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

Abstract Background: Waste sludge, as an inevitable by-product during wastewater treatment, is a valuable resource for nitrogen nutrient recovery (e.g. protein and ammonium). Electro-fermentation system (EFS) has a unique advantage in boosting sludge fermentation while recovering ammonium. However, the simultaneous recovery of protein and ammonium during sludge treatment has not yet been achieved. And it remains obscure how microbes cooperate regarding their molecular metabolic mechanisms during EFS treating sludge. Results: We proposed a novel approach for sequential recovery of protein and ammonium and simultaneous sludge treatment by pretreatment-EF cascading system. The results showed that 72.23% of protein was recovered by alkaline-thermal pretreatment of dewatered sludge, which was recycled as yeast extract and peptone substitute for bacterial growth. The cascading two-chamber EFS facilitated the ammonium recovery of 71−80% and 45−50% in cathode chamber of residual pretreated dewatered sludge (EFS-TS) and raw waste sludge (EFS-RS), respectively. Additionally, the EFS significantly enhanced the COD removal, where EFS-TS obtained the highest TCOD removal which increased by 6.39−14.53% over anaerobic digestion. Microbiome analysis demonstrated that EFS attributed to the enrichment and syntrophic interaction of electroactive bacteria ( Geobacter ), fermentative bacteria ( Rikenella , Lentimicrobium , and Petrimonas ), and nitrogen-fixing bacteria( Geobacter and Azonexus ). Metagenomics analysis uncovered that this syntrophic interaction facilitated the organic degradation pathways, intracellular and extracellular electron transfer, and nitrogen transformation. Conclusions: Our 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.