A new technology, biocover, has been developed to reduce greenhouse gas emissions from landfill sites.
About ten per cent of global man-made emissions of the greenhouse gas methane originates from landfill sites. Landfills are therefore estimated to be the third-largest man-made contributor to methane emissions—which, it should be noted, has a greenhouse effect that is approx. 25 times more potent than CO2.
Denmark has around 2,500 landfill sites which continue to emit methane for many years after the depositing of organic waste in landfills ceases. So even though Denmark has not landfilled organic waste since 1997, methane is still emitted. However, the precise emission level is unknown.
Biocover
At some landfill sites, the gas is collected with a view to utilizing the energy generated, while other landfills burn it. This is often not possible, however, also because the concentration of methane in the gas is too low—and this is where biocover technology comes in.
"Biocover is an inexpensive and very interesting technology aimed at reducing total greenhouse gas emissions."
Tage Duer, Administrative Officer, Danish Energy Agency
For a number of years, researchers at DTU Environment have conducted experiments on the technology, which have shown that oxidizing large parts of methane is possible by placing bioactive layers on top of the waste.
Many old landfill sites are only covered by limited amounts of soil, which means the gas can easily be released. In brief, the biocover technology involves sealing the surfaces of old landfills to prevent methane from penetrating them. Instead—by means of a gas drainage system—the gas is distributed through so-called 'biowindows', which are most comparable to a compost bed. Here the natural microorganisms of the compost transform methane into CO2 (see graphics at the bottom).
Among other things, the researchers conducted experiments at a major Danish landfill site, where they reduced emissions from 10 kg of methane per hour to one kg per hour. Therefore, it is assumed that methane emissions can be reduced by 80-90 per cent at landfills that implement biocover.
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At the Klintholm I/S landfill site, the surfaces have been sealed using the biocover technology, preventing methane from leaking out. Instead, 'biowindows' (1) have been established whereby methane is distributed through a gas drainage (2). Finally, the 'window' (3) is covered by compost. The microorganisms of the compost ensure that the methane is transformed into CO2.
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Photos: Jan Michael Rasmussen |
This has prompted the Danish government to spend approx. DKK 185 million over a three-year period on a subsidy scheme supporting the establishment of 100 biocovers at closed Danish landfill sites. In total, biocover is expected to reduce the equivalent of 300,000 tonnes of CO2 by 2020.
Consulting engineers needed
Associate Professor Charlotte Scheutz, who, together with Professor Peter Kjeldsen from DTU Environment, is responsible for developing biocover, says that while developing the technology itself, a measuring method was devised to identify the landfill sites where biocover will be most effective—and to document the effect.
"Biocover must be adapted to each landfill site. Measurements carried out within limited areas of the landfill will not provide the full picture, as the gas may leak out across the entire area. Therefore, we have developed a method whereby tracer gas is released on the surface of the landfill site, which can be measured at some distance from the landfill. By comparing the measurements of methane and tracer gas, we can calculate the total emission of methane. It is a simple approach, but it requires highly advanced equipment. We have measured around 20 landfill sites—however, bottlenecks can arise if 100 biocovers are established and, e.g., four measurements are to be carried out on each," says Charlotte Scheutz.
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At the Klintholm landfill site on Funen, the leaking of the aggressive greenhouse gas methane has been reduced by 90 per cent after introducing biocover.
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Photo: Klintholm I/S
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The development continues
DTU Environment is currently conducting yet another experiment—this time at the AV Miljø landfill site in Hvidovre, where they are working on a more efficient method for distributing gas to the biowindows. The experience gained from this work will contribute to the development of biocover.
"It allows us to monitor the obtained results and identify possible improvements" says Professor Peter Kjeldsen, adding that the technology is not only relevant in a Danish context.
He estimates that up to 80 per cent of all global waste is landfilled. In Europe, the figure is 40-60 per cent. Denmark and countries such as the Netherlands are at the top of the class in this area, only depositing a tiny percentage of the waste in landfills. Therefore, there is wide scope for biocover outside of Denmark.
Illustration Pernille Bech Larsen
Article from DYNAMO no. 41, DTU's quarterly magazine in Danish.