EVALUATION OF ALTERNATIVE OPERATION MODES FOR AEROBIC DIGESTER AT THE FOUNTAIN HILLS SANITARY DISTRICT WASTEWATER TREATMENT FACILITY

Recycle streams from sludge digestion facilities to activated sludge processes can be important in the overall performance of wastewater treatment facilities. The recycling streams at the wastewater treatment plant for the Fountain Hills Sanitary District Wastewater Treatment Plant include digested sludge dewatering centrate, waste activated sludge thickening centrate, and filter backwash water. This investigation evaluated alternative modes of operation for the aerobic digesters at the Fountain Hills plant with the purpose of reducing the total nitrogen concentrations in the combined recycle stream. The results showed that promoting denitrification in the digester, either by operating at a low dissolved oxygen concentration or operating in a cyclic aeration mode, can significantly reduce the total nitrogen concentration in the combined recycle stream and maintain the total nitrogen concentration in the effluent lower than 7.0 mg/L. The results also show that the full aeration operation of the digester without denitrification would result in a higher effluent total nitrogen concentration. However, the calculated effluent total nitrogen concentrations would still be lower than the plant effluent total nitrogen limit of 10 mg/L. The effluent total nitrogen concentration would approximate the plant's alert level of 8.0 mg/L. For this facility, the digestion operation with nitrification and denitrification is preferable.