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Biogas emissions from landfills are relatively well-known and regulated but in spite of the improvement of compulsory regulations regarding its collection in landfills and of the increasing performances of biogas collecting systems, a part of this biogas is still released without any treatment. These are fugitive emissions, diffusing through the landfill slopes and top covers. Due to the landfill wide-ranging typology, fugitive emissions are also highly heterogeneous in space, inside the landfill area, and in time, during the emissive landfill lifetime.

Until now, theoretical estimates were used to measure the fugitive emissions from landfills, with high uncertainties related to the assumptions made and the variability between the existing models. In order to achieve a real improvement in landfill sustainable performances and propose accurate remedial actions for the fugitive emission issue, an efficient monitoring is required. Therefore, landfill fugitive emissions metrology is the target issue to address since a clear quantification of this type of releases is a prerequisite to the total landfill biogas production knowledge and control.

Presently, several monitoring methods exist and provide a more or less accurate estimate of fugitive emissions fluxes from landfills but it appears that none serves as reference. Furthermore, very few comparison studies between methods have been undertaken in large scale. This study aims at evaluating the performance of five selected techniques (VRPM method, tracer gas method, differential absorption LiDAR method, micrometeorological method, flux chamber method) tested simultaneously on a real scale site and to determine which method(s) hold the most promise for further development/use. From the five techniques selected, all based on methane atmospheric dispersion assessment, three of them belong to surface emission factor methods category (VRPM method, micrometeorological method, flux chamber method) while the other two belong to the mass emission methods category (tracer gas method, differential absorption LiDAR method).

The study is divided in two main parts, each of them comprising specific objectives. The first part aimed at evaluating all five methods on a controlled methane release area to test their measurement accuracy and behavior towards a known amount of released methane flux. The second part was designed to test again the five methods but on real field conditions. The results obtained were compared in order to evaluate each method from a metrological, a technical and an economical performance point of view. For the field condition tests, two nearby landfill sites were selected, the WM-owned Metro Recycling and Disposal Facility and the Veolia-owned Emerald Park Landfill, both located in southeast Wisconsin. The measurement campaign took place from September 29th until October 12th of 2008, corresponding to a two weeks field measurement.

In terms of meteorological conditions during the campaign, the data shows that wind speed and temperatures were rather stable throughout the measurement operations, and rainfall and atmospheric pressure were subject to normal variations that had a limited impact on the levels of emissions across the campaign. This information was considered relevant as some of the applied techniques are sensitive to meteorological conditions.

For the controlled release test which occurred on October 7th, a calibrated amount of methane was released away from a chosen area, selected according to the wind conditions so that it would not interfere with the measurements by conveying methane from the landfill active areas nearby. For the test, four predetermined fluxes were released to the atmosphere at different rates and from three different sources. The teams conducting the measurements for each method were unaware of the release fluxes. Even so, they knew about the geometrical configuration of the release area and therefore, were free to achieve their most adapted equipment implementation. The large size and shape of the whole downwind test area enabled every team to perform their measurements under optimal conditions regarding their own requirements. Moreover, the test being conducted on the same day for all methods minimized potential differences induced by meteorological conditions.

For this controlled release test, the VRPM equipment measured three vertical planes, positioned at three different distances from the source (10, 50 and 100m). For the tracer gas method, concentration measurements were carried out on a perpendicular transect to wind direction, at a downwind optimal distance of about 10 times the source size (400m). The line of sight chosen for the DiAL measurement was positioned perpendicularly to the wind direction and two positions were used for the flux measurements (10m and 40m from the edge of the release area). As for the micrometeorological method, the team could not provide a complete measurement data for this test due to some difficulties in assessing the footprint with their model. Thereby, emission values were obtained but the emission area remained undefined. Finally, the flux chamber method was not applied in this preliminary test since it is an already well-known technique and it is not suited for global aerial measurements.

In terms of method results for this first test, the total flux released, to which each method measured flux can be compared to, is 7.1gCH4/s. The VRPM method measured 4.1 to 6.5gCH4/s. A systematic underestimation trend was noticeable. For the tracer gas method, the flux measured was 7.6gCH4/s, showing a systematic overestimation trend. As for the DiAL results, they ranged between 4.9 and 10.1gCH4/s, with no clear trend appearing. Since the VRPM equipment was positioned at three different distances, it was possible to conclude for this method that capture efficiency decreases rapidly as a function of the source distance. The DiAL team measured an abnormal flux during the third release, possibly caused by an intermittent external interference or an equipment malfunction. Thus, the final error for this release 3 was higher than the error obtained for the three other normal measurements. This phenomenon occurred only once and was only detected by the DiAL team. To conclude on this controlled release test, no clear conclusion could be drawn for the DiAL method and the VRPM method showed a large degree of variability and error. Finally, the tracer gas method presented the lowest amount of error and variability under the controlled release test conditions.

The second part of the study, dedicated to the assessment of total emission for each method, was conducted on both landfills selected for this study. In terms of percentage of landfill investigated, DiAL was able to assess the whole landfill in the two cases while the flux chamber method covered the less landfill surface in both cases (3 % investigated for the Metro LF and 1 % investigated for the Emerald Park LF). Thus, it seems difficult to grant a lot of credit to the extrapolated results from the flux chamber method. For the other three methods, a trend in the results was observed on both landfills: the lowest the surface area investigated, the higher the final result (VRPM measured the highest value based on the lower investigated area, followed by micrometeorological results, the tracer gas results and finally the DiAL results which were based on the whole landfills areas). Additionally, methods measuring mass fluxes provided lower flux values compared to methods measuring surface emission factors.

Finally, for Metro LF, the total fugitive emission estimates range from 290 to 1325kgCH4/h, with flux chamber method presenting the lower estimate and VRPM the higher one. For the Emerald Park LF, the total fugitive emission estimates range from 314 to 1244kgCH4/h, with DiAL method presenting the lower estimate and VRPM the higher one. Important disparities appear between all the results from each site when considering the whole landfill emission assessment.

Combining technical, economical and environmental criteria applied to each method, the final inter- comparison discussion indicates that tracer gas and DiAL methods appear to be the most promising approaches for landfill fugitive methane measurement. Considering the quantification objectives, a global assessment can be easily derived with tracer gas whereas a more comprehensive study of localized site specific performances can be undertaken with DiAL.