Combined Anaerobic and Aerobic Digestion: Effect on Nitrogen, Odors and Sludge Dewatering

Recent research in our laboratory has focused on combined anaerobic/aerobic digestion. There are many advantages to combined anaerobic/aerobic digestion and these include better solids reduction, improved sludge dewatering properties and reduction of nitrogen. In this study, an additional anaerobic digestion step was provided. Solids from the aerobic digester were thickened, the centrate discharged and the concentrated solids recycled back to the anaerobic unit. This is called anaerobic/aerobic/anaerobic digestion. The addition of the recycled sludge does not increase the reactor size but does require a separate recycle step. Combined anaerobic/aerobic/anaerobic digestion resulted in additional solids reduction, from 50% for conventional digestion to 62% for anaerobic/aerobic and 70% for anaerobic/aerobic/anaerobic.Total Kjeldahl nitrogen (TKN) removal was greater than 60% for both the anaerobic/aerobic and anaerobic/aerobic/anaerobic digesters. The combined anaerobic/aerobic and anaerobic/aerobic/anaerobic also resulted in lower polymer conditioning requirements compared to anaerobic digestion alone and lower organic sulfur odors were generated from the biosolids digested under the combined systems.