The more that is understood, the more there remains to be discovered. That seems to summarise the world of landfill technology and its environmental impacts.
Thinking on landfill gas has come a long way over the past 20 years. It was only in the 1980s, when landfill engineers began to use layers of clay and plastic to contain waste sites, that landfill gas problems began to emerge.
The engineers wanted to reduce both rainwater ingress and leachate discharges. Along the way they created a new problem as methane and other gases could no longer escape freely to atmosphere. The result was a whole new set of engineering challenges - to extract these toxic, explosive gases in a manageable way, and burn them safely. The mood in the late 1980s was very much one of an industry in crisis as it struggled to get to grips with serious gas migration hazards (ENDS Report 159, pp 9-13).
Today, most operational landfill sites taking biodegradable waste are fitted with active gas controls, and huge quantities of gas are being incinerated in flares or power generation plants.
The advances have helped to tackle some of the hazards of landfill gas, such as explosion risks, and significantly reduced the industry's contribution to greenhouse gas emissions. However, the process has also created a new set of environmental problems - in the shape of emissions from flares and engines.
These emissions are currently unregulated - indeed, they are not even monitored. But over the past few years, the landfill research community - with funding from both the Environment Agency and operators - has made big strides in characterising the range of emissions from typical equipment. At last, the Agency feels able to propose a set of emission limits and monitoring requirements which it believes to be reasonable - and the industry is now facing up to the task ahead (see box ).
A major obstacle for the Agency in planning any programme to upgrade existing equipment is the lack of reliable data under the waste management licensing regime. The Agency was, for example, unable to provide figures concerning the number of engines or flares in use, let alone the number of "open" flares - those that will stand no chance of meeting the new emission limits. Officials put the proportion of open to enclosed flares at around 50:50.
Neither is it clear how effectively landfill gas is being collected. The Agency's local area offices keep an eye on licensed sites, but the Agency has no data at national level to say how many sites are fitted with flares or gas utilisation equipment. The Agency has even less of a grip when it comes to sites that closed before 1994, because these sites fall outside the waste licensing regime.
It is the EU landfill Directive that is now forcing the hand of the industry and its regulators. The Directive requires that landfill gas be collected from all sites receiving biodegradable waste, and that this gas must be treated, and utilised to the extent possible. If the gas cannot be used to produce energy, it must be flared.
Importantly, the Directive also stipulates that the collection, treatment and use of landfill gas must be carried out in a manner "which minimises damage to or deterioration of the environment and risk to human health."
The Agency has a huge task in bringing the landfill sector into this new regime over the next four years. The first tranche of PPC permit applications is due in June. The Agency has prioritised the sites using a risk assessment.
New technical guidance
To assist with the implementation programme, the Agency has drawn up a daunting set of technical guidance notes. The package of seven documents - mostly issued for consultation through December and January - will replace the existing technical guidance, Waste Management Paper 27, issued in 1991.
An overarching document, Guidance on the management of landfill gas, sets the scene, with the full series1 comprising:
Taken together, the package represents a step change in standards for the industry. It is a considerable achievement for the Agency's now disbanded landfill gas working group and its research contractors, which have had to turn much of their work round in the space of 30 months.
"I don't think there's been such a fundamental review anywhere else in the world," says the Agency's waste policy adviser Chris Deed.
For the operators of the 49 landfill sites due to submit PPC permit applications by June, however, the picture is not rosy. They face the prospect of working to draft guidance documents which may change in certain key respects before they are finalised.
But Mr Deed is unrepentant: "We can't hold on for another five years before we set emissions standards. We don't see this as the end of the process," he adds. "I've no doubt that standards will be reviewed in the future."
The fundamental document is the new guidance on monitoring trace components of landfill gas. The paper builds on an Agency research project by Komex, published in December, which collected comprehensive new data from one landfill site and brought numerous other datasets into one place.2
It is the most significant contribution to the field since the end of the 1980s when the Department of the Environment sponsored a study of gas composition and measurements of trace gas emissions from three household waste landfills (ENDS Report 180, p 8).
The new paper notes that no fewer than 557 trace components have now been identified in various studies. The datasets span 79 landfills. But the bulk of these records were of "poor quality", the report says. Many fail to record non-detection of components, details of sampling methods and information on the age of the waste deposits.
The researchers also came to the view that most of the previously published information was gathered from "problem sites likely to generate gas with relatively high concentrations of a wide range of trace components." This has skewed the available data, they argue.
Nevertheless, on the basis of the data to hand, the researchers drew up a set of 26 priority substances with the highest potential to contribute to toxicological or odour impacts. Of the 16 components considered significant in terms of toxicity, only two appear on the list of 12 odour suspects.
The researchers describe their lists as a "coarse ranking" to distinguish substances with the greatest significance from those that do not justify detailed attention. Significantly, however, the available data were judged insufficient to consider any additive effects of the many trace compounds. This may become the focus of future research.
Risk assessments, accompanying operators' PPC applications, will have to address the potential health, environmental and amenity impacts associated with landfill gas. Preliminary assessments using the Agency's GasSim model can utilise default values for the "typical" concentrations of landfill gas components.
Where detailed risk assessments are considered necessary, however, operators will have to model the impacts of actual or projected trace component profiles based on field data.
Once a site is in operation, annual monitoring of the priority trace compounds will be required "as a minimum", and more frequently in the event of significant changes to waste composition or gas management systems. One use of these data will be in filing reports to the Agency's pollution inventory - the public web site which is now being extended to include landfill sites.
The key priority in designing gas collection systems will be to ensure that only a small proportion of raw landfill gas escapes into the environment. The Agency's new guidance states that, with best practice, operators should be able to achieve an annual gas collection efficiency of 85%. It also points out the importance of the speedy installation of active gas extraction.
As an "indicative standard," the guidance says, the first flare should be in operation within six months of waste acceptance into the site or cell concerned.
The industry is also funding research into ways of collecting landfill gas earlier in the life of a site. One study, funded by Shanks, Biffa and the Agency, is looking into the use of sacrificial collection systems, which would operate only briefly before being lost beneath the settling layers of compacted refuse.
The guidance also offers a benchmark to indicate when a gas utilisation scheme, rather than simply a flare, is likely to be required under the PPC regime - more than 200,000 tonnes of waste in place; more than 600m3/hour of gas; more than four metres depth of waste; and more than 25% organic waste composition.
Renewables success story
The new regulatory controls represent a radical change of direction for the landfill gas energy sector. Until now, the driver has been commercial exploitation of a renewable energy resource. Under the landfill Directive and the PPC regime, power generation looks more like a regulatory obligation - and one which must be balanced against the risks of causing impacts on local air quality.
When talk of emission limits for landfill engines first surfaced in the UK roughly four years ago, many in the industry were alarmed. The absence of monitoring data made it impossible to judge how readily the industry could fall in line. Coupled with the steady decline in renewable energy subsidies, the process threatened to undermine one of the UK's few success stories in renewable energy.
The first electricity generation scheme was installed in 1985, and by 1990 there were 12 in place. At that point, the industry's big push forward arrived in the shape of the Non Fossil Fuel Obligation, which offered stable premium prices for renewable energy developers. Each of the five NFFO rounds included reserved bands for landfill gas schemes.
Between 1990 and 1995, electricity generation rose four-fold. By 2000, it had increased by another factor of four. By 2001, output was 2,507GWh - roughly one-quarter of total UK renewables generation.
However, out of 306 landfill gas projects with NFFO contracts, only 200 are actually generating. The very low prices bid by landfill gas developers in the final NFFO round appear to have proved unrealistic. The average contract price under NFFO5 was 2.73p/kWh, down from 3.76p under NFFO3.
Fresh life has now been breathed into the sector, thanks to the Renewables Obligation which replaced NFFO. Electricity suppliers are obliged to supply a specified proportion of renewable electricity to all customers, with the proportion rising year by year.
According to the industry-commissioned report on landfill gas exhaust emissions,3 Renewables Obligation certificates (ROCs) were last year being valued at around 4.5p/kWh, bringing a total electricity price of 6.74p - more than double the landfill gas price secured under NFFO5. These figures are based on prices achieved under a complex auction mechanism under which electricity suppliers vie for rights to NFFO capacity.
The report concludes that there is "a particularly powerful economic incentive" for landfill gas schemes under the renewables obligation. "Much of the potential landfill gas resource that is not already contracted through the NFFO [is] already assigned to project developers keen to generate under the RO," it notes.
"The higher prices have led to increased interest in smaller landfill gas projects or projects that may be shorter lived," the report adds.
With excellent prices now available, many developers are looking for ways to wriggle out of their stingy NFFO contracts, and thereby sell ROCs themselves. The point helps to explain why more than 80 NFFO5 projects have yet to go live, representing a whopping 200MW of capacity.
The new emission controls and monitoring requirements might therefore prove a godsend for developers hoping to tear up their NFFO contracts. "If costs are increased due to regulatory requirements...then some of these contracts may no longer be economically viable," says the report. "The RO provides a stimulus to such projects."
Emissions from flares
Research into emissions from flares dates back to the early 1990s. Among the problems is that it is extremely difficult to sample emissions from "open" flare stacks. The work suffered a loss of focus when the Environment Department's Wastes Technical Division was transferred to the Agency in 1996 together with its research programme.
Eventually, in 1999, the Agency set out its proposed way forward in an interim guidance document (ENDS Report 290, p 43 ). The final version of this document was released with the Agency's new set of landfill gas guidance in January - although some of it has already been overtaken by other developments.The guidance states that no more "open" flares should be installed, except for "test and emergency purposes". All open flares are to be replaced by enclosed flares by the end of 2003.
In practice, however, many of these upgrades will not be enforced until sites fall under the PPC regime, a programme which stretches until 2007. The new permits will include emission limits and monitoring requirements. Such limits have already been set in a few permits issued for new landfill sites over the past 18 months (ENDS Report 319, p 28 ).
Under PPC, operators must also commission an environmental assessment of flare emissions. The Agency's Chris Deed explained that tighter controls might be required where an assessment indicated a potential problem with particular emissions.
Continuous monitoring of raw landfill gas will be required, together with annual monitoring against the new flare emission limits. The limits are designed to ensure good flare operation. They cover nitrogen oxides, carbon monoxide, total volatile organic compounds and non-methane VOCs. However, the limits proposed in 1999 have been adjusted so that existing flares will now have a less demanding target for carbon monoxide - 100mg/m3 - against the 50mg/m3 required of equipment installed after November 2002.
The new draft guidance on monitoring enclosed flares also makes it clear that operators will have to fit sample ports, and that, to ensure good dispersion, emissions must not be impeded by fixtures such as cowls. Existing flares will have to be upgraded to meet these requirements by November 2004.
Emissions from engines
The new controls on engines follow a similar approach to that for flares. The sector had a shock in 1999, when the first batch of Agency research recorded NOx emissions as high as 3,100mg/m3, but subsequent investigations have indicated that this is not typical.
Indeed the Agency believes that engines installed over the past five years should be able to meet a NOx limit of 650mg/m3. One factor is that newer engines operate on a lean-burn principle. This results in lower temperatures, and consequently less NOx in the exhaust.
The draft guidance on monitoring landfill gas engine emissions draws on collaborative research by the Agency and the Biogas Association. A key research paper was drawn up by Land Quality Management, with funding from Shanks, Biffa and Viridor as well as engine manufacturers.3The LQM paper is based on measurements taken from eight engines. It found that most 1MWe spark-ignition engines are tuned for around 650mg/m3 of NOx - working conditions which represent good thermal efficiency.
The range of NOx emissions from these engines was between 490 and 790mg/m3. This corresponds to CO emissions of 1,000-1,450mg/m3. A balance between NOx and CO must be struck, since measures to reduce the former tend to bring an increase in the latter.
The work also found that the German TA-Luft standard for non-methane VOCs - 150mg/m3 - was not exceeded. Measurements of sulphur dioxide generally came out below 200mg/m3, but higher, possibly problematic, emissions would be expected at sites that have taken large quantities of sulphate waste.
It also appears that most landfills will not have problems with halide emissions - but one co-disposal site which had taken chlorinated solvents recorded hydrogen chloride at 584mg/m3 as well as hydrogen fluoride at 45mg/m3.
The industry drew particular relief from the results on dioxin emissions. The highest recorded figure at household waste sites was 0.0046ng toxic equivalent/m3 - a factor of 20 below the new emission limit for municipal incineration plants.
Less room for complacency is available at co-disposal sites, however. Emissions of 0.013ng/m3 were recorded at the site involved in the study, and the researchers also concluded that there was a correlation between dioxin emissions and the chlorine loading in the raw landfill gas.
Compromise on emission limits
Translating the research findings into regulatory controls has proved tricky, particularly because of the remaining uncertainty about the performance of older engines.
The Agency also sought to compare its proposals with controls in Germany, Switzerland, Belgium and the USA. Regulators in some of these jurisdictions have set limits for a wide range of compounds.
The result in the UK is a pragmatic compromise, under which engines installed since 1998 will have to meet nationally prescribed limits (see Table 3), while older equipment will be subject to limits set on a site-specific basis in the light of risk assessments.
"It gives some dispensation for older engines as long as there is not going to be an environmental problem," says the Agency's Chris Deed.
Similarly, unlike the protocol in Germany, the standard set of emission limits covers only four parameters - NOx, CO, VOCs and non-methane VOCs. Where risk assessments indicate a potential problem with other substances - perhaps linked to high levels of chlorine or sulphur in the raw gas - regulators may stipulate limits for acid gases and possibly dioxins. Site-specific factors may also justify tighter limits for NOx.
If all goes to plan, then by 2007 all operational landfills should have been subjected to risk assessments of their impacts on local air quality, and will have emission limits in place. By contrast, closed landfills are likely to have escaped regulatory attention. This is despite the existence of powers available to the Agency under the waste management licensing regime. In the case of sites that are about to close - and those that have closed over the past eight years - a licence will remain in force.
Under the Environmental Protection Act 1990, the Agency is under a statutory duty to review licences from time to time. However, there remains no such programme in place, or resources to hand.
Chris Deed insists that the Agency will step in to set emission limits at closed landfills "where a risk is identified". But without a systematic programme, the situation does not offer much confidence. "The Agency has insufficient resources to undertake in-depth reviews of waste management licences," he said.
The Environment Department is about to undertake a review of the licensing regime. The issue of periodic reviews - and how to deal with closed landfills - may then be addressed. "The development of a planned programme to tackle emissions from closed licensed landfills would be inappropriate before DEFRA undertakes its review," Mr Deed said.
Then there is the picture for sites which closed before the licensing regime took effect in 1994. Many of these are fitted with gas flares installed by local authorities in their work on tackling contaminated land.
Such sites face no regulatory scrutiny from the Agency at all. "We are currently developing a policy on that matter and will be announcing a position in due course," Mr Deed said.
The Agency's view is that landfill gas is a "waste". However, the question as to whether flares on old landfills should be tackled under the waste licensing regime has been batting back and forth between Whitehall and regulators for several years. The problem is that forcing local authorities to install enclosed flares, and monitor emissions, would add a new burden to councils' already stretched work on contaminated land.