Environmental Effects of Sewage Sludge Carbonization and Other Treatment Alternatives

Carbonization is a newly developed process that converts sewage sludge to biocoal, a type of solid biomass that can partially substitute for coal during power generation. This study presents an assessment of the environmental effects of various sewage sludge treatment processes, including carbonization, direct landfills, co-incineration with municipal solid waste, and mono-incineration in Taiwan. This assessment was conducted using the life cycle assessment software SimaPro 7.2 and the IMPACT2002+ model. Results show that carbonization is the best approach for sewage sludge treatment, followed in descending order by co-incineration with municipal solid waste, direct landfills, and mono-incineration. The carbonization process has noticeable positive effects in the environmental impact categories of terrestrial ecotoxicity, aquatic ecotoxicity, land occupation, ionizing radiation, aquatic eutrophication, non-renewable energy, and mineral extraction. For the emission quantity of greenhouse gases, landfilling has the greatest impact (296.9 kg CO2 eq./t sludge), followed by mono-incineration (232.2 kg CO2 eq./t sludge) and carbonization (146.1 kg CO2 eq./t sludge). Co-incineration with municipal solid waste has the benefit of reducing green house gas emission (-15.4 kg CO2 eq./t sludge). In the aspect of energy recovery, sewerage sludge that has been pretreated by thickening, digestion, and dewatering still retains a high moisture content, and thus requires a significant amount of energy use when used as a substitute solid fuel. Therefore, the carbonization of sewage sludge would be a more sustainable option if the energy delivery and integration processes are made more efficient.