Human waste anaerobic digestion as a promising low-carbon strategy: Operating performance, microbial dynamics and environmental footprint

Human waste (HW) poses environmental and public health risk, thereby its sustainable management is becoming a serious growing challenge. Anaerobic digestion (AD) has long been introduced as an environmental-friendly and sustainable waste management technology with bio-methane as final product. Energy production via AD of HW would significantly contribute to low-carbon energy production systems and circular bio-economy. In this study, optimal conditions, inhibiting factors, and microbial community changes in continuously fed biogas reactors during anaerobic digestion of HW at elevated influent feedstock concentration (IFC) were investigated. The highest methane yield (327 ± 21 mL g VS⁻¹) was obtained at IFC of 3% TS. Increased IFC deteriorated the process and affected the microbiome dynamicity. Specifically, methane production was reduced by 50% with a concomitant increment of ammonia, Na⁺, and K⁺ concentration. Two archaeal species (Methanosaeta and WSA2) were dominating the microbial community at stable period. Two uncharacterized microbial groups (WWE1 and WSA2) were present and a potential syntrophic interaction between these two members was hypothesized to play a crucial role in achieving a well-performing process. AD process treating HW showed promising results for valorization of HW to clean energy-biomethane both in environmental and economic aspects. Specifically, 1 t of HW VS could obtain a greenhouse gases (GHG) mitigation of −54 to −272 kg CO_2,eq via the AD process. The LCA results demonstrated that such a bioenergy system would also bring about environmental savings in ecosystem quality and resource damage categories. Although ammonia inhibition at elevated IFC found as a potential inhibitory factor, it can be easily overcome using co-digestion strategies or nitrogen recovery at upstream.