Role of syntrophic acetate oxidation and hydrogenotrophic methanogenesis in co-digestion of blackwater with food waste

Blackwater collected from toilets represents a type of sustainable bioenergy resource in the modern sanitation system, while its biomethane recovery efficiencies through anaerobic digestion were limited by slow hydrolysis and inhibited methanogenesis due to a large fraction of solid organics and high free ammonia concentrations. In the current study, food waste and blackwater co-digestion was performed in an up-flow anaerobic sludge blanket (UASB) reactor (35 °C). Co-substrates with increasing food waste proportions were stepwise applied to demonstrate the threshold organic loading rate (OLR). Co-digestion effectively enhanced substrate hydrolysis efficiency by up to 86.1% and methanisation rate by up to 39.7% compared to blackwater mono-digestion. Hydrogenotrophic methanogens showed predominance in both feeding conditions. The dominant bacterial groups shifted from genus Bacteroides to T78, and methanogenic groups shifted from genus Methanogenium to Methanoculleus and Methanospirillum when the operation system shifted from blackwater mono-digestion to food waste co-digestion. The microbial community structures and the isotopic carbon analysis for CH4 and CO2 in the produced biogas indicated that a combined syntrophic acetate oxidation (SAO) and hydrogenotrophic methanogenesis (HM) pathway was established throughout the operation. The enhanced substrates’ properties including higher carbon/nitrogen (C/N) ratios and more readily biodegradable organics in food waste and blackwater co-digestion system contributed to the enhancements in biomethane recovery and microbial development compared to blackwater mono-digestion. The OLR stress under the overloaded condition negatively affected the microbial community structure and resulted in process deterioration.