Water management
Efforts are made to keep rain and waste separate. Most of the rainfall on the site is channeled through a series of surface drains to a sedimentation pond about 300 metres downstream in the Kate Valley. Here the soil sediments carried by the rainwater settle before the water is released to flow down to the water supply pond about half a kilometre down stream. The water in the supply pond is used for dust control around the Landfill, as a fire fighting water supply source if required, and to maintain a year round water flow into the Kate Stream that supplies water to the Kate Pond wetlands in the adjacent Tiromoana Bush. The existing natural wetlands have been greatly enhanced as a result of the development of the landfill. By managing the water flow year round the wetlands have grown to 12 hectares in size and now support a wide variety of birdlife, including rare species such as the spotless crane.
Only rainwater that soaks in to the landfill, mostly on the working areas of the Landfill, becomes part of the leachate, that is collected and drained from the Landfill.
Leachate
Leachate is the liquid created by decomposition of organic material in the Landfill, along with additional water that enters the land, such as rainfall. Leachate runs through the waste to a drainage blanket on top of the Landfill liner that seals the base of the Landfill. From there the leachate flows through a system of collection pipes to a sump. At the sump the leachate is pumped out into storage tanks.
From these tanks the leachate can either be used for surface irrigation (sun-powered evaporation) on top of the Landfill, injected back into the waste stockpile, where the rubbish absorbs some of the leachate, or evaporated by an onsite BeneVap. The BeneVap is a landfill gas powered boiler utilised to evaporate 90% of the leachate, largely the water component.
The concentrated leachate created by the BeneVap can be reinjected into the Landfill. If volumes get too great within the Landfill, the excess leachate can be stored in large onsite tanks, two of which are 250,000 litres each, or trucked to the Bromley Waste Water Treatment Plant in Christchurch.
Capturing GHG emissions
Transwaste engaged Toitū Envirocare to audit the Greenhouse Gas (GHG) emissions from the waste and operations at the Landfill. The audits, of the years to June 2022 and June 2023, showed an increase in the capture and destruction of methane from just under 90% in the first year to over 95% in the second year of audit. Anything above 90% capture and destruction is considered world's best practice.
At the landfill, as the waste is deposited and compacted, a series of gas extraction wells is installed and connected to a network of collection pipes. The organic material in the waste decays with minimal oxygen (the Landfill being an anaerobic digester) over several years generating Landfill gas, which includes a significant proportion of the high energy gas methane. By applying a modest vacuum to the pipe network, the Landfill gas is extracted in a controlled way rather than escaping as uncontrolled GHG emissions.
This Landfill gas is piped to the Gareth James Energy Park and because of its significant methane content is used as a fuel to run internal combustion engines to produce electricity, enough to power more than 2,000 homes last year. The volume of methane being captured and destroyed at the Landfill is equivalent to avoiding the emissions impact of some 120,000 cars per year.
The Landfill will continue generating electricity for several decades after it is closed. At the same time the carbon contained within the large plastics fraction of the waste will remain locked in the Landfill.