Landfill sites are used to dispose of a wide range of waste materials from domestic and industrial sources which contain organic materials. The composition of munici­pal solid waste (domestic) is given in Table 5.1. As the landfill is sealed, the conditions within the site soon become anaerobic, producing biogas. The stages of anaerobic degradation of the organic content are probably the same as for sewage sludge. In older landfill sites, the biogas generated was allowed to escape but at present the biogas is captured and used. In 2001 it was reported that there were 955 landfill sites globally where the gas was recovered.

The composition of municipal solid waste is very likely to change over time as recycling and reuse of waste continues to increase. In the UK, the composting of gar­den waste and the recycling of metals, glass and plastics along with degradable plastic will reduce the organic composition of landfill waste. The cost of landfill is also rising which will also reduce the amounts of waste which are placed in landfills.

In the construction of landfill sites an impermeable barrier to stop any leachate reaching the groundwater is required. The most suitable sites are abandoned quarries and opencast sites preferably with a non-porous substratum. The site is lined with clay, plastics and rubber and once sealed the site can be filled in cells or terraces (Fig. 5.6), each cell or terrace being covered with soil after compaction at the end of each day. The compaction reduces the amount of air trapped, helping anaerobic conditions form and avoiding spontaneous combustion by reducing oxygen content. In order to collect the biogas generated during construction, permeable horizontal trenches or perforated pipes are incorporated into the terraces and cells (Fig. 5.7). Once a landfill has reached its working level it is capped with clay, a drainage layer and soil. The drainage layer stops rainwater entering the landfill, reducing the leachate formed. Once capped the organic material in the landfill will begin to degrade and although aerobic at the start, oxygen will be soon exhausted and conditions will become anaerobic.

The biogas collected will, like that produced from sewage sludge, have about half of the calorific vale of natural gas because of the presence of carbon dioxide and the composition will vary depending on the waste composition in the landfill. The gas is normally extracted 1-2 years after capping of the site and at best yields 100 m3 gas

per tonne of waste. The total extracted is only 25% of the possible yield because of the slow rate of gas formation and migration within the site. Figure 5.8 shows the changes in the landfill gases as the site develops and the organic material is degraded. A typical timescale of gas production from a landfill site is given in Fig. 5.9.

Biogas from Anaerobic Digestion of Agricultural Wastes

Small anaerobic digesters have been installed on farms to treat excess animal slurries which cannot be placed on the land. The biogas formed is normally used for heating but can also be used in dual-fuel engines.

In a number of developing countries, such as China and India, both animal and human wastes are anaerobically digested on site. The biogas formed is used as a low pressure source of gas for domestic use.