An inexpensive way to reduce greenhouse-active methane emissions from existing Municipal Solid Waste (MSW) landfills is to exploit the natural process of microbial methane oxidation through improved landfill cover design. Landfill top covers, which optimise environmental conditions for methanotrophic bacteria and enhance biotic methane consumption, are often called ‘biocovers’ and function as vast bio-filters. Biocovers are typically spread over an entire landfill area. They are often waste materials, such as diverse composts, mechanically-biologically treated waste, dewatered sewage sludge or yard waste.
Methane oxidation in compost materials shows high oxidation capacity. Manipulation of landfill covers to maximise their oxidation capacity provides a promising complementary strategy for controlling methane emissions.
Simple but well-engineered biocovers can mitigate methane emissions from landfills. Mature composts show higher microbial methane consumption relative to conventional landfill soil, which can most probably be related to nutritional factors provided by the compost or to changes in the microbial ecology. Physical factors such as the increased porosity, water-holding capacity, or thermal insulation properties of compost seem to be responsible for much of the observed positive effects.
The minimum recommended thickness of a final compost cover to mitigate methane emissions on bioreactor landfills is 1.2m in the construction phase for climatic conditions in middle Europe (Huber-Humer and Lechner, 2008). Bogner et al. (2005) tried to determine a minimum biocover design made of recycled materials capable of mitigating methane emissions in subtropical environments.
The function of biocovers and their long-term durability and bio-active lifetime will reduce the rate of methane emission from MSW landfills. However, the temperature, moisture, gas fluxes and gas ratio may influence the role of compost cover for the mitigation of methane emissions.
Landfills exist all over the world. While soil is probably the most used landfill cover, a Google search reveals numerous reports of tests underway to determine the best approaches to utilise biocovers at specific sites in order to reduce GHG emissions. A lot of site-specific research needs to be done worldwide to determine availability of suitable materials, the thickness of material to apply, longevity and so on.
- Optimised and well-adapted biocovers are relatively less expensive in terms of operation and installation compared to a conventional gas collection system, whose cost can be high compared to the value of the captured fuel.
- These biocovers have low maintenance requirements and they can be maintained by a relatively untrained person. Thus, they are suitable for both high and low income countries.
- Biocovers need to be designed and modified for local, landfill site-specific conditions.
- Landfill chambers require homogeneity of gas fluxes and special cover material properties which still require significant research and development efforts.
- Due to the shifting of methane oxidation layer downwards, the mats get clogged due to microbiallyproduced biomass and therefore much labour is required to sweep the chamber’s basal gravel layer. However, the sweeping of the chamber basal gravel layer can be done by relatively untrained persons.
Future CH4 emission scenarios indicate rising shares of MSW and coal bed methane (CBM), where mitigation technologies have good penetration potential.
Optimised and well-adapted biocovers are relatively less expensive in terms of operation and installation compared to a conventional gas collection system, whose cost can be high compared to the value of the captured fuel.
Bogner J., K. Spokas, J. Chanton, D. Powelson, J. Fleiger, and T. Abichou, (2005): Modeling landfill Methane Emissions from Biocovers: A combined theoretical-empirical Approach. Proceedings Sardinia `05 – Tenth International Waste Management and Landfill Symposium, 3 – 7 October 2005, CISA, Cagliari, Italy.
Huber-Humer M. and Lechner P. (2008): Impact of Different Biocover Designs on Methane Mitigation. In: Pawlowska M., Pawlowski L. (Eds.): Management of Pollutant Emission from Landfills and Sludge. Selected Papers from the international workshop on Management of Pollutant Emission From Landfills and Sludge, Kazimierz Dolny, Polen, 16 - 19 September, 2006. Taylor and Francis Group, London, UK; ISBN13: 978-0- 415-43337-2 (hbk), 978-0-203-93218-6 (ebook); pp. 21-36.