Insights into sludge reduction in a sequencing batch reactor integrated with an anaerobic side-stream reactor

Abstract

Sludge production remains a major challenge in wastewater treatment and integrating an anaerobic side-stream reactor (ASSR) into the return activated sludge line is a promising technology for in situ sludge reduction. Thus, this research investigates the performance and internal mechanisms of an ASSR integrated with a sequencing batch reactor (SBR) under prolonged side-stream retention conditions for in situ sludge reduction. The results showed that this configuration reduced the observed sludge yield by 42% (0.288 → 0.166 g VSS/g COD), while maintaining stable COD removal of 85–90%. Ammonium removal remained high, although nitrate accumulation during part of the operation indicated that nitrogen conversion was not fully stable. Total phosphorus (TP) removal reached up to 98% during early operation, accompanied by phosphate release in the ASSR of up to 250 mg PO₄^3--P/L, indicating the formation of a phosphate-rich side-stream. However, TP removal and phosphate release declined during later operation, showing that this response was not sustained throughout the experiment. A simplified flow-based VSS balance indicated that the net VSS difference across the ASSR was equivalent to approximately 25% of the difference in sludge production between the two systems. Endpoint microbial profiles and predicted functional profiles differed among the reactors and were consistent with community differentiation under the integrated operating conditions. However, they did not establish the active metabolic pathways. Overall, the lower system-wide observed sludge yield reflected the combined effects of solids transformation in the ASSR, longer biomass retention, and repeated anaerobic–aerobic exposure.