Thermophilic/mesophilic digestion of sewage sludge and organic wastes

The performance of treating a mixture of sewage sludge and high strength food processing organic wastes was investigated by running a pilot-scale plant of two-stage anaerobic thermophilic/mesophilic digestion. Temperature and hydraulic detention time were controlled at 75 degrees C and 2.5 days for the first thermophilic digester and at 37 degrees C and 10 days for the second mesophilic digester, respectively. It was found that at the mesophilic stage, 16.8-53.2% of total solids (TS) and 33.8-57.6% of volatile solids (VS) were removed when the organic loadings were 6.62-16.32 kg VS/m(3)d and 1.65-2.92 kg VS/m(3)d for the first and second stages, respectively. Daily gas production was 10.02-23.73 m(3)/m(3) (equivalent to 1.511-1.861 m(3)/kg VS removed), 65-73% of which was methane in the mesophilic digester. Volatile fatty acids of 2600-9000 mg HAc/L produced at the first stage was removed down to 43-433 mg HAc/L in steady state mesophilic effluent. The ratio of volatile fatty acids to bicarbonate was less than 0.4 in mesophilic digester.This study showed that the proposed two-stage thermophilic/mesophilic digestion system was effective to treat sewage sludge combined with 20-50% potato processing concentrates, or 10% pig blood or 10% sausage processing floats.