Establishing the outputs of urban waste products is a key issue in quantifying urban materials flows. Many estimates of per capita waste production are found in the literature, but it is unclear whether such figures apply to total waste volumes or to domestic, or domestic and municipal, waste only. In total, domestic and municipal waste accounts for only 1.5 per cent of the estimated 10 billion MMT of waste produced annually in the USA. Of this waste, 75 per cent is related to the environmental 'rucksack' and other hidden flows of mining and oil and gas production, 9.5 per cent to industry, 13 per cent to agriculture and 1 per cent to sewage sludge (Miller 2000).
Generally, waste statistics are not totally reliable. For example, in the UK, despite increased regulation of waste flows, data on the volumes involved are still inaccurate and those available only apply to licensed waste disposal sites and do not take account of exempted waste disposal schemes and the, probably not inconsiderable, amount of uncontrolled, unofficial illegal dumping, known locally as 'fly-tipping', throughout the country. Controlled waste data from UK government sources and the Environment Agency of England, Wales and Northern Ireland will almost certainly underestimate the total waste volume. However, in terms of urban materials budgets, this underestimate may be counterbalanced by some possible double accounting in terms of construction waste movements, with a proportion of such waste being taken to landfill and this becoming part of both the materials movement during construction and the flow of waste materials.
Throughout the world per capita municipal and industrial solid waste generation continues to increase. However, the per capita figures vary widely from city to city, depending largely on average wealth. In Abidjian Africa in 1994, 200kg of waste per head were produced, while at the same time, Washington, DC produced 1246kg per capita (World Resources Institute 1996). In many cities, particularly in the poorer parts of the world, much solid waste is not collected properly, and accumulates in piles in streets or between dwellings and factories. Few effective inventories of the total waste produced can be made in such cities. However, sometimes artisanal or informal waste collection, or 'rag picking', can recycle large volumes of usable materials that would be dumped in more affluent cities.
Reclamation of paper and cardboard by small entrepreneurs is highly organized in cities as diverse as Nairobi, Calcutta, Cairo and Beijing. Recycling is increasing in many countries, but much more remains to be done (see Chapter 44). In Denmark, taxes on many types of solid wastes have increased recycling to over 61 per cent of household waste generation. In the UK, landfill and aggregate taxes have been introduced to encourage recycling of potential wastes but only some 8 per cent of household waste was being recycled or composted in 1998 (UK Department of the Environment, Transport and the Regions 1999). Some 53.6MMT of C&D waste are produced annually, but only some 5MMT per year of such materials are re-used for civil engineering and building construction. The potential, however, is not always matched by practical feasibility, within the context of both current standards/specifications and geographical location, where low price and the cost of transport to areas of substantial demand discourages use of these waste materials. The government aims to increase the use of secondary aggregates to 40MMT per year by 2001 and to 55MMT per year by 2006 (UK Department of the Environment 1994a). However, variations in composition are a major problem. While UK demolition waste comprises approximately 41 per cent by weight concrete, 24 per cent masonry, 17 per cent paper, cardboard and plastic and ceramics, metals and other materials, 15 per cent asphalt and 3 per cent wood-based products (Hobbs and Collins 1997; CIRIA 1997), construction waste may contain 45 per cent soil and other active surface materials (UK Department of the Environment 1994b).
Estimates of total amounts of demolition and construction waste produced in England and Wales vary (Howard Humphries and Partners 1994; Symonds Group Ltd. 1999). Of the approximately 53.6MMT per year:
• 27.4MMT per year are deposited in landfill. Perhaps 20 per cent of this material is employed in engineering works on site (haul and access roads, construction of cells, cover and so on);
• 21.2MMT per year are exempt from licensed disposal and are used in unprocessed form or coarsely crushed for use in demolition/construction sites and for sale/disposal off site for land modeling during the construction of projects such as golf courses and equestrian centers;
• 5MMT per year only comprise material which is either crushed to produce a graded product or is directly recovered.
In addition, material scraped from the surface of bituminous road pavements produces around 7.5MMT of material per annum and much of this finds its way to secondary uses as capping layers, public footpaths or haul/access road construction (UK Department of the Environment 1994b).
The destinations of urban waste are changing. Much legislation encourages recyling and discourages landfill. In Europe, the traditional dumping of sewage sludge in the sea ceased at the end of 1998 as a result of a European Community Directive. Some sludge is now converted into energy by incineration. Other sludge is spread on agricultural land while some ends up in landfill (Priestley 1998). Such changes in waste flow paths alter the destinations of substances contained in sludge, such as heavy metals like cadmium which may eventually find their way back into the food chain (see Chapter 33).
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