Marios Soutsos (Greg Keefe, Queen's University, Belfast)
The past decade has witnessed increasing interest in the study of urban metabolism. There are two related, non-conflicting, schools of urban metabolism: one describes metabolism in terms of energy equivalents; while the second more broadly expresses a city's flows of water, materials and nutrients in terms of mass fluxes. An "urban metabolism" needs to consider the inflows of material and energy resources, the outflows of wastes and emissions and the retention of materials as stock in the built environment and infrastructure. A study of the environmental and resource implications of the metabolism of a city requires a full life cycle analysis of all the flows of materials within or outside the city boundary.
It is common that waste management considers different forms of solid waste arising from urban areas, i.e. construction and demolition waste (C&DW), commercial and industrial waste (C&IW), and, domestic – municipal solid waste (MSW). Municipal solid waste and commercial and industrial waste represent materials with very short service lives. The flow of waste for final disposal can be reduced by reducing consumption, or by re-using or recycling materials from the waste. Re-use and recycling are, for example, particularly appropriate for packaging waste. However, much material is recovered for recycling from municipal solid waste and commercial and industrial waste, there will inevitably be a proportion which cannot be recycled due to contamination or degradation during reprocessing. While new technologies may reduce damage and contamination and thereby increase the proportion of material recycled with environmental and economic advantage, it is an inevitable that total recycling will be impossible. The material that must be "purged" from recycling loops includes mixed or contaminated plastics, paper and card and also the organic fraction, mainly food waste. Organic waste can be composted to reduce its volume, but demand for compost is limited. Energy recovery therefore represents the most beneficial use of the residual waste. Comparison of energy-from-waste technologies on a life cycle basis shows that the efficiency of energy recovery is the most significant consideration, with material recovery of secondary concern.
Recovering energy from waste, whether by thermal treatment or anaerobic digestion, avoids the emissions of methane (with its much higher greenhouse warming potential compared to carbon dioxide) which arise when degradable waste is landfilled and also offsets greenhouse gas emissions from use of fossil fuels to provide electricity and/or heat. Furthermore, organic waste is classified as "renewable" because its carbon content is mainly part of the renewable carbon cycle, so that the carbon dioxide emissions from using organic waste as an energy source do not contribute to the greenhouse gas balance. These elementary considerations underline the importance of treating waste flows as integral parts of the urban metabolism. The contribution of energy-from-waste to greenhouse gas mitigation can be significant, even though the proportion of energy consumption which can be met from waste is not large.
Roland Clift, Angela Druckman, Ian Christie, Christopher Kennedy & James Keirstead, "Urban metabolism: a review in the UK context", September 2015, Future of cities: working paper, Foresight, Government Office for Science, London, UK.