POSITIVE LIST ADVICE TO THE MINISTER FOR THE ENVIRONMENT
The Domestic Offsets Integrity Committee (DOIC) considers that the following activity
is suitable for inclusion on the positive list for the reasons outlined below.
Passive oxidation of landfill legacy waste emissions using phytocaps on landfills.
Definitions
Phytocap: A vegetated soil cap that promotes the activity of methanotrophic bacteria by
controlling percolation of water to optimise soil moisture storage and evapotranspiration
for a given climate.
Note: this differs from a vegetated traditional cap in that no compact clay, geosynthetic
clay or geomembrane layer is present.
Legacy waste emissions: Emissions from waste deposited in a landfill before 1 July 2012.
Background
In terrestrial environments, methanotrophic bacteria oxidize methane to water and organic
compounds (Hanson & Hanson 1996) which result in greenhouse gas abatement. Soils
covering landfill contain large amounts of these bacteria and under the right conditions can
significantly reduce greenhouse gas emissions from landfill (Abichou et al. 2004).
A phytocap comprises a layer of soil planted with vegetation which covers landfill and
creates an environment by providing favourable conditions for methanotrophic bacteria to
metabolise methane emissions and grow. The phytocap also reduces the percolation of
rainwater to the waste below, slowing the waste’s decomposition rate (Ashwath &
Venkatraman 2010).
Phytocaps can significantly reduce landfill gas emissions, achieving methane oxidation rates
which are up to five times greater than the traditional clay cap systems commonly used in
Australia (Ashwath & Venkatraman 2010)..To optimise the conditions for bacterial oxidation
of methane, the phytocap design must be calibrated to local climatic conditions, soil
characteristics and the transpiration characteristics of vegetation. Phytocaps are most
effective in drier areas where the phytocap is better able to regulate water balance, that is,
rates of percolation and evaporation (Waste Management Association of Australia (WMAA)
2011).
The use of phytocaps to reduce landfill gas emissions is a new technique in Australia. While
several sites have received approval to conduct small-scale trials, few full-scale phytocaps
have been constructed.
Reasons for advice
The DOIC considers that this activity is suitable for inclusion on the positive list.
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Passive oxidation using a phytocap can achieve methane oxidation rates higher than
traditional cap systems. The use of phytocaps to reduce landfill gas emissions is an emerging
technology in Australia and few full scale phytocaps have been established. Therefore the
activity is unlikely to be common practice.
References
Abichou T, Palueson D & Chanton, J 2004, ‘Bio-reactive cover system’, Florida Centre for
Solid and Hazardous Waste Management, Florida, 1-37.
Ashwath N & Venkatraman K 2010, ‘Phytocapping: an alternative technique for landfill
remediation’ International Journal of Environment and Waste Management. 6, 51-70.
Hanson RS & Hanson TE 1996, ‘Methanotrophic bacteria’ Microbiological reviews, June, 439471.
Waste Management Association of Australia (WMAA) 2011, Guidelines for the Assessment,
Design, Construction and Maintenance of Phytocaps as Final Covers for Landfills, June 2011.
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