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Enviros Research: Landfill gas bio-oxidation in active biofilters

Introduction  Select a linkIntroductionBackgroundBiofiltrationResearch at Betton Abbotts LandfillResreach at Ballymacvea Northern Ireland

     
As time passes, both the rate of landfill gas being produced in a landfill site and the proportion of methane in it decline and eventually reach a point where burning the gas in a flare becomes unreliable. Ultimately a point is reached where combustion cannot be sustained without the use of a support fuel such as propane. As well as incurring substantial costs for the landfill operator, the environmental acceptability of consuming fossil fuels in this way is questionable.

Combustion is not the only method of destroying methane. It can also be oxidised through microbiological activity. Therefore, Enviros is investigating the practical application of microbiological processes to convert the methane in extracted landfill gas to carbon dioxide.

The objective of the research is to define the design parameters for a biofilter that can replace an otherwise conventional gas extraction system when the methane content of the gas becomes insufficient to support combustion reliably. This will enable operators to reduce greenhouse gas emissions from landfill gas extraction systems without incurring the penalties associated with the use of support fuels at sites where gas production is declining.

Background

Both the rate of gas generation and the proportion of methane in the gas decline slowly after reaching a peak that generally occurs a year or two after emplacement. As well as sites where the methane content of the gas is declining, there are many others where the methane concentration remains low throughout because of the nature of the waste. In many instances the gas quality is marginal for flaring and there is a point where the methane content is too low to support combustion at all.

The EU Landfill Directive requires that all landfill methane must be burnt, in the interests of reducing Greenhouse Gas emissions, even if a support fuel is needed. But even before this stage is reached, flaring becomes unreliable and operators experience increasing callout attendance in an effort to avoid environmental risks.

Biofiltration

Biofiltration is the use of micro-organisms, usually bacteria, to remove unwanted substances from a stream of mixed gases. The technique is widely used, particularly by the food processing industry where it is often necessary to remove odorous compounds before process gas is released to the atmosphere. Biofilters have a proven track record for removing volatile organic substances from process emissions and there is a growing body of evidence that they can also be used for methane removal by incorporating naturally occurring methane oxidising bacteria (methanotrophs).

There are two basic types of biofilter;

Soil-bed biofilters

In these, the "dirty" gas is passed through a bed of organic "soil" such as compost, before being allowed to disperse to the atmosphere. Micro organisms living in the soil metabolise a range of undesirable compounds and so prevent their emissions to the air. Because soil will support a great diversity of micro organisms, soil-bed biofilters lend themselves to applications where the volume and composition of the gas varies and where a wide range of compounds need to be removed. Soil-bed biofilters are simple and very robust. They require little maintenance but they occupy a relatively large area of ground.

Below: Biofilter Research Cells at Betton Abbotts Landfill (Courtesy: SITA Waste Management)

Tower Biofilters

In these, a predetermined population of micro-organisms is established on an artificial substrate designed to present a very large surface area. In this way, the equivalent capacity of a soil-bed biofilter can be provided in a container small enough to be transported by road. Tower biofilters are best suited to applications where a relatively small number of compounds need to be removed from the gas and where changes in flow rate and composition occur relatively slowly.

Our research on two projects has concentrated upon the use of soil-bed biofilters.

Objectives of the landfill gas bio-oxidation research at Betton Abbotts Landfill

The research now in progress will concentrate on the construction and testing of a pilot scale soil-bed Biofiltration unit on a landfill site to determine the design parameters for full scale applications. It involves the active participation of the landfill operator (SITA) who is making gas extraction facilities available at their Betton Abbots Landfill for the duration of the programme.

It involves the construction, operation and monitoring of a series of pilot scale soil bed biofilters using different media. The trials are being conducted on a completed area of the site where landfill gas is already being pumped to a flare. This will provide a stable, measurable supply of landfill gas.

Below: An experimental Soil Bed Biofilter Cell at Betton Abbotts Landfill (Courtesy: SITA Waste Management)

A proportion of the landfill gas will be diverted from the flare supply, and passed through the biofilters. To ensure safety, the quality of the inert gas will be adjusted by dilution with air so that it remains below the Lower Explosive Limit. The quality of the treated gas will be monitored. A variety of compost based soil materials will be tested.

The specific aspects that will be investigated include:

• Definition of bioactive soil media
• Effect of changes in gas quality and pumping rates to determine points where the system becomes overloaded
• Inhibition of the process through build up of contaminants and/or depletion of nutrients
• Tolerance to changes in temperature and moisture content due to weather.

Acknowledgements: The project is funded under the UK Landfill Tax Credit Scheme, with contributions from SITA Environmental trust, and direct funding from Clark Energy, and Pipeline Services Ltd. We gratefully acknowledge the help given by all those who are assisting us in this work.

The Performance of Passive Biofilter Landfill Gas Vents at Ballymacvea Landfill, Northern Ireland

It has been estimated that landfill methane accounts for about 40% of the UK's total greenhouse gas emissions. About half of this is attributed to 400 or so large sites accepting predominantly household waste, with the balance accounted for by closed sites and operational sites taking small amounts of biodegradable waste.

The need to reduce greenhouse gas emissions has become almost an act of faith, and for UK landfill operators it has also become a legal requirement through implementation of the EC Landfill Directive which requires landfill gas to be collected and burned, preferably with energy utilisation. This is enforced through site licences and permits.

Owners of unlicensed, closed sites are under no such legal obligation and few have active gas extraction systems. Yet closed sites are estimated to account for about 25% of the total landfill methane emissions.

It would therefore seem logical that landfill gas extraction should be installed at closed sites but research recently undertaken by Enviros at the Ballymacvea landfill site in Northern Ireland suggests that an effective alternative is available.

In 1999, about 2 hectares of Ballymacvea landfill site, filled predominately with household waste, was completed and capped with clay. No gas venting or extraction was provided. Thirty gas monitoring standpipes were installed through the capping in three test areas at 10, 20 and 30m spacings. The standpipes just penetrated into the underlying waste. Weekly gas monitoring was carried out to assess the establishment of methanogenic conditions in the waste throughout each of the test areas. The gradual disappearance of oxygen and methane and carbon dioxide compositions attaining concentrations of 60%v/v and 40%v/v respectively, confirmed methanogenic conditions. This was reached in many of the standpipe locations in as little as 3 or 4 weeks from the capping being installed, although a few of the standpipes still contain detectable oxygen after a year.

Following establishment of methanogenic conditions in the waste, twenty-three passive vents were installed in the test cells, about two metre from the standpipes. Each vent comprised 1 metre diameter concrete rings, stacked and set into holes cut through the capping, resting on the exposed surface of the waste and protruding about 0.5 metres above ground level. Each was filled to ground level leaving a headspace to be used as a flux chamber. The material used for filling ranged from good quality green-waste compost, through a range of compost/soil/inert mixtures as well as one vent filled with the excavated capping material and another with fines free, crushed basalt

Fluxes of methane from the vents were been monitored at monthly intervals using, with slight adaptations, the Landfill Methane Measurement Protocol published by the Environment Agency.

The results were very encouraging. Fluxes of methane from many of the vents were very small confirming the ability for soil micro-organisms to bio-oxidise methane. Laboratory and theoretical studies suggest that methane bio-oxidation may well be able to cope with the rates of generation encountered at many landfill sites though so far as we are aware, this is the first time that the theoretical and laboratory research has been put to test under full-scale landfill conditions.

The Ballymacvea landfill site is an operational site and the area used for the investigation is close to its peak in methane generating capacity. Even so the results indicate that the media in some of the vents has the capacity to oxidise the methane and flammable trace gases to a level where they become undetectable using a flame ionisation detector capable of 0.1 part per million resolution.

Most old landfills as well as recent ones that contain a low proportion of degradable wastes, will have a much lower rate of landfill gas generation than Ballymacvea. With the lower methane production rates at these sites it should be possible to assure the level of performance achieved by the most effective of the vents at Ballymacvea and passive biofilter vents may offer an effective means of reducing greenhouse gas emissions from many landfill sites. This will be especially relevant where the statutory requirement to collect and burn the gas does not apply, or where methane rates of production are not sufficient to maintain combustion without the need for costly support gases.

A full scientific paper on this project was published in the Proceedings of the International Landfill Symposium, Sardinia, 2003. Click here to request a free copy.

Acknowledgements: Enviros gratefully acknowledges the assistance given to this two year project, which has been funded jointly by Ballymena District Council and The Northern Ireland Environment and Heritage Service, DoE, Northern Ireland.

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