Effects of oxygen exposure on anaerobic digester sludge

Recuperative thickening of anaerobic digester sludge (thickening with solids return) yields increased digester capacity. Common thickening methods cause oxygen exposure to the digester sludge. This study evaluated the effects of various levels of oxygen exposure on the acetoclastic methanogens. Gravity belt thickening had no detrimental effect on the acetoclastic activity. From a 7-day batch test with continuous oxygen exposure of digester sludge, a 12% loss in acetoclastic activity was predicted for a digester with a 20-day solids retention time (SRT) and 100% recycle with recuperative thickening via dissolved air flotation thickening. However, a greater loss (27%) was found from a long-term, bench-scale digester operated under similar conditions. This loss did not affect the digester performance, as measured by volatile solids destruction. This research suggests that recuperative thickening may not affect digester performance at a long SRT with constant operation, but may change the reserve capacity of the anaerobic community.