Pilot‐ and full‐scale two‐phase anaerobic digestion of municipal sludge

Abstract

Two‐phase anaerobic digestion tests were conducted at DuPage County, III., 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% CO 2 , 30% CH 4 , 2% nitrogen, 1% H 2 S compared with 30% CO 2 , 69% CH 4 , 0% N 2 , and 0.1% H 2 S 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.