Mesophilic and Thermophilic Anaerobic Digestion of Municipal Sludge and Fat, Oil, and Grease
Abstract: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.
Document Type: Research Article
Publication date: 2009-05-01
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Water Environment Research� (WER�) publishes peer-reviewed research papers, research notes, state-of-the-art and critical reviews on original, fundamental and applied research in all scientific and technical areas related to water quality, pollution control, and management. An annual Literature Review provides a review of published books and articles on water quality topics from the previous year. Published as: Sewage Works Journal, 1928 - 1949; Sewage and Industrial Wastes, 1950 - 1959; Journal Water Pollution Control Federation, 1959 - Oct 1989; Research Journal Water Pollution Control Federation, Nov 1989 - 1991; Water Environment Research, 1992 - present.
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