Settling and Dewatering of Fecal Sludge: Building Fundamental Knowledge for Improved Global Sanitation

Abstract

More than 1/3 of the global population relies on onsite sanitation systems, and in some cases entire urban areas in low- and middle-income countries are not served by sewers. Improved options for the management of fecal sludge that accumulates in these onsite systems are required immediately, as the majority of it is currently discharged untreated into the urban environment, placing a huge burden on public and environmental health. One of the greatest obstacles to establishing more reliable and accessible fecal sludge treatment is inconsistent and unpredictable solid-liquid separation. Extreme influent variability in influent fecal sludge poses operational problems with settling and dewatering, reducing the capacity of existing centralized and semi-centralized treatment facilities and hindering the transfer of low-footprint technologies which could extend decentralized and community-scale sanitation coverage in high-density urban areas. In order to develop robust and reliable treatment solutions for fecal sludge, we need to understand the factors governing solid-liquid separation. In this thesis, the influence of extracellular polymeric substances (EPS), solution properties, particle size distribution, and stabilization in onsite containment on solid-liquid separation performance in fecal sludge was investigated, and a conceptual model for fecal sludge settling and dewatering was developed. Based on this research, field predictors of fecal sludge characteristics and dewatering performance were identified and predictive models and an app were developed that can use photographs and probe measurements to predict dewatering performance of influent sludge. The aim of this thesis was to understand fundamental drivers of solid-liquid separation in fecal sludge and use knowledge gained to inform transfer of treatment technologies, develop methods for rapid characterization of influent, and predict dewatering performance to facilitate responsive process control for dewatering at treatment facilities. In chapter two, the relationships between physical-chemical parameters including EPS and cations and settling and dewatering performance from fecal sludge field samples were evaluated. Higher concentrations of EPS appeared to contribute to turbid supernatant and worsened filtration by clogging pores but were not associated with differences in bound water in dewatered sludge. Fecal sludge had different physical-chemical characteristics and displayed different dewatering and settling behavior compared to wastewater sludges, and the existing conceptual model for sludge dewatering may need to be edited and expanded to include fecal sludge. In chapter three, the results of fundamental research in chapter two was applied to advise practitioners on how to develop, transfer, and scale-up dewatering and drying technologies for fecal sludge treatment. In chapter four, low-cost and simple field measurements and questionnaire data were used to predict fecal sludge influent characteristics and solid-liquid separation performance with different types of empirical models. Color and texture information from photographs and conductivity and pH measurements from probes were good predictors of total solids, ammonium concentration, settling efficiency, and dewatering time when combined with linear and machine learning models. Accuracy of models based on photos and probe measurements could be sufficient for estimating conditioner dosing for dewatering technologies. In chapter five, the results of fundamental research in chapter four were applied to develop the prototype Sludge Snap app. The app automates image processing of field photographs and runs the predictive models developed in chapter 4 to make real-time predictions of influent characteristics and dewatering performance for use by researchers and practitioners in the field. In chapter six, field samples and controlled anaerobic storage experiments were used to determine the relationships between (1) stabilization and time in onsite containment, (2) stabilization and particle size distribution, and (3) particle size distribution and dewaterability. The common perception that stabilization and dewatering of fecal sludge are linked to storage time in onsite containments did not hold up to scientific investigation. However, although time was not a predictor, particle size and dewatering performance were related to stabilization. Particle and aggregate size distribution, especially the concentration of small particles <10 µm, was a driver of dewatering performance. This thesis provides insight into the fundamental drivers of fecal sludge solid-liquid separation and informs suggestions about how to improve and adapt treatment technologies. Overall, suspended small particles were identified as responsible for poor dewatering and settling in fecal sludge, which suggests that treatment efforts should focus on technologies that remove small particles. This currently cannot be reliably achieved during storage in containment, but could be accomplished with treatment options that promote flocculation (e.g. conditioners) or destruction of small particles (e.g. hydrolysis pretreatment followed by controlled anaerobic digestion). Predictive models based on photographs and probe measurements could help to facilitate adaptive process control to allow for these treatment technologies to successfully function with highly variable influent fecal sludge.