Biological nutrient - removal processes perform poorly when the influent contains insufficient carbon, and supplementation with external chemicals or carbon sources greatly increases operating costs. Side-stream enhanced biological phosphorus removal (S2EBPR) has therefore gained attention as a cost-effective alternative for low-carbon wastewaters. In this study, a sequencing-batch reactor (SBR) equipped with an S2EBPR side stream was operated at various readily biodegradable COD-to-phosphorus ratios (rbCOD/P), sludge diversion percentages, and side-stream sludge-retention times (SRTSS). With 20% of the return sludge diverted to the side-stream and SRTSS maintained at 72 h, the system achieved stable performance at an influent rbCOD/P of 18.3: phosphate-removal efficiency averaged 90.9% and the P-release/acetate-uptake ratio reached 0.41 mol P mol^-1 C. Increasing the diversion ratio to 25% while shortening SRTSS to 57.6 h impaired phosphorus removal under the same low-carbon conditions owing to weakened microbial selection and reduced activity of polyphosphate-accumulating organisms (PAOs). Although in-situ sludge fermentation increased the ammonium load to the mainstream reactor by about 13%, the additional carbon generated offset this impact. High-throughput sequencing showed that fermentative Tetrasphaera species dominated the PAO community and likely acted synergistically with glycogen-accumulating organisms. Maintaining SRTSS > 72 h favoured PAO dominance at low rbCOD/P values. Overall, the results demonstrate that S2EBPR can achieve effective phosphorus removal from carbon-limited wastewaters without external supplementation.