Peter Kjeldsen

Mitigation of methane emission from Fakse landfill using a biowindow system

Landfills are significant sources of atmospheric methane (CH(4)) that contributes to climate change, and therefore there is a need to reduce CH(4) emissions from landfills. A promising cost efficient technology is to integrate compost into landfill covers (so-called biocovers) to enhance biological oxidation of CH(4). A full scale biocover system to reduce CH(4) emissions was installed at Fakse landfill, Denmark using composted yard waste as active material supporting CH(4) oxidation. Ten biowindows with a total area of 5000 m(2) were integrated into the existing cover at the 12 ha site. To increase CH(4) load to the biowindows, leachate wells were capped, and clay was added to slopes at the site. Point measurements using flux chambers suggested in most cases that almost all CH(4) was oxidized, but more detailed studies on emissions from the site after installation of the biocover as well as measurements of total CH(4) emissions showed that a significant portion of the emission quantified in the baseline study continued unabated from the site. Total emission measurements suggested a reduction in CH(4) emission of approximately 28% at the end of the one year monitoring period. This was supported by analysis of stable carbon isotopes which showed an increase in oxidation efficiency from 16% to 41%. The project documented that integrating approaches such a whole landfill emission measurements using tracer techniques or stable carbon isotope measurements of ambient air samples are needed to document CH(4) mitigation efficiencies of biocover systems. The study also revealed that there still exist several challenges to better optimize the functionality. The most important challenges are to control gas flow and evenly distribute the gas into the biocovers.

Biodegradation of methane and halocarbons in simulated landfill biocover systems containing compost materials.

The attenuation potential of methane (CH(4)) and of selected volatile organic compounds (VOCs) was compared in four types of compost materials using dynamic flow column experiments over a period of 255 d. Garden waste compost mixed with wood chips showed the highest steady-state CH(4) oxidation rate (161 g m(-2) d(-1)), followed by a commercial compost product Supermuld (110 g m(-2) d(-1)). In the column containing the highest fraction of compost (compost/sand mixed in 1:1), CH(4) oxidation declined significantly during the period of operation, probably due to clogging by formation of exopolymeric substances. After 40 d of operation, CH(4) production was observed. All the VOCs tested were degraded. CFC-11 (CCl(3)F) and HCFC-21 (CCl(2)FH) were anaerobically degraded by reductive dechlorination, generating HCFC-31 (CClFH(2)) and HFC-41 (CFH(3)), which were both aerobically degraded in the oxic portion of the columns. Overall, the highest removal of VOCs was observed in the column containing the compost/wood chip mixture. This study demonstrates that biocovers consisting of compost materials have the potential to attenuate trace gas emissions from landfills.

Availability and properties of materials for the Fakse Landfill biocover

Methane produced in landfills can be oxidized in landfill covers made of compost; often called biocovers. Compost materials originating from seven different sources were characterized to determine their methane-oxidizing capacity and suitability for use in a full-scale biocover at Fakse Landfill in Denmark. Methane oxidation rates were determined in batch incubations. Based on material availability, characteristics, and the results of batch incubations, five of the seven materials were selected for further testing in column incubations. Three of the best performing materials showed comparable average methane oxidation rates: screened garden waste compost, sewage sludge compost, and an unscreened 4-year old garden waste compost (120, 112, and 108 gm(-2)d(-1), respectively). On the basis of these results, material availability and cost, the unscreened garden waste compost was determined to be the optimal material for the biocover. Comparing the results to criteria given in the literature it was found that the C/N ratio was the best indicator of the methane oxidation capacity of compost materials. The results of this work indicate that batch incubations measuring methane oxidation rates offer a low-cost and effective method for comparing compost sources for suitability of use in landfill biocovers.

Evaluation of respiration in compost landfill biocovers intended for methane oxidation

A low-cost alternative approach to reduce landfill gas (LFG) emissions is to integrate compost into the landfill cover design in order to establish a biocover that is optimized for biological oxidation of methane (CH(4)). A laboratory and field investigation was performed to quantify respiration in an experimental compost biocover in terms of oxygen (O(2)) consumption and carbon dioxide (CO(2)) production and emission rates. O(2) consumption and CO(2) production rates were measured in batch and column experiments containing compost sampled from a landfill biowindow at Fakse landfill in Denmark. Column gas concentration profiles were compared to field measurements. Column studies simulating compost respiration in the biowindow showed average CO(2) production and O(2) consumption rates of 107 ± 14 gm(-2)d(-1) and 63 ± 12 gm(-2)d(-1), respectively. Gas profiles from the columns showed elevated CO(2) concentrations throughout the compost layer, and CO(2) concentrations exceeded 20% at a depth of 40 cm below the surface of the biowindow. Overall, the results showed that respiration of compost material placed in biowindows might generate significant CO(2) emissions. In landfill compost covers, methanotrophs carrying out CH(4) oxidation will compete for O(2) with other aerobic microorganisms. If the compost is not mature, a significant portion of the O(2) diffusing into the compost layer will be consumed by non-methanotrophs, thereby limiting CH(4) oxidation. The results of this study however also suggest that the consumption of O(2) in the compost due to aerobic respiration might increase over time as a result of the accumulation of biomass in the compost after prolonged exposure to CH(4).