Denitrification and phosphorus release under anoxic conditions in an anoxic-anaerobic-aerobic BNR process

An anoxic‐anaerobic‐aerobic biological nutrient removal process was used in this study. The kinetic aspects of denitrif ication and phosphorus release under anoxic conditions were investigated by conducting a pilot‐scale plant operation under various SRTs (solids retention times), HRTs (hydraulic retention times) and internal recycle ratios. The process was capable of completely denitrifying the NOx −‐N (the sum of NO2 −‐N and NO3 −‐N) in the nitrified recycle, resulting in an NOx −/‐N concentration of less than 1.0 mg l−1 N in the anoxic zones. Denitrification and phosphorus release were accomplished due to abundant organic substrates in the anoxic zone at the head end of the process and achieved approximately equivalent rates with respect to influent SCOD loading in the zone. Phosphorus release continued without any nitrate inhibition due to low NCx‐N concentrations of less than 2.0 mg l−1 N in the anaerobic zone.