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Insufficient influent carbon source affects the performance and stability of the biological nutrient removal (BNR) process in wastewater treatment plants (WWTPs). To solve this problem, a side-stream activated sludge hydrolysis (SSH) process that utilizes the internal carbon source through hydrolysis and fermentation of return activated sludge (RAS) was proposed as an alternative. In this study, two laboratory-scale pilot reactors were run to compare the pollutant removal performance in SSH and conventional anaerobic/anoxic/aerobic (A2/O) processes at various influent food-to-microorganism (F/M) ratios (0.07–0.71). When the average F/M ratio increased to > 0.35, the chemical oxygen demand (COD), ammonia (NH4+-N), total nitrogen (TN), and total phosphorus (TP) removal efficiencies in SSH reactor reached ~ 94%, > 95%, > 90%, and > 85%, respectively. Under the same influent conditions, the TN removal in SSH reactor was 23% higher than that in A2/O reactor, which is related to the additional carbon generated via RAS fermentation and mitigated substrate competition among various heterotrophic organisms. A relatively high polyphosphate accumulating organism (PAO) activity was also observed in SSH reactor. Meanwhile, the relative abundances of denitrifiers (e.g., Thiothrix, Azospira, and Dechloromonas) and PAOs (i.e., Accumulibacter, Tetrasphaera, and Dechloromonas) were higher in SSH reactor than those in A2/O reactor. These results will provide a better understanding of the mechanisms involved in SSH process, leading to more effective nutrient removal for treating wastewater with limited carbon sources. [ABSTRACT FROM AUTHOR]