Optimization denitrifying phosphorus removal at different hydraulic retention times in a novel anaerobic anoxic oxic-biological contact oxidation process

A modified two-sludge system consisting of AAO and BCO (Anaerobic anoxic oxic-biological contact oxidation) was developed. The unique advantage was the high efficient utilization of influent carbon sources in AAO reactor with NO3−-N produced in BCO reactor, which achieved the simultaneous removal of both nitrogen and phosphorus. With real municipal wastewater at low carbon/nitrogen ratio (COD/TN = 3.50), the influence of hydraulic retention time (HRT = 6 -12 h) in the AAO reactor on denitrifying phosphorus removal was observed. The mass balance analysis demonstrated that the system saved about 30-35% carbon sources compared with traditional biological nutrient removal (BNR) systems. HRT of 10 h was found to be optimal for denitrifying phosphorous removal due to the limitation of poly-β-hydroxyalkanoates (PHA) utilization and denitrification capacity (NO3−-N/PO43−-P ratio). It was also determined that the NO3−-N concentration of the middle settler should be kept around 1.50-2.50 mg/L to prevent secondary phosphorus release. Finally, anoxic duration and NO3−-N loading were the major limiting factors for denitrifying phosphorus accumulation organisms (DNPAOs) growth.