In order to intensify anaerobic stabilisation of sewage sludge, several pretreatment processes have been tested. Pretreatment processes investigated are: prepasteurisation, separate acidification, short-term anaerobic digestion. Anaerobic stabilisation was performed in 500 1 digestors. All experiments were performed in parallel to a process operated with untreated raw sludge, the later serving as a reference process. Prepasteurisation turned out to improve solids degradation and dewaterability of the digested sludge, whereas acidification was found to be a rather slow and uncomplete process due to the rate limiting function of hydrolysis of sludge particulates. Using a short-term anaerobic digestion process before anaerobic stabilisation, the easily degradable organics were readily converted to methane. It was shown that this two-stage process improves stabilisation efficiency in comparison to a 20 day one-stage process very significantly.

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