Rainwater that falls onto the working areas of the Landfill soaks into the refuse and becomes part of the leachate that is collected and drained from the Landfill. However, 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 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, 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.
Leachate is the liquid created by a combination of decomposition of organic material in the Landfill and some rainfall. Leachate runs through the waste to a drainage blanket on top of the Landfill liner. 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, where the organic matter in the leachate settles. From these tanks the leachate can either be used for surface irrigation on top of the Landfill, or can be injected back into the waste stockpile, where the rubbish may absorb some of it. This process can also enhance the decomposition process that creates Landfill gas for electricity generation. If volumes get too great within the Landfill, it is sent for treatment at the Bromley sewage treatment plant in Christchurch.
Decomposing organic material in the Landfill produces gas, predominantly methane. To prevent methane, which is a greenhouse gas, escaping into the environment it is collected by a network of pipes and used as fuel to drive generators to produce electricity for the national power grid at the Gareth James Energy Park. At the end of 2014 two generators were installed producing about 2MW of electricity. Two more generators installed in June 2019 increased the electricity production to 3MW. Any surplus gas is destroyed in a high temperature flare.
You can download our Renewable Energy booklet here.