Partial Nitrification and Denitrifying Phosphorus Removal in a Pilot-Scale ABR/MBR Combined Process

Simultaneous nitrification-denitrifying phosphorus removal (SNDPR) at low DO for treating carbon-limited municipal wastewater

This study investigated simultaneous nitrification-denitrifying phosphorus removal in a sequencing batch reactor (SBR) activated sludge process. The process consisted of an extended anaerobic period (180 min) followed by a low DO (0.3 ± 0.05 mg/L) simultaneous nitrification-denitrifying phosphorus removal. The reactor was operated within a wide range of COD/N ratio (5-10) without any volatile fatty acids (VFA) supplementation. N and P removal efficiencies were as high as 91% and 96%, respectively. The process was efficient even at a very low COD /N ratio of 5, with N and P removal efficiencies of 70% and 90%, respectively. The N and P removal efficiencies improved to more than 90% at a COD/N ratio 8. It was found that the initial filtered flocculated COD (ffCOD)/[total oxidized Kjeldahl Nitrogen (TKN_oxidized) + NO_x-N_intitial] ratio in the reactor played a significant role in the process efficiency. It was observed that N-removal efficiency decreased with a decrease of [ffCOD_initial/ (TKN_oxidized + NO_x-N_initial)] ratio even at high COD/N ratio of 10. Simultaneous nitrification denitrification (SND) efficiencies varied between 60%-88% depending on the influent COD/N ratio and [ffCOD_initial/ (TKN_oxidized + NO_x-N_initial)] ratio in the reactor. Cyclic studies showed a distinct two step phosphorus release in the extended anaerobic period in contrast to the more conventional single step phosphorus release. During the aerobic period, low DO favored denitrifying P-removal without significant accumulation of NO₃-N, and NO₂-N until all endogenous carbon was consumed. Denitrifying phosphorus accumulating organisms (DPAOs) played a vital role in simultaneous denitrification and phosphorus removal. Aerobic and anoxic P-removal represented about 55% and 45% of the overall phosphorus removal, respectively. Cycle tests showed that DPAOs have a competitive advantage over NOB for nitrite consumption at low DO. The process was found to be carbon efficient as evidenced by the COD/NO_x-N ratio of 4.2 for denitrification. Compared to traditional enhanced biological phosphorus removal (EBPR) coupled with exogenous denitrification, this process reduces carbon and oxygen demand for combined N and P removal from municipal wastewater by about 45%, and 35% respectively.