Strategic environmental assessment of alternative sewage sludge management scenarios

Strategic environmental assessment (SEA) of sewage sludge management in a Danish municipality (Aalborg), with 160,000 inhabitants using alternative methods for aggregation of environmental impacts was performed. The purpose is to demonstrate the use of SEA in relation to sludge management and to improve SEA methodology. Six different scenarios for management of sewage sludge within the Aalborg municipality involving thermal treatment, composting and landfilling of sludge were evaluated. Environmental impact categories considered were global warming, non-renewable resources (nutrients and fossil fuels) and land use. Impact categories human health, ecotoxicity and soil quality were excluded as methodology for their assessment is not yet fully developed. Thermal sludge treatment with energy utilisation was shown to be a promising option for sewage sludge management in Aalborg. Sensitivity of the relative environmental impacts with respect to calculation methodology and input parameter values were evaluated to identify important parameters and calculation methods. The analysis showed that aggregation procedures, sludge bio-gas potential and sludge production were very important whereas sludge transport was not.