Experiences with the dual digestion of municipal sewage sludge

The dual digestion process was investigated using sludge samples collected at the WWTP of Tomaszow Mazowiecki (Poland). Mixed sludge was treated in a laboratory setup under batch and semi-continuous conditions. Dual digestion with a 1 d SRT aerobic thermophilic pretreatment followed by an anaerobic step with 20 d of SRT turned out to be optimal, since a 44–46% VS reduction and a biogas yield of 480 dm3/kg VS fed were achieved. In the course of the process, the concentration of nitrogen in supernatant increased over 5 times and its major portion was converted into ammonia. Phosphorus also entered the supernatant, reaching over 200 g/m3. The dual digestion noticeably deteriorated the sludge dewaterability. Following completion of the process, capillary suction time measurements averaged 64 s for the raw sludge, 400 s for aerobically pretreated sludge and 310–360 for the anaerobically digested sludge. Aerobic pretreatment consistently reduced Enterobacteriaceae content to below detectable limits.