Continuous Pre‐Anoxic and Aerobic Digestion of Waste Activated Sludge

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Abstract
Conventional aerobic sludge stabilization requires extensive energy input. Incorporation of endogenous nitrate respiration (ENR) into the aerobic‐digestion system offers the energy‐saving advantage. In the ENR system, nitrate instead of oxygen is utilized for the biomass destruction. In this study, continuous preanoxic and aerobic digestion of waste‐activated sludge is studied to evaluate its effectiveness for volatile suspended solids (VSS) destruction and nitrogen removal. The ENR occurs in the anoxic digester because of the recycling of nitrate from the aerobic digester. The anoxic effluent, in turn, provides a high pH and adequate buffer capacity for enhancing the subsequent aerobic decay and nitrification. Two experimental runs, differing in hydraulic‐detention time in anoxic and aerobic tanks, are conducted with the same recycle rate. The results indicate that total nitrogen removal is approximately 25%, with 40% VSS destruction. The denitrification/nitrification efficiencies and rates are also determined. A mathematical model developed from the mass balance of biomass, ammonium, and nitrate reasonably predicts transient responses of these parameters.