Fate of Nutrients During Aerobic Digestion

Laboratory scale aerobic digestion experiments were performed and nutrient balances were maintained in order to evaluate nutrient transformations during digestion of waste activated sludge. Reactors loaded at 0.03 ft³ and 0.081 lb VS/day/ft³, were operated under both batch and daily feed conditions. Significant nitrification occurred, resulting in substantial reductions in pH and alkalinity. Denitrification occurred when dissolved oxygen concentrations dropped below 1.0 mg/l. Phosphorus and nitrogen were released to the liquid in approximately a stoichiometric ratio to the volatile solids destroyed. As much as 37% of the phosphorus and 41% of the nitrogen in the feed sludge was found in the supernatant after digestion. The solid phase of aerobically digested waste activated sludge had a nitrogen content of 7% of the suspended solids and a phosphorus content of 2% of the suspended solids.