Bioaugmentation for low C/N ratio wastewater treatment by combining endogenous partial denitrification (EPD) and denitrifying phosphorous removal (DPR) in the continuous A2/O-MBBR system

Endogenous partial denitrification (EPD) and denitrifying phosphorous removal (DPR) were combined in a novel A2/O - MBBR (Anaerobic Anoxic Oxic - Moving Bed Biofilm Reactor) system for low carbon/nitrogen (C/N) ratio wastewater treatment. The DPR performance was compared and the nutrient metabolism was elucidated based on the optimization of hydraulic retention time (HRT, 4-12 h) and nitrate recycling (R, 200%-600%). In the continuous-flow, the nitrate (NO3- ) denitrification accompanied by nitrite (NO2-, via EPD) accumulation with the nitrate-to-nitrite transformation ratio (NTR) of 35.87%-43.31% in the anoxic zones. At HRT of 12 h with R of 500%, batch test initially revealed the DPR mechanism using both NO3- and NO2- as electron acceptor, where denitrifying phosphorus accumulation organisms (DPAOs) and denitrifying glycogen accumulation organisms (DGAOs) were the main contributors for EPD with incomplete denitrification (NO3- -> NO2- ). Furthermore, stoichiometry-based functional bacteria analysis displayed that higher bioactivity of DPAOs (NO2- -> N2, 57.30%; NO3- -> N2, 35.85%) over DGAOs (NO3- -> N2, 6.85%) facilitated the anoxic NO3- reduction. Microbial community analysis suggested that Cluster I of Defluviicoccus-GAO group (-4%) was responsible for stable NO2accumulation performance via EPD, while increased Accumulibacter-PAO group (by -15%) contributed to the advanced nutrient removal. Based on the achievement of NO2- accumulation, the application feasibility of integrated EPD - DPR - Anammox for deep-level nutrient removal was discussed.