Metagenomic and transcriptomic insights into microbial activity maintenance strategies in a pilot-scale...

Sewer overflow is a widely recognized issue in urban water environment pollution. Traditional in situ treatment technologies based on filtration and flocculation often fail to remove soluble pollutants effectively. Conventional in situ biological systems also struggle to maintain activity under fluctuating and nutrient-imbalanced influent conditions. Here, a compact in situ biological treatment process based on biosorption-biodegradation technology with a shortened hydraulic retention time (HRT) is proposed. During a 180-day pilot-scale experiment integrating ballasted flocculation, the system achieved average removal efficiencies of 75-94% for COD_Cr, NH₄⁺-N, TP, BOD₅, and SS. Effluent concentrations met Chinese surface water quality standards. The system maintained stable performance during wet weather events and after multiple dry periods of up to 30 days, demonstrating effective microbial activity maintenance. Based on the metagenomic and transcriptomic analyses, this stability is potentially related to nutrient supplementation through carbon metabolism of mixotrophic organisms and pollutant adsorption by biosorption sludge. Additionally, the reduced HRT prevents endogenous respiration and sludge degradation. The compact biosorption-biodegradation process offers an efficient and space-saving strategy for maintaining microbial activity during dry periods. It provides a promising solution for mitigating sewer overflow pollution in high-density urban areas.