For operational purposes the site is divided into a number of cells, typically covering an area of between 1 and 2 hectares. The base and sides of each cell are lined with an engineered layer to contain the landfilled waste and any degradation products – landfill leachate generated by rainwater falling on the waste and landfill gas produced as the organic content of the waste degrades over time. At Westmill II the lining system comprises dense asphaltic concrete, underlain by a layer of clay and overlain by a drainage blanket of granular material.
Waste is deposited progressively in each lined cell, typically in a sequence of layers and lifts moving around the area of the cell. Infrastructure is installed to enable the leachate to be removed for treatment and disposal to sewer, and for landfill gas to be collected and used to generate renewable energy. Following infilling to final levels, plus an agreed allowance for settlement, the waste is capped with an engineered layer and restored with soils and planted. Following restoration, the site enters a period of aftercare extending for a period of at least 30 years.
Monitoring of the environment around the site, including ground and surface water, soil and air is conducted according to a plan agreed with the Environment Agency, the Government body responsible for ensuring that the site is operated in compliance with all relevant environmental requirements.
In addition to the engineered containment and capping systems described earlier, a number of other environmental control systems are employed to mitigate or reduce any adverse environmental impacts arising from the operation of the site. These systems are designed to control, but are not limited to:
- Dust
- Mud and debris
- Litter
- Flies
- Vermin
- Odour
