Effect of low solids retention time and focused pulsed pre-treatment on anaerobic digestion of waste activated sludge

The interacting effects of Focused Pulsed (FP) treatment and solids retention time (SRT) were evaluated in laboratory-scale digesters operated at SRTs of 2–20 days. Anaerobic digestion and methanogenesis of waste activated sludge (WAS) were stable for SRT ⩾ 5 days, but the effluent soluble organic compounds increased significantly for SRT = 2 days due to a combination of faster hydrolysis kinetics and washout of methanogens. FP treatment increased the CH4 production rate and TCOD removal efficiency by up to 33% and 18%, respectively, at a SRT of 20 days. These effects were the result of an increase in the hydrolysis rate, since the concentrations of soluble components remained low for SRT ⩾ 5 days. Alternately, FP pre-treatment of WAS allowed the same conversion of TCOD to CH4 with a smaller SRT and digester size: e.g., 40% size savings with a CH4 conversion of 0.23 g CH4–COD/g CODin.