Open Access

Wenxin Shi, Tao Zhang, Daniela Almeida StreitwieserORCiD, Yibo WangORCiD, Yiming Han, Chengrong Li, Mengda Feng

• Waste activated sludge (WAS) management remains a critical challenge in wastewater treatment. This study evaluated sludge reduction in an anaerobic/oxic (A/O) process using a biological reduction agent, ultrasonication, ozonation, and their combined application. The combined treatment achieved a sludge reduction efficiency of 65.34% while maintaining stable chemical oxygen demand (COD) and total nitrogen (TN) removal performance. Multiple lines of evidence were used to interpret the underlying mechanisms. Ultrasonication was associated with sludge floc disruption and increased organic release, whereas ozonation was associated with enhanced floc degradation and higher soluble organic content. EEM-PARAFAC analysis showed treatment-dependent shifts in protein-like, soluble microbial by-product-like, and humic-like dissolved organic matter fractions, consistent with a coupled release oxidation–restructuring pathway under combined treatment. The observed changes under ultrasound–ozone treatment were further consistent with intensified sludge disintegration and enhanced cell lysis and lysis-cryptic growth. In parallel, the biological reduction agent was associated with reduced sludge production, accompanied by shifts in microbial community composition and suppression of filamentous and sludge-producing bacteria. 16S rRNA gene sequencing identified treatment-dependent changes in the abundance of Chloroflexi and Patescibacteria. A preliminary OPEX-focused assessment indicated configuration-dependent disposal-cost savings of 3.50–55.77%, while COD and TN removal were largely maintained despite reduced TP removal under intensive physicochemical conditioning. The apparent GHG benefit was strongly baseline-dependent, with substantial avoided emissions under an incineration-displacement boundary but much smaller benefits under a representative lower-carbon anaerobic-digestion reference.