A novel in-situ sludge reduction strategy: Bio-promoter assisted low-MLVSS operation for reducing sludge production while maintaining nitrification.

Excess sludge production and subsequent treatment remain major challenges in activated sludge-based wastewater treatment. However, biological strategies for efficient in-situ sludge reduction remain limited. In this study, a composite bio-promoter was developed to support low-MLVSS operation. At 15% lower MLVSS, the bioreactor maintained 88.37% ammonia nitrogen removal, while the observed sludge yield decreased by 19.78% within a cycle. Long-term operation showed that bio-promoter addition activated the metabolic activity and key enzyme functions, thereby reducing the net sludge increase by 13.46%. The lower sludge production response was accompanied by higher biomass-specific nitrifying activity. The specific oxygen uptake rate of ammonia-oxidizing bacteria and nitrite-oxidizing bacteria increased by 7.39% and 11.55%, respectively. The specific activities of ammonia monooxygenase and hydroxylamine oxidase increased by 13.66% and 27.71%, respectively. Metagenomics analysis revealed that Nitrosomonas and Nitrospira became the dominant functional bacteria in the community. The relative abundance of key nitrification genes amoA and hao increased by 6.25% and 40.80%, respectively, suggesting that the bio-promoter enhanced the functional activity of retained nitrifying biomass and helped maintain nitrification under reduced sludge concentration. This study created a novel bio-promoter technology scheme for in-situ sludge reduction and provided a theoretical basis and practical strategy for achieving energy-saving and efficient operation.