Improved dewatering by hydrothermal conversion of sludge

Abstract

A growing demand for renewable energy resources, together with increasing criticism on the competition with food crop production caused by current production methods, has stimulated new interest for fuel production from sewage sludge. One of the processes to recover fuel from sludge is hydrothermal conversion (HTC), whereby the wet sludge undergoes a high temperature/high pressure treatment, with the possible addition of catalysts. In this study, the influence of subcritical HTC treatment with regard to sludge dewaterability was investigated. A range of different temperatures (150-240 degrees C), pH values (3-11), and peroxide additions (0-150% of COD) were studied. It was shown that after hydrothermal treatment of sludge or predewatered sludge cake and subsequent mechanical dewatering, dry matter contents of up to 60% could be obtained, and that the cake mass after HTC decreased by a factor of 5 on average, compared to the untreated sludge. The HTC process also resulted in an enrichment of the inorganic matter in the final cakes, by solubilisation of the organic matter into an easily biodegradable form. The energy demand of the HTC process was low compared to drying by evaporation and might be partially provided by biofuels generated in the process, according to energetic calculations.