Pulse Electric Field as a Pre-treatment Method of Wastewater Sludge prior to Anaerobic Digestion
Abstract: This work analyzed the impact caused by different intensities of Pulsed Electric Fields (PEF) pre-treatment on the biogas production from small-scale anaerobic digesters of wastewater sludge under thermophilic conditions. PEF technology consists of passing the substrate through an electric field which generates a potential difference across the cells, resulting in electroporation. Mixed sludge from Källby wastewater treatment plant was used and the biogas yield (Nm3/kg VS) obtained from the semi-continuous lab digesters with untreated sludge was similar to the yield registered by the plant’s full-scale version. Substrate submitted to higher intensities exhibited higher biogas yield improvements. The digestion of PEF-treated sludge reached values of biogas production up to 7.4% higher than values obtained from the digestion of untreated sludge with the same solids content. No relevant difference was seen in the methane content of the biogas produced by treated and untreated substrate. PEF treatment causes an increase in temperature in the substrate. That energy can be used to reduce the external heating requirements of the digester, contributing to offset the energy input required from the treatment. In this study, treatments with intensity around 70 kJ/kg were shown to increase the temperature of the sludge by around 18°C. Higher cumulative biogas production and VS degradation were achieved by increasing treatment intensity. Increases of 7.3% in cumulative biogas production and 7% in VS reduction were obtained from the anaerobic digestion of mixed primary and secondary sludge pre-treated with intensity of 95 kJ/kg. The increase in substrate temperature caused by the pre-treatment can decrease the energy required for heating the subsequent anaerobic digester and contribute to offsetting the energy required from the pre-treatment. The treatments conducted in this work only differed in treatment intensity. However, PEF treatment design consists of many other parameters. Thus, the relationship between these parameters and anaerobic digestion should be further investigated so that the true impact of Pulsed Electric Fields can be determined. Additionally, the performance of PEF varies from substrate to substrate, so methods to determine optimal parameters based on sludge type should be developed.
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