A continuous mesophilic co-digestion of sewage sludge and softened rice straw was conducted and the dewatering characteristics of digested sludge were evaluated by a dewatering experiment using a belt press. The digestion was operated with solid retention time (SRT) of 25 days, and the feeding ratio of sludge to rice straw was 1:0.5 (total solids base). After 129 days of stable operation, the properties of digested sludge were analysed; then five kinds of cationic coagulants were tested to select the optimal coagulants for dewatering, and two coagulants were selected and used in the dewatering experiment because of lower doses and lower moisture of sludge cakes. Sludge property analysis showed that by the addition of rice straw, the fibrous materials in the digested sludge increased remarkably and the normalized capillary suction time (CST) decreased significantly, indicating that the dewatering properties was improved. The results of dewatering experiment showed that by the addition of rice straw, specific filtration rate of digested sludge increased by 81.2% and 174.6%, respectively; water content of dewatered sludge cakes decreased by 8.2% and 13.4%, respectively. The dewaterability of digested sludge was suggested to be improved due to rice straw addition.

A continuous high solid co-digestion experiment of dewatered sludge from an oxidation ditch process and waste fried tofu was conducted at 55°C in the present study. Sludge retention time was 10 days, and the mixing ratio of sludge, dry and raw fried tofu was 1:0.39:0.06 (dry weight basis). Total solids (TS) of substrate was increased gradually (30, 50, 70, 100 g/L) in the sludge digestion, while TS was increased and then decreased in the co-digestion (43.5, 72.5, 101.5, 72.5 g/L). In the sludge digestion, performance was stable when the TS was below 70 g/L (organic loading rate (OLR) was 5.9 kg-volatile solids (VS)/(m 3 ·d)), and methane yields of sludge were 0.05-0.08 L/g-TS. In the co-digestion, biogas production significantly increased by the fried tofu addition. When total TS was increased to 101.5 g/L (OLR 8.8 kg-VS/(m 3 ·d)), accumulation of ammonia and volatile fatty acid was observed, inhibition was indicated. Methane fermentation failed to recover when the TS was decreased to 72.5 g/L (OLR 6.3 kg-VS/(m 3 ·d)). Results of the denaturing gradient gel electrophoresis analysis of the microbial community revealed a significant difference in the bacterial community with the fried tofu addition, while no major difference in the archaeal community was recognized.