Mitigation of methane emissions in a pilot-scale biocover system at the AV Miljø Landfill, Denmark: 1. System design and gas distribution

Abstract

Greenhouse gas mitigation at landfills by methane oxidation in engineered biocover systems is believed to be a cost effective technology, but so far a full quantitative evaluation of the efficiency of the technology in full scale has only been carried out in a few cases. A third generation semi-passive biocover system was constructed at the AV Miljø Landfill, Denmark. The biocover system was fed by landfill gas pumped out of three leachate collection wells. An innovative gas distribution system was used to overcome the commonly observed surface emission hot spot areas resulting from an uneven gas distribution to the active methane oxidation layer, leading to areas with methane overloading. Performed screening of methane and carbon dioxide surface concentrations, as well as flux measurement using a flux chamber at the surface of the biocover, showed homogenous distributions indicating an even gas distribution. This was supported by results from a tracer gas test where the compound HFC-134a was added to the gas inlet over an adequately long time period to obtain tracer gas stationarity in the whole biocover system. Studies of the tracer gas movement within the biocover system showed a very even gas distribution in gas probes installed in the gas distribution layer. Also the flux of tracer gas out of the biocover surface, as measured by flux chamber technique, showed a spatially even distribution. Installed probes logging the temperature and moisture content of the methane oxidation layer at different depths showed elevated temperatures in the layer with temperature differences to the ambient temperature in the range of 25-50°C at the deepest measuring point due to the microbial processes occurring in the layer. The moisture measurements showed that infiltrating precipitation was efficiently drained away from the methane oxidation layer.

Mitigation of methane emissions in a pilot-scale biocover system at the AV Miljø Landfill, Denmark: 2. Methane oxidation

“…In both experiments a composite compost sample (obtained by mixing equal amounts of compost from all sampled depths; -20 cm, -40 cm, -60 cm, -80 cm, -90 cm) were incubated at eight different temperatures (4°C, 10°C, 15°C, 20°C, 30°C, 40°C, 55°C, 70°C) with the purpose of analyzing the temperature dependence of CH 4 oxidation and respiration of a standard compost sample. The tested temperature range was comparable to the temperature range measured in the biocover (3 to 60°C) (Cassini et al, 2017). The moisture content on the incubated compost sample was 0.4 g g DM -1 , thus the samples were sufficiently moist, so moisture was not limiting CH 4 oxidation and temperature was the main controlling factor.…”

“…The first composite sample consisted of a mix of compost from five depths (-20 cm, -40 cm, -60 cm, -80 cm, -90 cm), whereas the second composite sample consisted of compost from three depths (-20 cm, -40 cm, -60 cm). The sampling depth of 40 cm was chosen as gas concentration profiles and temperature measurements in the biocover indicated optimal conditions for CH 4 oxidation at this depth (see section 3.3 and Cassini et al, 2017).…”

“…A tracer gas (HFC-134a) was injected during several measuring campaigns performed at AV Miljø biocover to calculate the CH 4 oxidation efficiency of the bacterial community (see section 2.4) and to study the gas distribution quality of the system (confer Cassini et al, 2017 for more details on the tracer experiment and gas distribution quality). In order to study the possible inhibitory effect of the tracer gas on the microbial methane oxidation activity, two laboratory experiments were carried out.…”

“…Two transects each having 50 gas ports were constructed at two different distances from the gas inlet. More details of the biocover system design are given in Cassini et al (2017).…”

“…This paper is the second in a series of two. The first paper presents the design of the biocover system and investigations on the gas distribution in the biocover system (Cassini et al, 2017).…”