Two-stage, thermophilic-mesophilic anaerobic digestion of sewage sludge

Three configurations for dual anaerobic digestion were examined. Each configuration consisted of two completely stirred tank reactors (CSTRs), a 5 litre thermophilic digester linked to a 10 litre mesophilic digester. The first-stage thermophilic (55 degrees C) digesters were operated at 4, 8 and 12 hours retention time and these were used to provide the feed to each of the second-stage mesophilic (35 degrees C) digesters. These second-stage digesters were operated at hydraulic retention times of 10 days. A single-stage mesophilic digester (10 litre capacity), which was operated at a 20 day retention time, was used as the digestion bench mark. The results showed that dual digestion can be operated with retention times in the first-stage digesters much lower than suggested previously. Also, varying the retention time in the first-stage digester had little effect on the properties of the sludge produced. Although the volatile solids' (VS) reduction was not as great as reported by other workers, the specific methane yield (m(3) CH4 per kg VS removed) suggested that dual digestion had an advantage over conventional mesophile digestion.