Sustainable hydrogen and vivianite recovery from waste activated sludge in electro-fermentation: Perspectives of product regulation and microbial interaction.

Zero-valent iron mediated electro-fermentation (EF) has recently emerged as a promising strategy for the synchronous hydrogen and vivianite recovery from waste activated sludge (WAS), while the mechanism of production regulation and microbial interaction still remains unclear. In this study, a comprehensive analysis of hydrogen and phosphorus recovery from prefermented sludge via EF was performed. The substrate reduction (∼2500 mg COD/L, fermentation liquid was diluted 1:1) resulted in the highest hydrogen yield (25.5 mmol/g COD) and complete phosphate recovery at 2 d, the recovered vivianite accomplished the largest crystal size (138.8 μm) with layered structures. The highest utilization efficiency of short-chain fatty acids (SCFAs) and organics was also achieved in 1:1 group. Anaerobic fermentation bacteria (AFB), electroactive bacteria (EAB), homo-acetogens, and nitrate reducing bacteria (NRB) were the predominant microbes in the plankton and bio-cathode, forming a mutually beneficial and stable symbiotic network. Further analysis of metabolic pathways revealed that the 1:1 group exhibited higher abundance of key functional genes involved in hydrolysis, acidification, and hydrogen production. This study may provide the theoretical and technical foundation for sludge valorization in the future implementation of EF in wastewater treatment plants.