Landfill - How it Works
Mixed waste is brought to our Waterbeach site in refuse collection vehicles or skip lorries and is dropped off in a purpose built transfer building, so that none of these vehicles need to go onto the landfill itself. Our own dumper trucks take this waste to the tipping face in the landfill cell and a heavy compactor vehicle (weighing nearly 50 tonnes) drives over the waste crushing it down to ensure we get as much in the cell as possible. At the end of each working day, the waste deposited that day must be covered with material (called ‘day cover’) to help reduce odours, litter and vermin. We reuse the reject material from our In-Vessel Composter or low quality soils generated on site that otherwise would have gone to waste as day cover.
Once the cell is full and at Waterbeach we typically dig cells that take a year to 18 months to fill, the landfill has to be covered (capped) with a metre of clay, to stop rainwater getting in and to control landfill gas escape and a metre of soil, which we seed with grass. Another small benefit is that Kimmeridge clay is a marine clay formed in the Jurassic period and we recover a range of fossils, particularly ammonites, when we dig new cells.
LANDFILL – RENEWABLE ENERGY
However, this is not the end of the story. The waste we receive for landfill contains a lot of organic material, mostly food waste (which attracts rats and seagulls) and paper and cardboard. This material rots and in our compost system would produce a useful material. Unfortunately in a landfill site, because the waste is crushed and constantly covered, very little air is available so normal composting bacteria cannot break this organic matter down, but bacteria that can live in these anaerobic conditions (no oxygen) do break down this waste. A major by-product of this anaerobic bacterial action is methane (which is produced in the form of landfill gas, mostly a mixture of methane and carbon dioxide) which is flammable and a very potent green house gas (23 times more potent than carbon dioxide).
This can make the aftercare of landfill sites difficult, and in the past, when less was known about the green house effect, the methane was either vented into the atmosphere or burnt with a flare. However, at Waterbeach we now capture the landfill gas, including methane, produced within the landfill and use the gas to produce renewable electricity. As the landfill is being filled with waste, we lay pipes across the waste to capture the gas. These pipes go to a special combustion unit, provided and operated by Summerleaze that burns the methane and uses the heat to generate electricity. The Waterbeach site produces enough renewable electricity to power 3,000 homes. As the methane is burnt it creates a negative pressure in the pipes, which ‘sucks’ more gas out of the landfill.
LANDFILL RECLAMATION AND MBT PLANT
Completed landfills are not particularly suitable for building or agriculture (except grazing), due to the waste under the soil cap and the gases produced, but because the topsoil remains undisturbed it can be a haven for wildlife. The restored landfill at Waterbeach is home to a variety of plants and animals including bee orchids, barn owls, tawny owls, skylarks and the brown hare. Two County Wildlife Site ditches run through the site, attracting kingfishers and dragonflies and we have a Habitat Creation & Monitoring Plan for the site.
Before the new MBT plant opened all unsorted household waste in Cambridgeshire (black bin/sack waste for those in Cambridge City, East and South Cambridgeshire and Huntingdonshire; green bin waste for those in Fenland) went to landfill, along with unsorted business waste that was brought to our site: we used to landfill over 160,000 tonnes of waste a year. The MBT plant is completing commissioning, but when it is fully operational all unsorted household waste will go through the MBT plant, where materials still in waste, like cans, glass and plastics are removed for recycling and organic material, like kitchen waste and cardboard, is composted. In addition AmeyCespa (East) has an extensive recycling system for the mixed skip waste we collect and for Building Waste brought to our site and we extract metals, timber, soil and brick and concrete rubble for recycling from this waste. So now we send a much smaller proportion of the waste that comes to our Waterbeach site to landfill, which has huge environmental and economic benefits.