Organic wastes of a city arising from food processing, food waste, sewage, packaging, paper, textiles, etc. can typically reach around It/person/yr in OECD countries. The large volumes involved, after collection, can be treated and linked to heat production and application in a variety of ways including separation and production of refuse derived fuels (RDF); combustion of the dry waste fraction; and anaerobic digestion of the wet wastes. Many examples exist of waste-to-energy projects in cities including MSW incineration (as in Vienna, where the architecturally designed, locally accepted, 270 000 t/yr plant at Spittelau in the centre of the city provides heat for the district heating scheme and publishes flue gas emission information on its web site daily50). Full details of the technologies (including incineration of sewage sludge) and related issues such as emissions to air can be found from the IEA Bioenergy task "Integrating energy recovery into solid waste management".51
The concept of minimising organic waste production by reducing, reusing and recycling helps a city save on the costs of collection and treatment of refuse. Future societal and technical changes in the production of waste and its conversion to energy need to be anticipated by a city when planning and designing future waste-to-energy projects, since the present volumes incinerated or sent to landfills may decline over time.
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