Unraveling roles of the intermediate settler in a microaerobic hydrolysis sludge in situ reduction process

The roles of the intermediate settler in the sludge process reduction activated sludge process (SPRAS), and the influences of its hydraulic retention time (HRT_ST) on pollutant removal and sludge reduction were investigated. Prolonging HRT_ST from 3.0 to 4.5 and 6.0 h resulted in sludge reduction efficiencies increased from 46.8% to 61.5% and 62.7%. The sludge accumulation in the intermediate settler formed an anaerobic zone but inhibited methane production, and the alternating microaerobic and anaerobic environment in the sludge process reduction (SPR) module increased the microbial diversity and enriched the hydrolytic and fermentative bacteria. Prolonging HRT_ST accelerated dissolved organic matter release and elevated the degradation of refractory fraction, and improved the sludge properties of the SPRAS. Metagenomic analysis showed that the SPR module enhanced the glycolysis pathway and decoupling metabolism for sludge reduction. The results revealed that the intermediate settler plays dual roles in solid–liquid separation and sludge reduction metabolism.