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In local cities with decentralized small wastewater treatment plants (WWTPs), centralized anaerobic digestion at a WWTP that accepts more sludge from others can serve as the energy hub of the region. In this system, transportation of sewage sludge with a high solid content by vehicles and the use of digested sludge as fertilizer are promising. The present study evaluated the effect of high solid content on anaerobic digestion and on the quality of the digested sludge as a fertilizer in a 1,100-day continuous laboratory experiment. Mixed sewage sludge with 10% total solids was subjected to mesophilic anaerobic digestion with a volatile solids (VS) removal ratio of approximately 0.6. The acceptable organic loading rate was lower than 3.2 kgVS/(m 3 ·d). The viscosity increased to 8.5 dPa·s, but computational fluid dynamics analysis confirmed that mixing need not be upgraded under conditions of high solid content. Component and eluted material analyses of the digested sludge as a fertilizer confirmed that the high solid content in the sewage sludge does not limit the use of digested sludge as a fertilizer according to the Japanese Fertilizer Control Act. The results showed that this centralized anaerobic digestion system is useful for sludge management in small facilities.Keywords: anaerobic digestion of high solid sewage sludge, computational fluid dynamics (CFD), fertilizer

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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.

Microwave pretreatment was applied to highly concentrated dewatered sludge produced from an oxidation ditch process (OD sludge) to improve its degradation and biogas production in mesophilic anaerobic digestion. In a batch experiment using methane fermentation, the biogas production potential of OD sludge was increased after microwave irradiation. The increase in the biogas potential depended on the irradiation period and the temperature. Continuous digestion of the pretreated OD sludge was carried out in mesophilic conditions using a laboratory-scale digester at 25 days of hydraulic retention time (HRT). Although the biogas yield in the control digester was 0.12 NL/g-total solids (TS), that fed with OD sludge pretreated with max 100 W of irradiation at 80°C for 60 min was 0.15 NL/g-TS. Subsequently, dewatered OD sludge was pretreated by a novel continuous microwave irradiation device at 400 W for 20 min and fed into a pilot-scale co-digester with six kinds of waste biomass. The biogas yield of the reactor was increased from 0.25 NL/g-TS to 0.28 NL/g-TS. Assuming that the biogas yield of each co-digesting biomass was constant, the biogas yield of the OD sludge was estimated to increase by 42%.