The impact of faecal sludge-derived biochar as an additive on anaerobic degradation of synthetic human excreta

Abstract

Pit latrine filling poses a major challenge in developing countries. This study investigated the effect of faecal sludge-derived biochar (FSB) on anaerobic degradation, the dominant process in pit latrines, using synthetic human excreta under laboratory conditions simulating wet pit environments. Results show that FSB addition significantly increased hydrolysis rate and shortened lag phase by up to 1.5 days. However, this enhancement did not translate into improved overall degradation, as volatile solids (VS) reduction decreased under FSB addition, while only minor changes were observed in methane and total gas production. This decoupling is attributed to redistribution of organic carbon from gaseous products toward microbial biomass and extracellular matrices, as reflected by elevated extracellular polymeric substance (EPS) carbohydrate concentrations. Microbial community analysis revealed a targeted restructuring toward biofilm-associated hydrolytic and methanogenic populations under FSB amendment, characterised by enrichment of metabolically versatile methanogens (Methanosarcina) and proteolytic fermenters. Overall, these findings indicate that FSB promotes microbial aggregation and stabilised anaerobic processes but does not enhance net solids degradation, and therefore the findings do not support the recommendation of FSB as a pit latrine additive for reducing pit filling.