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 nitrogenfixing 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.