A full-scale investigation was conducted to evaluate the performance of an egg-shaped digestion tank in treating sewage sludge of high concentration. The experiment was continued over a half year at 36°C, and the influent concentration of sewage sludge to the digester was controlled at about 50 g/L (5%) with a centrifugal thickener. The total solids (TS) in the raw sludge contained 20% inorganic matters and 80% volatile solids (VS), which were further composed of carbohydrates (45.4%), proteins (34.8%), lipids (13.1%) and volatile fatty acids (about 9.2%). At the mesophilic egg-shaped digestion tank with the retention time of 30 days, the removal efficiencies of carbohydrates, lipids, proteins and VFA reached 75%, 50.6%, 46% and 100%, respectively, and the reduction efficiencies of TS, VS, SS, VSS were 44.2%, 56.6%, 46.6% and 61.9%, respectively. The result of gas production showed that 1 m3 of influent concentrated sludge could produce 21 m3 of digestion gas containing 61% of methane. Of all influent sludge COD, about 57% was converted to methane and as a result, 1 kg of influent VS produced 0.531m3 digestion gas. There was no noticeable scum and foaming problems in the egg-shaped digesters, although they have been continuously operated for over ten years. The metabolic activities and populations of individual trophic groups of bacteria in the digester were also elucidated in this study.

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