A survey of municipal anaerobic sludge digesters and diagnostic activity assays

A plant site survey was made of 30 municipal wastewater treatment plants concerning the operational characteristics of their anaerobic sludge digesters. Design information, operating data and analytical data were tabulated. Samples of each sludge were then assayed to determine the residual gas production rate, the maximum potential acetate and propionate utilization rates, the 5 and 30 day biochemical methane potential (BMP 5) (BMP 30) and the possible limitation in bioavailability of iron, cobalt or nickel. The average solids content of the raw sludge fed to the digesters was 4. 7%. Eight of the 30 sludges showed stimulation in the gas production rate when iron, cobalt or nickel was supplemented and acetate was unlimiting. Nine of the 30 sludges showed stimulation in the gas production rate when iron, cobalt or nickel was supplemented and propionate was unlimiting. The average BMP 5 was 0. 7 volumes of methane per volume of sludge. The average BMP 30 was 1.9 vol CH 4/vol sludge. On average 87% of the biodegradable fraction of the sludge was converted to methane during digestion. The average maximum potential acetate utilization rate (MPAUR) was 0.93 vol CH 4/vol sludge-day. The average maximum potential propionate utilization rate (MPPUR) was 0.18 vol CH 4 /vol sludge-day. The average H 2S in the digester gas was 2200 ppm. The average digester capacity was 0.14 million gallons/million gallons per day (MG/MGD) of raw wastewater. On average the raw sludge pumping rate was 4300 gal per MG of raw wastewater. The average digester gas production was 0.066 vol gas/vol of raw wastewater (8800 ft 3 per MG). The average unit gas production rate was 0. 64 volumes of digester gas per volume of digester per day. The average volume of digester gas produced per volume of raw sludge feed was 15. 8 v/v. The average gas production per pound of volatile solids added was 0. 46 m 3 kg −1 VS (7. 4 ft 3 lb −1 VS). The average gas production per pound of volatile solids destroyed was 0. 94 m 3 kg −1 VS (15. 0 ft 3 lb −1 VS).