Anoxic‐aerobic digestion of waste activated sludge: Part I‐solids reduction and digested sludge characteristics

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Abstract
This research demonstrated the feasibility of incorporating, at regular intervals, non‐aerated conditions in aerobic sludge digestion. Part I of this paper outlines the benefits associated with solids reduction and digested sludge characteristics. Three digestion modes (anoxic‐aerobic, aerobic with lime control and aerobic) were investigated at 20 °C and 10 °C. For each mode and temperature, solids retention times (SRTs) of 10, 15 and 20 days were examined. The results showed that anoxic‐aerobic digestion yielded comparable percent solids reduction, despite using only 42 percent as much air. Anoxic‐aerobic digestion also maintained neutral mixed‐liquor pH levels at ‘no cost’. In addition, significant nitrogen removal (up to 32 percent) and reduced phosphorus release were realized.