PILOT-SCALE AND FULL-SCALE 2-PHASE ANAEROBIC-DIGESTION OF MUNICIPAL SLUDGE

Two-phase anaerobic digestion tests were conducted at DuPage County, Ill., to ascertain if this process configuration would alleviate severe foaming and overloading conditions experienced by the existing high-rate digestion system. Pilot-scale two-phase digestion of activated sludge exhibited aggregated carbohydrate-protein-lipid and volatile solids (VS) reductions of more than 70%, and gas yield and gas production rate of 0.29 m(3)/kg VS . d and 2.2 volume per culture volume per day, respectively, at an hydraulic retention time 12 days and a loading rate of 4.7 kg VS/m(3) . d without any evidence of digester foaming; this performance was considerably better than that of high-rate digestion. Acid- and methane-phase digester culture development was completed in approximately a month. The acid-digester gases contained 67% CO2, 30% CH4, 2% nitrogen, 1% H2S compared with 30% CO2, 69% CH4, 0% N-2, and 0.1% H2S in the methane-digester gases. The methane digester generated 90 volume percent to 95 volume percent of the system methane production. An enrichment culture of fermentative acidogens was established in the acid-phase digester, whereas the methane digester harbored a dominant population of methanogenic organisms. The existing high-rate digestion system was converted to a two-phase digestion process.