Sidestream Treatment of Full-Strength Dewatering Centrate Using Activated Sludge Sequencing Batch Reactors (AS-SBRs)

Sidestream treatment of dewatered anaerobically digested sludge liquor, referred to as dewatering centrate (or centrate), has gained interest in recent years since wastewater treatment plants (WWTPs) are required to meet stricter effluent nutrient limits while maintaining performance and stability of the mainstream process. This study investigated separate sidestream nitrification of centrate from a WWTP in Toronto, Canada, using activated sludge sequencing batch rectors (AS-SBRs). Two identical SBRs were seeded with return activated sludge (RAS) and operated with and without supplemental alkalinity in sequential batch mode on a 24 h cycle time at ambient temperature (25–27 °C) in the laboratory using a nitrogen loading rate of 0.18–0.20 g/L.d. Complete nitrification was observed in the SBR with supplemental alkalinity as ammonia–nitrogen (NH₃-N) removal during stable operation was 100 ± 0% (26th to 93rd day), while the SBR without supplemental alkalinity achieved 61 ± 2% (18th to 52nd day). Insufficient alkalinity adversely affected the performance due to the decreasing pH during the SBR cycle, which translated to incomplete nitrification and inhibition of ammonia oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB). Results showed that addition of excess sodium bicarbonate (NaHCO₃) as a source of alkalinity during start-up extended the acclimation period. Furthermore, cycle activity test performed on the SBR with supplemental alkalinity during stable operation resulted in a nitrification rate (NR) and specific nitrification rate (SNR) of 23.21 mg N/L.h and 13.8 mg N/g VSS.h, respectively. Overall, this study confirmed complete nitrification of centrate and provided support in the development of a reliable sidestream treatment process.