Methane oxidation in a landfill cover with capillary barrier

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Abstract

The methane oxidation potential of a landfill cover with capillary barrier was investigated in an experimental plant (4.8 m × 0.8 m × 2.1 m). The cover soil consisted of two layers, a mixture of compost plus sand (0.3 m) over a layer of loamy sand (0.9 m). Four different climatic conditions (summer, winter, spring and fall) were simulated. In and outgoing fluxes were measured. Gas composition, temperature, humidity, matrix potential and gas pressure were monitored in two profiles.

CH4 oxidation rate within the investigated top cover ranged from 98% to 57%. The minimum was observed for a short time after irrigation. Temperature distribution, gas concentration profiles and lab-scaled batch experiments indicate that before irrigation the highest oxidising activity took place in a depth of about 30 cm. After irrigation the oxidising horizon seemed to migrate upwards since methanotrophic bacteria develop better there due to an adequate supply with oxygen.

It can be assumed that the absence of oxygen is one of the most important limiting factors for the CH4 oxidation process. Abrupt cross-overs between horizons of different soil material may lead to zones of increased water saturation and decreased soil respiration.

Introduction

The worldwide anthropogenic emission of the greenhouse gas methane is actually about 360 TgCH4/a (Wuebbles and Hayhoe, 2002). Compared with CO2, the global warming potential of CH4 is 21 times higher over a time horizon of 100 years. Thus, CH4 is the second largest contributor to global warming behind CO2 (EPA, 2001), although the absolute emission lies well below those of CO2. Atmospheric concentration of methane in pre-industrial time was estimated at 0.7 ppm, whereas that in 1998 was 1.745 ppm (IPCC, 2001). Waste dumping means an important source of anthropogenic CH4 emissions (EPA, 2002). Behind agriculture, waste landfills contributes to the worldwide anthropogenic CH4 emission with 17% (Wuebbles and Hayhoe, 2002). In Germany, landfills contribute with 24% of the total methane emissions, which is the second largest fraction under the antropogenic emission sources (UBA, 2002).

Methane coming out from landfills could be oxidised by methanotrophic bacteria. Those bacterias are ubiquitous and were already found in landfill soils (Dunfield et al., 1993, Fornés et al., 2003, Gebert and Kussmaul, 1998, Hanson and Hanson, 1996, Heyer, 1990, Higgins et al., 1981, Wise et al., 2001). The microbial oxidation of methane in landfills contributes to diminish the emissions of this gas in the atmosphere. As of 2005 German landfill regulations give limits for carbon content in deposed residues a pre-treatment of waste will be necessary before dumping (AbfAblV, 2001). In view of these changes the project focuses especially on landfills with a reduced CH4 emission potential. In order to investigate the CH4 reducing potential for such applications, the German Ministry for Education and Research (BMBF) launched the project “Technological development of a landfill cover with capillary barrier for methane oxidation”. This paper deals with the study of the methane oxidation potential in a experimental plant, working with a two-layers cover soil.

Capillary barriers for landfills and waste dumps are an alternative to conventional surface sealing systems. They consist of a layer of fine material (capillary layer) which is built above a layer of coarse material (capillary block) on a slope. The sealing effect is based on capillary forces and the principles of unsaturated flow (Kämpf et al., 1999). One of the objectives of this project was to examine the property of capillary barriers as gas distribution layers under different conditions. This part of the project is worked out by the Institute of Hydraulic Structures and Water Resources Engineering, Technical University of Darmstadt. First results are presented in Wawra and Holfelder (2003).

Section snippets

Experimental plant

The core of the experiments is formed by a 4.8 m × 0.8 m × 2.1 m testing reactor which is positioned in a slope in order to simulate a landfill cover. The plant consists of a capillary barrier at the bottom and a soil layer above which functions as the CH4 oxidising biofilter (Fig. 1).

The cover soil consists again of two layers: a 30 cm mix of compost and sand over a 90 cm layer of sand. The properties of the cover soil material used are shown in Table 1.

The experimental plant is placed in an

Methane oxidation

Fig. 2 shows methane degradation and the development of temperature in selected profiles (environmental chamber, cover soil in depths of 10 and 30 cm) over the time of examination. After a stabilisation phase the initial reference soil samples were taken on day 19 followed by the first climate phase simulating a warm and dry summer climate. In this phase, it lasted 3 weeks till CH4 oxidation leveled off to a degradation of 94–98%. On day 68 temperature was decreased to 10 °C (phase 2) which led

Conclusions and outlook

There are two basic demands on a landfill cover system in terms of gas and water management. Firstly, to prevent rainfall from infiltrating the waste body so that water cannot be a medium of transport and reaction for contaminants. Secondly, to collect and treat the landfill gas which contains the greenhouse gas methane. The technology of a landfill cover for CH4 oxidation in combination with a capillary barrier is supposed to perform these functions and could therefore be a cost-effective

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