Yields of anaerobic digestion (AD) of sewage sludge carried out at mesophilic and thermophilic conditions at the optimum hydraulic retention times (HRT) found for the semi-dry AD of the mechanically selected organic fraction of municipal solid waste (OFMSW) (15 and 12 days, respectively) are compared. Due to the characteristics of the sludge used in thermophilic conditions (partially stabilized) the specific biogas production was lower. However, stability parameters show a secure process. Based on experience gained digesting OFMSW and hypothesizing a similar ratio between yields at mesophilic and thermophilic conditions, an economic analysis has been carried out. It shows that the increased biogas production is large enough to compensate the additional consumption to heat the feed flow rate to 55°C instead to 35°C.

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