Life cycle assessment of sewage sludge carbonization
Date Issued
2012
Date
2012
Author(s)
Wang, Ning-Yi
Abstract
Sewage sludge production in Taiwan is rapidly increasing as a result of continuous population growth. Wastewater treatment plants are estimated to generate 1040 ton/d of dewatered sludge by 2014. Therefore, evaluating currently available dewatered sludge management practices is critical for reducing the negative impacts of sludge management and disposal. Currently, landfill and co-incineration with municipal solid waste are the 2 most common approaches for managing dewatered sludge from wastewater treatment plants in Taiwan. However, the scarcity of available land for landfill and the prevention of secondary pollution as a result of incineration are pressing problems without a satisfactory solution at this moment. One recently proposed method that is increasingly receiving the attention of researchers is carbonization, because it not only recycles sewage sludge but also reduces greenhouse gas emissions. Carbonization is a newly developed process that converts sewage sludge to biocoal which is a type of solid biomass that can be used to partially substitute coal for power generation. Carbonization offers the advantages of reducing sludge volume, exploiting its thermal content, increasing sludge grindability, removing odor, reducing N2O emission and easy handling. However, during the carbonization process, the input and output of energy and materials may create some adverse environmental impacts. Therefore, the main objectives of this study were to compare and evaluate the application of carbonization, landfill, co-incineration and mono-incineration processes for sewage sludge treatment in Taiwan regarding various environmental impacts. The life cycle assessment software SimaPro 7.2 was used and the IMPACT2002+ methodology was applied to evaluate functional unit of 1 ton municipal sewage sludge.
Noticeable positive results regarding terrestrial ecotoxicity, aquatic ecotoxicity, land occupation, ionizing radiation, aquatic eutrophication, non-renewable energy and mineral extraction were observed when co-firing of biocoal and coal was applied for power generation. Overall, carbonization is the optimal treatment approach followed by co-incineration, landfill and mono-incineration. Although carbonization is the ideal solution for sludge treatment, even after sludge pre-treatment (condensation, digestion and dewatering), the sludge still contains high content of moisture, thus requiring a significant amount of energy consumption. Therefore, carbonization of sewage sludge could be more effective once the energy delivery efficiency is enhanced. Moreover, it is imperative for policy makers to consider environmental impacts and evaluate the environmental feasibility of bioenergy recovery.
Subjects
Sewage sludge
carbonization
life cycle assessment
co-firing
bioenergy
Type
thesis
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