Table 1. Municipal solid waste composition for different income levels.

Waste component Countries income High Medium Low

Organic Food waste 6-30 20-65 40-85 Paper and cardboard 20-45 8-30 1-10 Plastics 2-8 2-6 1-5 Textiles 2-6 2-10 1-5 Rubber and leather 0-2 1-4 1-5 Wood and yard waste 10-20 1-10 1-5 Miscellaneous organics < 1 < 1 < 1

Inorganic Glass 4-12 1-10 1-10 Tin cans 2-8 1-5 1-5 Aluminium 0-1 1-5 1-3 Other metals 1-4 1-5 1-5 Dirt, ash, etc. 0-10 1-30 1-40

Sanitary landfills are engineered disposal facilities, which make use of physical barriers designed to isolate solid waste from the biosphere in order to minimise public health and environmental impacts (Allen, 2001; Tchobanoglous and Kreith, 2002). These barriers (landfill bottom liners and caps) prevent the entry of water, into the waste bulk, which is essential for the degradation process of waste (Westlake, 1995; Allen, 2001). Consequently, the waste is contained and it remains practically intact for long periods of time. Nevertheless, water will always be present within the landfill due to the inherent water content of waste (Westlake, 1995; Allen, 2001). This water, together with previously infiltrated water (e.g. rainfall percolating during the filling process of the landfill), percolates and interacts with the waste and air trapped within the solid matrix, providing favourable conditions for microorganisms development. By their metabolic activities, microorganisms degrade waste until their nutrient sources are depleted and the residues are no longer capable of supporting microbial growth. This process is known as the biological degradation process of organic matter (Buswell and Mueller, 1952; Parkin and Owen, 1986; Zehnder 1988; Palmisano and Barlaz, 1996). This multi-sequential phase process produces gases and releases contaminant substances into the water that percolates through the landfill, resulting in a liquid called leachate. The duration of

these phases may vary depending on several factors (e.g. climatic and environmental conditions, waste characteristics, operational factors, etc.) and may last from decades to centuries (Wall and Zeiss, 1995; Townsend et al, 1996; Johannessen, 1999; Tchobanoglous and Kreith, 2002). Leachate and landfill gases contain substances that are harmful to human health and the environment (El-Fadel et al, 2002). The leachate escaping from landfills can contaminate soils, aquifers, and surface waters; landfill gas contains greenhouse gases that contribute to the global warming effect when they are released into the atmosphere.