Optimisation of Operating Strategies for Return Sludge Sidestream (RSS) EBPR Process in Pilot-Scale Systems Treating Real Domestic Wastewater

Return sludge sidestream (RSS) enhanced biological phosphorus removal (EBPR) processes currently lack clear design guidelines for sidestream tank (SST) hydraulic retention time (HRT) and return activated sludge (RAS) diversion ratio, which have often been evaluated as interdependent parameters. This study evaluates a pilot-scale Modified Ludzack–Ettinger (MLE) reactor with and without an RSS treating real domestic wastewater, independently assessing sidestream tank HRT and RAS diversion ratio. Nutrient removal performance, nitrous oxide (N₂O) emissions, sludge production, and microbial community dynamics were investigated. A sidestream tank HRT of 24h led to improved phosphorus (P) removal performance and stability (93.3 ± 7.0% versus 75.7 ± 17.3% at 16 h), though anaerobic mass fraction was controlled by either sidestream tank HRT or RAS diversion ratio. The optimised RSS phase eliminated chemical P precipitation requirements and reduced total sludge yield by ∼31-47 % via in situ fermentation as compared to MLE operation, comparable to reductions previously reported at sidestream tank HRTs >48 h, with substantially larger anaerobic tank volumes. Coupling RSS with intermittent aeration enabled stable nitrite accumulation while maintaining low N₂O emissions, with an average emission factor of 1.1 ± 0.7%). Microbial community analysis revealed enrichment of polyphosphate-accumulating organisms (PAOs) (notably Dechloromonas ) and suppression of glycogen-accumulating organisms (e.g., Competibacter <1 %), consistent with sustained EBPR stability. Collectively, these findings provide practical design guidance for RSS retrofits, including selection of sidestream tank HRT, RAS diversion ratio, and anaerobic mass fraction, enabling stable EBPR performance, reduced sludge production, and minimal environmental footprint, thereby supporting cost-effective full-scale implementation.