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Methane oxidation

In the summer of 2009 Afvalzorg has constructed two test fields for research of methane oxidation in cover soil. The research is carried out with support of Agentschap NL(programme Reduction of Other Greenhouse Gases), the Institute of Soil Science of the University of Hamburg and the Technical University of Hamburg-Harburg in the framework of the MiMethox project (www.mimethox.de). 

Methane oxidation 
Methane oxidation is the process through which soil microorganisms in a cover soil degrade methane from landfill gas into carbon dioxide. It is well known that methane is an important greenhouse gas. At some point it is no longer feasible to extract and utilise or flare landfill gas actively. In that situation a "passive"approach to neutralise the remaining methane emission of the landfill is required. With a methane oxidising cover soil a significant reduction of the greenhouse gas emission of landfills can be realised. Insufficient knowledge on the effectiveness of methane oxidation hampers inclusion in green house gas accounting. The research contributes to recognition of methane oxidation as suitable option to reduce methane emission on landfills. In addition, knowledge is generated on the water balance in a cover soil and the optimisation of water storage of a cover soil. 

Conditions 
Catchment for water and gas injection tubeIn order to ‘evenly feed’ the soil microorganisms it is important to evenly distribute the landfill gas over the entire surface. Two aspects are very important. First of all a gas distribution layer is required. Secondly the soil needs to have a sufficient porosity for even gas transport through the soil. This imposes requirements on the composition and the construction of the cover soil. The soil needs to have a relatively high percentage of sand and it should not be compacted. The pore volume available for gas transport is strongly influenced by moisture. Rainwater has to flow down through the same pores as the gas has to flow up. Too much moisture means that the gas cannot find an even way out. Gas pressure increases and the gas will move through a "preferential pathway". The speed and the amount of landfill gas will be too high to give the microorganisms an opportunity to degrade the methane. An uncontrolled methane emission will occur. It is important to find the required pore volume that enables both adequate methane oxidation and adequate water holding capacity and water discharge under most of the conditions that can occur during the year. The research therefore addresses both methane oxidation and water balance.

Test fields
Construction of cover soil with long-stick excavatorThe test fields are constructed on a membrane with sidewalls. This enables catchment and measurement of water flow through the cover soil. The membrane blocks the ‘natural gas supply’ from the landfill. A gas distribution unit has been installed to supply landfill gas to the test fields. This enables measurement of incoming and outgoing gas flows and establishment of the oxidation efficiency. Two test fields have been constructed with a different technique. One test field is constructed with a long-stick excavator. Thus a very "fluffy" cover soil was constructed. The other test field is constructed in a more traditional way with a bulldozer.

Results after two years 
In the first year a methane load to the cover soil of 0.25 liter methane per m2 and hour was imposed. In both testfields no methane emissions could be observed. Methane was not detectable in the soil profile except very deep. In the second year the methane load to the cover soil was increased to 1 liter methane per m2 and hour. After a short period of "habituation" methane again could only be observed at significant depth in the soil profile. 94 - 97% of the injected methane was oxidised. At most Afvalzorg landfills gas generation is less then 1 liter per m2 and hour. Teh project can already be called succesful. The testfield built with the long-stick excavator did slightly better than the testfield built with the bulldozer. In july 2011 the methane load to the cover soil was increased to 2.5 liter methane per m2 and hour.

3D differential GPS measurements on ‘low soil pressure’ cloggs
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