Energy Efficient Combination of Sewage Sludge Treatment and Hygenization After Mesophilic Digestion–Pilot Study

Abstract

Biogas production is probably the most feasible way of utilizing sewage sludge as energy source, simultaneously with nutrient recovery by recycling the biogas digestate (i.e. the residue) to arable land. However, the sludge commonly contains high amounts of human pathogenic bacteria excreted in faeces and urine. To use sludge as fertilizer on food producing land is therefore a controversial issue, partly because of the risk of spreading disease-causing pathogens. The Swedish environment protection agency (SEPA) pre-approved two hygenization methods for the treatment of the sludge due to their positive effects on the sludge quality. One of them, conventional pasteurization (70°C, 1h), was investigated for its feasibility in Uppsala, Sweden, and it was found that the heat consumption was very high. The other method has the advantage of potentially increase the produced biogas. This hygenization method has been investigated in the present study through a pilot experiment where thickened mesophilic digested sludge is digested once more at thermophilic conditions (55°C). The aim of the study was to investigate the possibility to develop this self-sufficient (in heat and electricity) hygenization method. The results showed an increase in the gas production from 430 dm3/kg VSin to 610 dm3/kg VSin by adding the thermophilic step. This increase gave an energy balance with an excess of both heat and electricity. Sludge hygenization was sufficient with the method and another important result is the significant decrease digestate volume.