Environmental and economic assessment of waste-to-energy treatment paths for organic waste

Abstract

Waste wood, food waste, the organic fraction of municipal solid waste (OFMSW) and sewage sludge represent the main organic waste streams, which are disposed of as waste in Germany. Given that reuse of these four waste streams is not applicable, the European Waste Framework Directive suggests recycling, recovery or ideally a combination of both. The respective treatment paths would return nutrients to the nutrient cycle (recycling) and/or exploit energy from the waste substrate (recovery). Within this work, organic waste was primarily considered as a resource for thermal and electrical energy production and the treatment paths were outlined as waste-to-energy systems. On a meta level, this work aimed at finding preferable treat- ment paths for the respective substrates. According to the mindset of a circular economy, a preferential treatment path is characterized by minimum impairments to the human health and the ecosystem quality, while considering the entire life cycle of the waste treatment. As a subordinate criterion, it must be ensured that the economic feasibility is not compromised. To quantify these aspects, the methodologies of environmental life cycle assessment (eLCA) and levelized costs of exergy (LCOE) were utilized. When outlining potential treatment paths for the four organic waste streams, a treatment according to five principal technologies was considered. Herein two conventional technologies, i.e., anaerobic digestion and incineration and three “alternative” technologies, i.e., hydrothermal carbonization (HTC), pyrolysis and gasification were comprised. The technologies were either considered individually or in a cascade. In total 35 potential treatment paths were designed, inventoried and quantified with regards to their environmental and economic burdens. The results showed that alternative treatment technologies did not hold the potential to reduce environmental or economic impacts considerably and conventional technologies stated suitable solutions to comply with the demand in a high-quality treatment of organic waste. For waste wood, incineration was an apt solution, for food waste anaerobic digestion or a combination of anaerobic digestion and incineration was preferable, for OFMSW, either anaerobic digestion or incineration were applicable and for sewage sludge, a combination of anaerobic digestion and incineration stated an acceptable treatment path. The economic feasibility was given for all solutions. Alternative treatment systems were only found to result in increased benefits for niche solutions.