International Review of Waste Management Policy - Department of ...

LQM did not consider the amount of energy generated from LFG within their analysis,
although they estimated the total flaring back-up capacity to be around 60% of
generation capacity. It is usual for landfill operators to maximise energy generation as
this represents a revenue stream. We assume within the current analysis that 40% of
the recovered gas will be flared. Although it is acknowledged that there is some
uncertainty here, the impact of this uncertainty (in terms of CO2 equivalent offsets
associated with energy generation from landfill) is relatively small.
63.2.1.4 Oxidation of Landfill Gas
Some of the uncaptured landfill gas will be oxidised as it passes through the cap to
the surface, the proportion being dependent upon the nature of the cap. The USEPA
suggests a range of 10% to 25%, with clay soils at the lower end of the range and topsoils
being at the higher end. This reflects a figure proposed by Brown et al in 1999 in
a study on behalf of what was then the DETR. 1224 A similar value was proposed by the
IPCC.
However, a recently published review of the wider literature on this subject suggests
that the mean fraction of methane oxidised was 36% (an average across 42 studies
taken in a variety of locations). 1225 We have retained the 10% figure assumed by the
IPCC and USEPA, but acknowledge that this is likely to overestimate fugitive
emissions of methane occurring from landfill in many cases.
63.2.1.5 Landfill of Pre-treated Waste
Under the very low fluxes of landfill gas assumed to occur when pre-treated wastes
are landfilled, the methanotrophic bacteria within the soil cover can oxidise a much
larger portion of the methane delivered them, oxidising up to 95-100% of the
emission. 1225 The very low gas production resulting from landfilling MBT residues in
German and Austrian cases, especially for methane, is estimated to be around 1 l
CH4/m 2 *h in a typical landfill. 1226 In this case, no active landfill gas collection is
possible. To prevent gas emissions into the atmosphere, a passive method of
oxidation of the residual emissions is necessary. A suitable approach is the use of a
biologically active oxidising landfill cover, consisting of biologically active material like
compost. This oxidises the methane gas during its passage through the layer.
As a survey of several experimental results indicate, the oxidising capacity of soils and
landfill layers are in the range of 0.01 to 16.8 l CH4/m 2*h. 1227 Most values are
1224 K.A. Brown, A. Smith, S.J. Burnley, D.V.J. Campbell, K. King and M.J.T. Milton (1999) Methane
Emissions from UK Landfills, A Report for the UK Department of the Environment, Transport and the
Regions.
1225 J.P. Chanton, D.K. Powelson and R.B. Green (2009) Methane Oxidation in Landfill Cover Soils, is a
10% Default Value Reasonable? Journal of Environmental Quality, 38, pp 654-663.
1226 Soyez, K., and Plickert, S. Mechanical-Biological Pre-Treatment of Waste – State of the Art and
Potentials of Biotechnology, University of Potsdam, mimeograph.
1227 K. Hoering, I. Kruempelbeck, H-J. Ehrig, (1999) Long-term emission behaviour of mechanicalbiological
pre-treated municipal solid waste. In: Proceedings Sardinia 99. Seventh International Waste
992
29/09/09