Mesophilic and thermophilic anaerobic digestion of municipal sludge and fat, oil, and grease.

The anaerobic biodegradability of municipal primary sludge, thickened waste activated sludge (TWAS), and fat, oil, and grease (FOG) was assessed using semi-continuous-feed, laboratory-scale anaerobic digesters and compared with the ultimate degradability obtained from 120-day batch digestion at 35 degrees C. In run 1, combined primary sludge and TWAS (40/60%, volatile solids [VS] basis) were fed to digesters operated at mesophilic (35 degrees C) and thermophilic (52 degrees C) temperatures at loading rates of 0.99 and 1.46 g-VS/L x d for primary sludge and TWAS, respectively, and a hydraulic retention time (HRT) of 12 days. The volatile solids destruction values were 25.3 and 30.7% (69 and 83% biodegradable volatile solids destruction) at 35 degrees C and 52 degrees C, respectively. The methane (CH4) yields were 159 and 197 mL at the standard temperature and pressure (STP) conditions of 0 degree C and 1 atm/g-VS added or 632 and 642 mL @ STP/g-VS destroyed at 35 degrees C and 52 degrees C, respectively. In run 2, a mix of primary sludge, TWAS, and FOG (21/31/48%, volatile solids basis) was fed to an acid digester operated at a 1-day HRT, at 35 degrees C, and a loading rate of 52.5 g-VS/L x d. The acid-reactor effluent was fed to two parallel methane-phase reactors operated at an HRT of 12 days and maintained at 35 degrees C and 52 degrees C, respectively. After an initial period of 20 days with near-zero gas production in the acid reactor, biogas production increased and stabilized to approximately 2 mL CH4 @ STP/g-VS added, corresponding to a volatile solids destruction of 0.4%. The acid-phase reactor achieved a 43% decrease in nonsaturated fat and a 16, 26, and 20% increase of soluble COD, volatile fatty acids, and ammonia, respectively. The methane-phase volatile solids destruction values in run 2 were 45 and 51% (85 and 97% biodegradable volatile solids destruction) at 35 degrees C and 52 degrees C, respectively. The methane yields for the methane-phase reactors were 473 and 551 mL @ STP/g-VS added, which is approximately 3 times larger compared with run 1, or 1040 and 1083 mL @ STP/g-VS destroyed, at 35 degrees C and 52 degrees C, respectively. The results indicate that, when co-digesting municipal sludge and FOG, a large FOG organic load fraction could have a profound effect on the methane gas yield.

Methane recovery from the anaerobic codigestion of municipal sludge and FOG.

The anaerobic biodegradability of a mix of municipal primary sludge (PS), thickened waste activated sludge (TWAS) and fat, oil, and grease (FOG) was assessed using semi-continuous feed, laboratory-scale anaerobic digesters operated at mesophilic (35 degrees C) and thermophilic (52 degrees C) temperature. Addition of a large FOG fraction (48% of the total VS load) to a PS+TWAS mix, resulted in 2.95 times larger methane yield, 152 vs. 449 mL methane @ STP/g VS added at 35 degrees C and 2.6 times larger methane yield, 197 vs. 512 mL methane @ STP/g VS added at 52 degrees C. The high FOG organic load fraction was not inhibitory to the process. The results of this study demonstrate the benefit of sludge and FOG codigestion.

The anaerobic biodegradability of municipal sludge and fat, oil, and grease at mesophilic conditions.

The anaerobic biodegradability of municipal primary and secondary sludge with increasing levels of partially dewatered fat, oil, and grease (FOG) was assessed using a mixed methanogenic culture at 35 "C. Under batch conditions with an acclimated and enriched microbial population, the sludge loading was 3 kg volatile solids/m3 and the highest FOG loading tested was 1.5 kg volatile solids/m3, resulting in a methane yield of 245 mL methane/g sludge volatile solids added at 35 degrees C and 1010 mL methane/g FOG volatile solids added at 35 degrees C. Under semicontinuous feeding conditions, the sludge and sludge plus FOG loading tested were 3 and 3.75 kg volatile solids/m3-d, respectively. Within 23 days of operation, the volatile fatty acid concentrations were reduced below 200 mg chemical oxygen demand/L (187 mg/L as acetic acid). Enhancement of sludge digestion was observed in those reactors where codigestion of sludge and FOG took place, which was attributed to a higher level of microbial activity maintained in these reactors as a result of FOG degradation. The results of this study demonstrate that beneficial use of FOG through codigestion with municipal sludge is feasible.