Effects of ultrasonic sludge reflux in an improved anaerobic/anoxic/oxic(A2/O) system for municipal wastewater treatment plant: Pollutant removal, sludge reduction, and microbial community

Abstract

Municipal wastewater treatment plants (MWWTPs) are currently facing challenges like low carbon sources of influent and excess sludge. Utilizing ultrasonic sludge disintegration can help supplement the carbon source for the biochemical tank and achieve in-situ sludge reduction. A full-scale experiment was conducted at an MWWTP to investigate the impact of reflux points on the performance of an improved A²/O system, and the reflux points included pre-anoxic (PrAX), anterior anoxic (AAX) and anaerobic (AN) tanks. Results showed that the use of ultrasound did not impact the effluent quality of the improved A²/O system. The optimal reflux point was AN, resulting in a 37.4 % sludge reduction while enhancing sludge settleability (SVI decreased by 13.39 mL/g). Nitrogen balance calculations revealed that refluxing disintegrated sludge to AAX increased its denitrification and nitrogen removal efficiency (DNR) by 12.22 %. The supplementation of organic matter also increased the relative abundance of Acidobacteriota (3.19 %) in the AAX. Refluxing to AN increased the denitrification phosphorus removal efficiency (DPR) of AAX by 32.02 %. AAX fully utilized the more storable carbon sources provided by ultrasonic sludge for AN. Refluxing to AN increased the relative abundance of Bacteroidota in AN by 5.47 %. This study investigated the synergistic effects of different sludge reflux points on pollutant removal and sludge reduction, providing a practical basis for implementing sludge ultrasonic treatment in MWWTPs.