REFUSE-DERIVED FUEL
RDF is the product of the treatment of MSW to create a fuel that can be burned easily in a combustion boiler. To produce RDF, waste must first be shredded and then carefully sorted to remove all noncombustible material such as glass, metal, and stone. Shredding and separating is carried out using a series of mechanical processes that are energy intensive. The World Bank has estimated that it requires 80–100 kWh to process one tonne of MSW and a further 110– 130 kWh to dry the waste.
After the waste has been shredded and separated, the combustible portion is formed into pellets that can be sold as fuel. The original intention of this process was to generate a fuel suitable for mixing with coal in coal-fired power plants. This, however, led to system problems and the modern strategy is to burn the fuel in specially designed power plants. An alternative is to mix the RDF with biomass waste and then burn the mixture in a power plant. Since RDF production must be preceded by careful sorting, this type of procedure is best suited to situations where extensive recycling is planned.
ENVIRONMENTAL ISSUES
Urban waste, its production, and its fate are major environmental issues. As already noted, modern urban living produces enormous quantities of waste in the form of paper, plastic, metals, and glass, as well as organic materials. How these wastes are processed is a matter of increasing global concern. Wastes such as paper, glass, and metal can be recycled, as—in theory—can plastics. From an environmental perspective it makes sense to reuse as much waste as possible, so environmentalists generally favor maximum recycling. Many governments now promote recycling too. However, the economics of recycling are not clear-cut and there are critics who consider it economically ineffective. Since such debates pitch sustainability against economics, the issue is not easily resolved.
While recycling offers the ideal solution, in practice there are frequently neither the facilities nor the infrastructure to recycle effectively. Even where recycling is employed there is still a residue of waste that cannot be reused. Thus, there remains a considerable volume of waste for which an alternative means of disposal is required. The only options currently available are burial in a landfill site or combustion.
The combustion of waste would seem initially to be the ideal solution. Combustion reduces the quantity of waste to 10% or less of its original volume. At the same time it produces energy as a by-product and this energy can be used to generate electricity, heating, or both. Unfortunately, waste often contains traces of undesirable substances that may emerge into the atmosphere as a result of combustion. Other hazardous products may result from the combustion itself, with the waste providing the chemical precursors. So, while solving one environmental problem, waste combustion can generate others.
In the face of this, the combustion of waste has become subject to strict legislation. This sets limits on amounts of different hazardous materials that can be released as a result of the process. Chief among these are heavy metals, such as mercury, cadmium, and others, and potent organic compounds, such as dioxins and furans.
Modern WTE plants aim to meet the regulatory requirements imposed on them. In spite of this they are often extremely unpopular and even when the technology appears capable of limiting emissions to extremely low levels, it can be difficult gain permission to build plants. New waste conversion technologies such as gasification and pyrolysis may be able to help overcome popular objections if they can be shown to generate negligible toxic emissions.