Enhancement of Biogas Recovery from Fecal Sludge using Anaerobic Co-Digestion Process

Abstract

With enhanced focus on global sanitation, access to toilets at the household level is increasing in developing countries although the provision of sewer networks is not expanding at the same pace. This is resulting in the adaptation of non-sewered sanitation (NSS) facilities to contain the fecal sludge. The fecal sludge generated by these facilities requires proper handling before its disposal into the environment. The issue of fecal sludge has not been recognized until recently and the understanding of treatment mechanisms for fecal sludge are not fully understood. This study investigates the anaerobic digestion (AD) potential of fecal sludge through three objectives. Objective 1A assessed the sanitation situation and fecal sludge management (FSM) needs in Karachi, Lahore, and Islamabad, Pakistan. Using tools like excreta flow diagrams (SFD) and service delivery assessments (SDA), the findings revealed sewer network coverage of 60%, 63%, and 50%, respectively. However, wastewater treatment remains minimal: 10% in Karachi, 9% in Islamabad, and none in Lahore. Non-sewered systems provide safe sanitation for 8% of Lahore and 25% of Islamabad, relying on onsite sludge containment. National level sanitation programs exist but are limited to reducing open defecation and containments of fecal sludge. The inclusion of complete FSM related framework, guidelines, and policies can help achieve the goal of safe sanitation for all. The objective 1B conducted an integrated FSM assessment by combining SFD and quantity and quality (Q&Q) analyses to evaluate fecal sludge volumes and characteristics. Physical-chemical characterization for 50 samples from pits latrines and septic tanks was performed. The study highlights the significant variability in fecal sludge characteristics, with pit latrine sludge exhibiting 10-100 times higher concentrations of total solids (TS), volatile solids (VS), and chemical oxygen demand (COD) compared to septic tank sludge. The findings reveal that the indicators related to fecal sludge characteristics are containment type, income level, and emptying practices. Helminth eggs, notably Ascaris lumbricoides, were found in sludge, posing health risks. The findings underscore the need for improved FSM planning and treatment solutions to address pathogen risks, contributing to public health improvements. The wide variation in the fecal sludge characteristics makes it difficult to design treatment processes and requires careful investigation at laboratory scale before xxi implementing decentralized to semi-centralized scale anaerobic digesters. In the 2nd objective of this study, anaerobic mono- and co-digestion of synthetic and real fecal sludge was studied. For mono-digestion, the sludge from pit latrines and septic tanks was mixed in 100:0, 75:25, 50:50 and 0:100 ratio respectively and their performance was compared to the synthetic surrogates. Concurrently, the co-digestion performance of synthetic and real fecal sludge was also evaluated using fruit vegetable waste as co-feed. The experiments revealed that synthetic fecal sludge is significantly different from real fecal sludge. The anaerobic co-digestion for real fecal sludge is successful and produces approximately 47 – 3,655% more biogas in comparison to its mono-digestion. During co-digestion the organic removal improved by by 8% for TS, 18% for VS and 14% for COD removal. In comparison, instability was observed during co-digestion of synthetic fecal sludge owing to its highly biodegradable organic fraction coming from the synthetic sludge and fruit vegetable waste. The 3rd objective of the study focused on biomethane potential (BMP) tests from fresh (not stored at all), stored (pit and septic tank sludge), and dewatered (septic tank sludge) fecal sludge, together with co-digestion with fresh foodwaste. This objective revealed that fresh fecal sludge produced similar cumulative biogas (CBG) to fresh foodwaste (615–627 mL/gVS), whereas dewatered fecal sludge produced (396 ml/gVS) while stored fecal sludge showed a wide range of gas production (13–449 mL/gVS). Co-digestion significantly enhanced the CBG production of fresh (1.2×), dewatered (1.5×), and stored (29–36×) fecal sludge. In BMP tests with the higher range of gas production, a biphasic CBG production was observed, with degradation of readily biodegradable organics occurring during the first week. The first-order rate coefficients indicated hydrolysis limitation, which was also confirmed by the presence of slow-growing methanogens (Halobacterota). Priming with co-digestion significantly enhanced CBG from stored fecal sludge. The physical–chemical metrics VS/TS and TOC/TN were not predictors of biogas production, while BOD/COD and sCOD were better indicators, suggesting that metrics of stabilization representing biologically available fractions are more representative than metrics of entire pools of organic matter. This study suggests that biogas production from anaerobic digestion is viable for fresh fecal sludge, whereas for stored fecal sludge, this requires co-treatment or pretreatment.