Innovating In The Waste Sector: Landfill Gas
Landfill Gas migration poses a significant health and safety risk as methane (one of the major constituents of landfill gas) is explosive at concentrations above 5% volume and below 15% volume. In addition, landfill gas can cause vegetation stress on the surrounding environment, as well as corrosion of underground assets.
To mitigate these potential problems, operational and closed landfills need to carry out gas risk assessments, and require ongoing management.
The EPA have included a requirement in the new Post Closure Pollution Abatement Notice (PC PAN), Closed Landfill Guidelines, Publication 1490, December 2012 for assessment of landfill gas risks as part of the aftercare management plan for landfill sites.
If Councils followed Best Practice Environmental Management (BPEM 2010) guidelines to the letter for a “standard” landfill, the cost of drilling and construction of landfill gas bores at recommended intervals could run into tens of thousands of dollars. Utilizing a combination of technical expertise, industry knowledge and open dialog with the site EPA Environmental Auditor can reduce this potentially onerous burden, without increasing the risks to the surrounding environment.
As part of landfill gas risk assessments, methane emissions are modelled for the lifespan of the landfill, plus the 30 year aftercare period. The BPEM requires potential landfill gas treatment technologies for a range of gas generation rates, from power generation (>1000 m3/hr) to passive flares (<100 m3/hr). However, if the gas generation rate is calculated to be below the lowest generation range, it can be argued that landfill gas generated at the site is likely to be naturally venting through the cap. Therefore the installation of costly passive venting infrastructure, biofilters and bicocover is unlikely to be required.
As per the BPEM, the aim of a landfill gas risk assessment and monitoring bore system is to intercept any landfill gas escaping laterally from the site and identify its location and preferred pathway off site. As such, landfill gas monitoring bores must be installed at appropriate locations, drilled to depths suitable to intercept gas movement paths and should be determined based on the findings of the landfill gas risk assessment.
The BPEM states, a landfill gas monitoring bore system should target sensitive receptors (example dwellings), encircle the entire landfill gas waste mass and be installed into the local geology (not into waste or fill materials). As such, the recommended bore spacing around the landfill is based on the subsurface geology (low or high permeability) and the distance to sensitive receptors.
The minimum bore spacing recommended in the BPEM is 5m intervals surrounding the waste mass, for a landfill identified as having high-permeable geology and a development within 150m. The maximum bore spacing is 150m, for landfills surrounded by low-permeable geology, with no development within 250m.
By identifying all the potential pathways for landfill gas to migrate, negotiations can begin with the EPA regarding the actual number of bores really required.
We have been able to place bores at intervals of more than 200m, while at some landfills, due to their low risk location, low-permeable geology and small gas generation rate, we have not needed any bores at all on some sides of the site.
While it may be quick and easy to simply follow guidelines – assessments can be technically complex – but by investing a little time and money into proper assessments, significant long-term savings can be made.
It is worth operational landfills thinking about this now. They may currently have an income to support these required assessments, they need to be prepared for when they are closed. Costs at closure can come as quite a shock, but early assessments can mitigate this impact later.
Today’s guest blog is from Megan Stanley, Environmental Scientist at Meinhardt.