Nitrogen Removal in a Staged Anaerobic-Aerobic Sludge Digestion System.

  A mixture of primary and secondary municipal sludge was treated in the digestion system consisting of a first-stage mesophilic anaerobic and a second-stage aerobic digester. The study was to monitor solids and nitrogen removal efficiencies under different operational conditions in the aerobic digester.In the first phase, the aerobic digester was batch fed under continuous aeration. More solids and ammonia were removed as the aerobic retention time increased. About 90% or greater ammonia was removed at 3 days or longer aerobic retention times. However, higher oxidized nitrogen was measured at the longer aeration times.In the second phase, the aerobic digestion system was operated for 3 to 5 days while continuously fed with anaerobically digested sludge under different intermittent aeration times. About 90% ammonia removal and low effluent oxidized nitrogen were achieved at 40 minutes of aeration over a 60-minute cycle at a 4-day retention time.