Sequential recovery of protein and ammonium from waste sludge and functional metabolism in a combined process of nutrient recovery electro-fermentation (NREF)

Waste sludge is a valuable resource for nitrogen recovery, while electro-fermentation system (EFS) can boost sludge fermentation. However, the simultaneous recovery of nitrogen nutrient during sludge treatment has not been achieved. Herein, we proposed a novel approach for sequential protein-ammonium recovery during sludge treatment by pretreatment-EFS. The results showed that 72.23 % of protein was recovered by alkaline-thermal pretreatment of dewatered sludge. The cascading EFS facilitated the ammonium recovery of 45−80 % in catholyte of pretreated dewatered sludge (EFS-TS) and raw waste sludge. The EFS-TS obtained the highest total chemical oxygen demand removal which increased by 6.39−14.53 % over anaerobic digestion. Metagenomics demonstrated that EFS attributed to the enrichment and interaction of electroactive, fermentative and nitrogen-fixing bacteria, which facilitated the organic degradation pathways, electron transfer, and nitrogen transformation. This study provides insights into syntrophic interaction between biofilm and suspension in the nutrient recovery electro-fermentation and presents a promising approach for processing sludge with full-form nitrogen recovery.