Phytocapping our Landfills to Reduce Greenhouse Gases
We all know that our huge landfill sites produce tons of greenhouse gases, methane and carbon dioxide as the waste decays. Phytocapping is growing plants and trees on top of a landfill aimed at reducing the production and release of these gases. Legislative pressures throughout the developed world are to reduce landfill use, but in certain parts of the world it remains the most economical and simplest method of waste disposal.
Biodegradation of organic matter in a landfill site occurs most rapidly when water comes into contact with the buried waste, explains Kartik Venkatraman and Nanjappa Ashwath of the Department of Molecular and Life Sciences, Rockhampton, Australia.
They point out that conventional approaches to reducing this effect involve placing clay over the top of a landfill to form a cap that minimizes access of water to the landfill. The use of clay capping has generally proved ineffective in trials in America, the researchers say. The problem being that in arid regions the clay cap dries and the cracks allow water to access the landfill. Equally problematic, methane gas collection is an expensive option for many landfills that do not reach the methane production threshold to enable efficiency.
Along comes phytocapping, which involves placing a layer of top soil and growing dense vegetation on top of a landfill. Phytocapping was successfully tested at Rockhampton's Lakes Creek Landfill not far from Central Queensland University. Research was conducted by Kartik Venkatraman and Nanjappa Ashwath (CQU) in conjunction with the Rockhampton Regional Council and Phytolink Pty LTD.
Plants are established on an unconsolidated soil placed over the waste. The soil acts both as storage and sponge and the plants as biopumps and rainfall interceptors. For an effective site water balance, it is important that appropriate plant species are chosen and the soil depth optimized.
The team's studies of the benefits of a landfill phytocap show that the approach can reduce surface methane emission four to five times more than an unvegetated site. The team also looked at the effects of nineteen tree species, including acacias, figs, eucalyptus, and other native species, growing in the phytocap to determine which species are most effective at reducing water percolation and methane emissions. The root system acts as a good substrate to methanogens, which oxidizes methane thereby reducing methane emission into the atmosphere.
The benefits of phytocapping include, cutting in half the cost of landfill remediation and providing biodiversity corridors along which wild species can travel. The process also inverts the aesthetic qualities of landfills adjacent to urban communities, and in some cases, introduces economical benefits such as timber and fodder. "The establishment of phytocaps would offer an additional and economical way of reducing methane emission from landfills," the researchers conclude.
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