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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°C. In run 1, combined primary sludge and TWAS (40/60%, volatile solids [VS] basis) were fed to digesters operated at mesophilic (35°C) and thermophilic (52°C) temperatures at loading rates of 0.99 and 1.46 g‐VS/L · 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°C and 52°C, respectively. The methane (CH 4) yields were 159 and 197 mL at the standard temperature and pressure (STP) conditions of 0°C and 1 atm/g‐VS added or 632 and 642 mL @ STP/g‐VS destroyed at 35°C and 52°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°C, and a loading rate of 52.5 g‐VS/L · d. The acid‐reactor effluent was fed to two parallel methane‐phase reactors operated at an HRT of 12 days and maintained at 35°C and 52°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 CH 4 @ 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°C and 52°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°C and 52°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.