title: 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 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.
authors.affiliation: Carollo Engineers, Sarasota, Florida 34232, USA. jkabouris@carollo.com
authors.forename: John C
authors.initials: JC
authors.lastname: Kabouris
authors.forename: Ulas
authors.initials: U
authors.lastname: Tezel
authors.forename: Spyros G
authors.initials: SG
authors.lastname: Pavlostathis
authors.forename: Michael
authors.initials: M
authors.lastname: Engelmann
authors.forename: James A
authors.initials: JA
authors.lastname: Dulaney
authors.forename: Allen C
authors.initials: AC
authors.lastname: Todd
authors.forename: Robert A
authors.initials: RA
authors.lastname: Gillette
mesh_terms: D001421:Bacteria, Anaerobic / Q000378:metabolism*; D001673:Biodegradation, Environmental; D019149:Bioreactors; D005223:Fats / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D009821:Oils / Q000378:metabolism*; D012722:Sewage / Q000737:chemistry / Q000382:microbiology*
chemical_list: D005223:Fats; D009821:Oils; D012722:Sewage