Nitrite-shunt and biological phosphorus removal at low dissolved oxygen in a full-scale high-rate system at warm temperatures.

This study presents results from the City of St. Petersburg's (Florida) Southwest Water Reclamation Facility. This high-rate BNR plant (SRT ~ 5 days; HRT < 8 hr) achieves combined bioP and shortcut simultaneous nitrification and denitrification (SND) via nitrite in a simple BNR configuration-an anaerobic-aerobic (A/O) process without mixed liquor recycle and a 25% unaerated fraction. N removal to low effluent and nitrate and nitrite (  +  ) concentrations occurs mainly via SND by operating the aerated zone at low DO, but still achieving near-complete ammonium ( ) removal. Despite the low DO operation, very good bioP performance is achieved. Full-scale performance data and detailed bench-scale testing were conducted to assess the nitrogen and phosphorus removal at low DO conditions. Full-scale results showed that the plant achieves effluent total inorganic nitrogen (TIN) and total phosphorus (TP) concentrations of approximately 2.0 mgN/L and 0.5 mgP/L, respectively, at an average influent C:N ratio of 7:1 mgCOD:mgN. PRACTITIONER POINTS: Simple anaerobic-aerobic (A/O) process demonstrated combined N and P removal Ammonia oxidation was not hampered by low DO (<0.5 mg/L) operation Low DO (<0.5 mg/L) operation sustained SND via nitrite pathway in a high-rate process (HRT < 6 hr) P uptake was demonstrated at low DO which counters to the widely held understanding that high DO (>1.5 mg/L) is necessary Heterotrophic consumption of nitrite at low DO was the key to the out-selection of nitrite-oxidizing bacteria.