Determination of methane and carbon dioxide surface emissions in a Spanish landfill

Abstract

Fugitive emissions from the surface of landfills constitute an important source of carbon dioxide and methane that are emitted to the atmosphere and contribute to the greenhouse effects. Quantifying those emissions can give valuable information, not only, on the amount of those gases which are released into the atmosphere but on their environmental consequences. They can also help to evaluate another aspects related to the landfill management like the seal efficiency or the gas recovery.

Quantifying these emissions is not an easy task, due to the heterogeneity of the waste into the landfills, their huge dimensions and the influence of others factors like the site meteorology. Several measurement methods as well as prediction models for determine this type of emissions can be found in the bibliography. In this study a process in two steps, as described on the Guidance on monitoring landfill gas surface emissions (EA 2010), was followed in order to obtain the fugitive emissions of two landfill sites in Madrid (Spain). The first step consists on a walkover survey. At this stage the gas concentration close to the surface is monitored and zones with equal concentrations are defined. At the same time, significant emission sources and failures in the gas collection system are identified. All this information is used to choose the sample points for the second step, the flux box study. A flux box or flux chamber is a well known volume dispositive that allows to accumulate and to quantify over time emissions emanating from a given area of the landfill. To measure the composition of methane inside the flux box, a portable detector based on Infrared (IR) Absorption Spectroscopy (Sensit PMD) in combination with an electronic narrow band pass filter technology is being used. Carbon dioxide emissions were obtained using a passive IR sensor for CO2 (Kimo HQ210). Some preliminary results show the possibility of quantifying emissions rates from less than 1 mg/m2h to 104 mg/m2h or even higher. From a number of measures of this type in the selected emissions points the total amount of the surface emission of the landfill could be estimated.

This method has been used in two landfills. The first one is a closed landfill that has been exploited for almost 40 years and was closed in 2000. In this case the landfill gas extracted is being used to generate electricity. The other one corresponds to a landfill which is still in use and the extracted gas is being burnt in a flare. Another critical difference between the two landfills is the type of cover used. In the closed landfill there is a high density polyethylene film while in the other one there is only a clay and sand barrier as cover. The walkover survey results show lower concentrations of methane on the closed landfill. This is an evidence of the effectiveness of a good seal combined with the active extraction and use of the gas to reduce methane release.