Orthophosphate addition to enhance degradation of synthetic human excreta in simulated wet pit latrines

Filling of pit latrines remains a major challenge for non-sewered sanitation. This study investigated the anaerobic digestion of synthetic human excreta under laboratory conditions simulating dry and wet pit latrine environments, with orthophosphate applied as an additive to enhance degradation. At low total solids content (∼4 %), representative of wet pit conditions (e.g. connected to pour-flush toilets), orthophosphate addition did not significantly affect methane yield but enhanced early steps of the degradation, resulting in a 10-21 % increase in volatile solids reduction. Microbial community analysis showed that orthophosphate acted as a selective pressure that reshaped microbial community structure across multiple functional groups in anaerobic digestion. Enrichment of proteolytic hydrolysers and acidogens, promotion of syntrophic acetogens and inhibition of methylotrophic methanogens by orthophosphate addition indicate a clear shift toward protein-oriented degradation and CO₂ production pathway rather than CH₄. Extrapolation to pit latrine conditions suggests that orthophosphate addition could expand the pit lifetime by up to 17.2 % in a typical well-drained pit. These findings highlight orthophosphate as a nutrient-based additive that can enhance sludge stabilisation and reduce pit filling rate, with limited impact on methane production, and underline the need for field validation under real faecal sludge and pit conditions.