PHASED NITRIFICATION-DENITRIFICATION PROVIDES FLEXIBLE NITROGEN REMOVAL

Phased nitrification-denitrification (PNDN) refers to a design where nitrification and denitrification is achieved sequentially within the same basin by simply turning the aeration system on and off. One of the limitations of conventional activated sludge nitrogen removal processes using compartmentalized zones is the inability of the operator to adjust the anoxic/aerobic time sequence. Once the various anoxic and aerobic zones are constructed, the plant operation is set and the anoxic and aerobic times are set, depending on the flow and basin volumes. Designers can overcome this limitation by installing both mixers and aeration equipment in the anoxic zones. Another alternative is to construct multiple parallel trains and design the system to take trains offline to increase or decrease the anoxic fraction.The PNDN design is based on a continuous flow, cyclic air on/air off system to achieve nitrification and denitrification in the same basin. Predicted performance from this process shows that on/off operation produces stable effluent nitrate and ammonia concentrations. This design eliminates the need for internal recycles to achieve substrate level denitrification.Performance data from three full scale PNDN facilities show very good nitrification and denitrification. Effluent total inorganic nitrogen concentrations below 5 mg/L are measured. The operating conditions can be optimized for nitrification or denitrification. Ammonia and nitrate will bleed exit the PNDN reactor during the aeration off and on cycles, respectively. The effluent quality is affected by the cycle frequency, the aeration on/off period, and the hydraulic retention time.