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

Three configurations for dual anaerobic digestion were examined. Each confi guration consisted of two completely stirred tank reactors (CSTRs), a 5 lit re thermophilic digester linked to a 10 litre mesophilic digester. The firs t-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 diges ters were operated at hydraulic retention times of 10 days. A single-stage mesophilic digester (10 litre capacity), which was operated at a 20 day ret ention time, was used as the digestion bench mark. The results showed that dual digestion can be operated with retention times in the first-stage dige sters much lower than suggested previously. Also, varying the retention tim e in the first-stage digester had little effect on the properties of the sl udge produced. Although the volatile solids' (VS) reduction was not as grea t as reported by other workers, the specific methane yield (m(3) CH4 per kg VS removed) suggested that dual digestion had an advantage over convention al mesophile digestion.