Sustainable Hydrogen Production via Dark Fermentation of Protein- and Lipid-Rich Municipal Organic Waste: Digestate Reuse and Ammonia Mitigation Strategies

A major limitation to the conversion of OFMSW via anaerobic fermentation is the high concentration of animal-derived wastes, which can inhibit the process due to ammonia accumulation. This study assessed the reusability of ammonia-loaded, dark fermentation (DF) liquor at two reuse cycles, derived from the fermentation of protein/lipid-rich substrates. Mitigation strategies such as ammonia stripping and biochar addition were evaluated against unstripped and unrecycled bioreactors. The initial slurry was generated from DF of substrates with varying compositions of proteins and lipids, which yielded characteristic results, and subsequently applied under four operational variations namely biochar addition, ammonia stripping, and unstripped and unrecycled bioreactors. Biochar addition effectively mitigated against ammonia accumulation across both cycles. In the first cycle, it produced the highest hydrogen yield, outperforming stripped and unstripped bioreactors by 53.8%, and 76.9%, respectively. In cycle 2, biochar further outperformed stripped and unstripped bioreactors by 44.1% and 42.4%. Despite a higher ammonia exposure, microbial consortia in the unstripped bioreactors acclimatised more effectively than stripped bioreactors in the second cycle. The main limiting factor was not conversion efficiency but electron diversion due to competing metabolic pathways. This study provides new insights into biohydrogen enrichment from animal-derived wastes, while emphasising mitigation strategies amid freshwater savings.