Improvements in Environmental Performance of Biogas Production from Municipal Solid Waste and Sewage Sludge

Abstract

Management of municipal solid waste is an efficient method to increase resource efficiency as well as to replace fossil fuels with renewable energy sources. This is due to that (1) waste to a large extent is renewable in itself as it contains food waste, paper, wood etc. and (2) when energy and materials are recovered from waste treatment, fossil fuels can be substituted. In this paper some of the results from a comprehensive system study of future waste management in the Gothenburg and Boras regions are presented. Emphasis is put on biological treatment of easy degradable waste such as food waste, by-products from food industry and sewage sludge. The project has been performed in cooperation between Kretsloppskontoret (The municipal office for waste and water management), Goteborg Energi (The energy company in the city of Gothenburg), Renova (The waste management company in the Gothenburg region), Gryaab (A water management company in Gothenburg) and researchers from Profu (Environmental and Energy Consultancy). Several treatment options for the organic waste have been investigated. Different collection and separation systems for food waste in households have been applied as well as technical improvements of the biogas process as to reduce environmental impact. The biogas replaces fossil fuels and the solid residue is pelletised and either used as fertiliser or as fuel. The method used is computer modelling with the ORWARE (Organic Waste Research) model for the waste management system. Deliverables from the model are environmental impact categories as developed within life cycle assessment and financial costs and revenues. The results show that central sorting of a mixed fraction into recyclables, combustibles, biowaste and inert is a competitive option compared to source separation. The result is however based on several crucial assumptions. Separation and utilisation of nitrogen in the wet part of the digestion residue is made possible with a number of technologies which decreases environmental impact drastically, however to a s ubstantial cost in some cases. There are several advantages with pelletisation of the solid digestion reside. Use of pellets is beneficial compared to direct spreading as fertiliser. Fuel pellets seem to be the most favourable option, which to a large extent depends on the circumstances in the energy system. Waste management integrated with local energy supply, wastewater treatment, agriculture and vehicle fuel supply is thus a cost efficient method to decrease greenhouse gases and promote the use of waste as a renewable fuel.