the varying levels of stabilization. Since the microbial community in fecal sludge has not been thoroughly investigated, this thesis further explored the microbial community structure of 135 fecal sludge samples obtained from Lusaka, Zambia, with emphasis on the associations between microbial communities and demographic environmental factors, characteristics of fecal sludge and treatment performance such as dewatering and stabilization. Generally, fecal sludge had similar microbial communities at higher taxonomic levels; however, differences in community structure occurred at lower Summary iii taxonomic levels, which was dependent on the differences in containment type, which is dictated mainly by differences in water usage, ammonia, total solids concentration and total organic content. This variation in microbial communities at lower taxonomic levels may arise from environmental conditions selecting for different microorganisms or different microbial communities themselves creating different environmental conditions. On the contrary, the similarity at the higher taxonomic levels stems from the dominant metabolic pathways for microorganisms that select for communities like firmicutes and Bacteroides that mainly fermentative organisms. This thesis provides fundamental knowledge of the composition of feces or fecal sludge and how these components influence dewatering performance. Overall, EPS, cellulose and lipids influenced dewatering performance, which sheds light on governing mechanisms of fecal sludge dewaterability. Furthermore, differences in dewatering between fresh and stored fecal sludge are attributed to the levels of stabilization. This is informative for practitioners on selection of appropriate management and treatment solutions as fresh feces are likely to cause clogging in drying beds due to the higher concentrations of EPS and cellulose than stabilized fecal sludge. These outcomes suggest that to improve dewatering performance, fresh fecal sludge that has not been stored in containments, should undergo some level of stabilization, which can be achieved through anaerobic pretreatment.