Post-endogenous denitrification and phosphorus removal in an alternating anaerobic/oxic/anoxic (AOA) system treating low carbon/nitrogen (C/N) domestic wastewater

A novel post-endogenous denitrification and phosphorus removal (post-EDPR) system was developed by enriching denitrifying glycogen accumulating organisms (DGAOs) and phosphorus accumulating organisms (PAOs) to treat low carbon/nitrogen (C/N) wastewater (C/N = 4.4). The unique feature of the post-EDPR system was the continuous alternating anaerobic/oxic/anoxic zone (3.1 h/3.9 h/7 h) with suspended biofilm carriers in oxic zone. The 132-day test results showed that extended anaerobic and anoxic stage strengthened the intracellular carbon storage and endogenous denitrification. Low dissolved oxygen (DO) concentration (1-2 mg/L) in oxic zone promoted the simultaneous nitrification and denitrification (SND) and saved aeration energy, nitrifiers were enriched with the biofilm carriers and nitrification performance was strengthened, so oxic hydraulic retention time (HRT) could be shortened to avoid excessive polyhydroxyalkanoate (PHA) consumption which would be used for the latter anoxic denitrification. In a steady state, the system achieved advanced nitrogen and phosphorus removal (TIN (total inorganic nitrogen) ≤3 mg/L, PO43− − P ≤ 0.2 mg/L) with a high removal efficiency of 92.15% and 92.67%, respectively, and PHA played a major role for P-uptake and endogenous denitrification. Microbial community analysis demonstrated that PAOs and GAOs were enriched and performed aerobic P-uptake and anoxic endogenous denitrification, respectively.