Blinding of a well: The entrainment of fines because of pumping, which results in the partial or complete blockage of the deliberately perforated or slotted areas of a gas-well pipe.

Cover thickness: The thickness of clay cover used to “cap” the landfilled refuse. Clay is relatively cheap and impermeable with respect to the transmission of water, compared with other cover materials. In general, the greater the thickness of the clay cover, provided that it has not dried out and cracked, the better the isolation of the lfg production process within the landfill site from external influences; this leads to less air contamination of the harnessed lfg, which is sucked out of the site.

Differential pressure: Either the positive-pressure increment above, or the suction-pressure increment below, atmospheric pressure.

Estimated mean static differential-pressure: The positive-pressure increment above atmospheric pressure within the non-pumped landfill.

Gas yield: The total yield of lfg over the lifetime of a landfill site.

Initial lfg reservoir: The landfill gas initially stored in the voids of the landfill, i.e. before any pumping ensued.

Leachate thickness to total tip depth ratio: Ratio of the depth of leachate to the total thickness of the horizontal refuse layer.

Long-term testing: In the present context, the continuous lfg-abstraction trials lasting for four or more weeks.

NFFO: An imposed by the Government upon the electricity-supply companies via the 1989 Electricity Act, whereby those companies must ensure that a predecided proportion of their electricity is generated using . The resulting levy, imposed on customers of mains-supplied electricity, is used to subsidise the installers of non-fossil-fuel electricity-generation plant during the early years of investment in the required plant, The percentage subsidy per unit of electricity supplied from a particular non-fossil-fuel source of energy (e.g. wind or refuse) decreases with succeeding tranches of the NFFO.

Perched level: The level of the local zone of leachate.

Pseudo steady-state: In the present context, this is the state in which no detectable changes occur during the period of a single day. (In practice, a refuse tip is never in a steady state because of the changes occurring in bacterial reactions, moisture content and temperature.)

Refuse-type ratio: Indicates the solid-inert content relative to the site’s total refuse input. The solid inert-content can be considered in terms of the domestic, commercial, industrial, civic amenity and actual delivered inputs.

Suction pressure: The pressure increment below that of atmospheric pressure.

Well-head suction: The mean suction difference between atmospheric pressure and the pressure at the well-head during the brief pumping trial.

Currently, more than 80% of domestic, commercial and hazardous wastes are landfilled in the UK.

Simultaneously, with acceptable empty landfill sites near major sources of refuse generation becoming fewer, the EU regulations governing the burial of refuse, (ii) the monitoring of sites and (iii) groundwater-quality regulations have become more stringent in the UK. Also the Environmental Protection Act (1990), which in part deals with lfg sites, is being implemented more rigorously. In addition, with the implementation of the Landfill Tax (which was introduced in October, 1996), landfill costs per tonne of refuse are set to rise rapidly. So it is not surprising that the current trends are towards the reduction, reuse and recovery of discarded materials, prior to final (usually landfilled) disposal. Even though considerable pre-treatment will be required for hazardous and special wastes, once a refuse “stream” has been created, ultimately, at present, there is no economically-viable alternative to landfill for the residual product.

The UK Government’s White Paper on waste outlines plans for implementing more environmentally-sustainable waste management systems. As the Secretaries of State for the Environment and for Wales point out, the main objects of the UK Government are, “to reduce the amount of waste that [the UK produces], to make the best use of that which is produced, and to adopt associated practices which (try to) minimise risks to the environment and to human health”. The White Paper also highlights the “strategy and targets for delivering these objectives”. The targets set are:

• To reduce the proportion of controlled waste (as defined in the Environmental Protection Act, 1990) going to landfill, to 60% by the year 2005.

• To recover value from at least 40% of municipal waste by 2005 AD.

• To set a target for overall waste-reduction by the end of 1998.

• A range of secondary targets relating to individual waste streams. These include recycling or composting 25% of household waste by the year 2000. This should be achieved by encouraging recycling and composting schemes, and hence lead to 4 further targets:

1. 40% of domestic properties with a garden to carry out composting by the year 2000;

2. all waste-disposal authorities to cost and consider the potential for centralised composting schemes by the end of 1997;

3. easily-accessible recycling facilities should be available for 80% of households by the year 2000; and

4. 1×10⁶ tonnes y^?1 of organic household waste is to be composted by 2001 AD.

Resulting as it does from the anaerobic digestion of landfilled refuse, lfg consists principally of methane (a potent greenhouse gas), carbon dioxide and approximately 170 other trace gases. The rate of lfg evolution from a landfill site depends on severity tractors, such as the age and composition of the landfilled refuse, its moisture content, the geology of the site, the leachate level, the temperature distribution within the landfill and the effectiveness of capping of the site. Properly engineered landfall sites, where the leachate is contained and the lfg emissions are harnessed, can be environmentally acceptable. The most modern sites have either plastic liners or layers of low-permeability clays, or both, to (i) prevent the escape of leachate and lfg and hence (ii) widespread contamination. If all the leachate (i.e. the liquid in the refuse site) were allowed to drain from the landfill, the methane-producing microbial activity there would be less, and so lower lfg yields would ensue. Thus, the depth of leachate for a site should be optimised in order to maximise the rate of lfg production, although the permitted depth is the subject of site licence conditions. The lfg extracted has been used for heating in boilers, furnaces and kilns, and also to fuel gas engines or turbines for the generation of electricity. The UK Government is firmly convinced that lfg is a profitable renewable energy-resource and as such has published Landfill-Gas Development Guidelines, which are designed to encourage the widespread harnessing of lfg.