A full-scale investigation was conducted to evaluate the treatment performance and the bacterial system in an egg-shaped digester treating sewage sludge. The experiment was continued over one year to measure the methanogenic activities using various substrates and the population levels of various trophic groups of methanogenic bacteria, as well as the degradation of each component of organic matters. A total of 10 full-scale egg-shaped sludge digesters fed with concentrated sludge were investigated in this study. At a retention time of 30 days and mesophilic condition of 36 °C, the average reduction efficiencies of carbohydrates, proteins and lipids, which are the three main components in raw sludge, were 74.2%, 55.2% and 42.2%, respectively. As a result, the removal efficiencies of volatile solids (VS) and total CODcr reached 54.2% and 60%, respectively, and 1 m3 influent concentrated sludge produced 23.3 m3 digestion gas in which methane content was about 60%. The comparison between the specific methanogenic activity and the actual specific methane production rate in the digester suggestes that the retention time of the digester could be reduced from 30 days to 15 days without affecting their performance. In addition, both the hydrogen and acetate-consuming methanogenic bacteria were enumerated on the order of 107 MPN/ml.

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