Since 1995, Government policy on landfill has been to move towards the unproven flushing bioreactor technology in which wastes would be saturated with water and flushed repeatedly to speed their degradation and remove contaminants. The Government believes that, unlike current landfill practices which will require pollution control and monitoring at many sites for hundreds of years, a bioreactor could be compatible with sustainable development by achieving stabilisation within a generation or so (ENDS Report 265, pp 11-14 ).
In part, the policy has been driven by UK opposition to EC landfill policy. The latest draft of the landfill Directive, issued earlier this year, would require pre-treatment of wastes before landfilling and a phased reduction in the proportion of biodegradable municipal waste going to landfill (ENDS Report 265, pp 40-41 ).
The new Government has yet to state its position on the proposal. Environment Minister Michael Meacher said in June that he is "still considering the environmental and cost implications".
Sections of the landfill industry are hoping that investment in a flushing bioreactor demonstration project may help to broker a redrafting of the Directive to permit the addition of liquid wastes to landfills - needed for leachate recirculation in the flushing bioreactor - and to ease the requirements on biodegradable wastes.
A key issue in developing a flushing bioreactor is whether it would be cost-competitive with alternative solutions such as incineration or anaerobic digestion. According to an Institute of Wastes Management (IWM) working group on sustainable landfill, a bioreactor located in clay geology may impose additional net costs of only £9.90 per tonne compared with a conventional site. This rises to £23.22 for landfills in non-clay mineral extractions.
The results, presented by UK Waste's Chris Parry at the IWM's annual conference, take into account benefits - such as reduced aftercare costs, lower risks of uncontrolled releases, and better use of landfill void space - which had not been considered in earlier assessments. One of the biggest additional costs for a bioreactor would be the leachate recirculation system.
The extra costs, though substantial compared with current landfill prices - usually less than £20 per tonne including landfill tax - indicate that a bioreactor might in some cases be competitive with alternatives such as incineration. However, Mr Parry said that, given market forces, the "quantum leap" to the bioreactor would only take place if driven by regulation.
Access to water supplies could prove a major obstacle. At the IWM conference, Richard Beavan of Southampton University estimated that 500 million cubic metres of water per year would be needed for the 100 million tonnes of UK waste that would require flushing. This is equivalent to 4.25% of the water supply in England and Wales, although untreated water and sewage effluent could be used.
The IWM working group now plans to join forces with the Environmental Services Association (ESA) to develop the bioreactor further. "It's practical and it's possible," said Terry Bradley of Lancashire Waste Services, who chairs the IWM group. "We've got to set up a demonstration in the next 12 months."
A new research funding body planned by the ESA may help to fund the demonstration. It will enroll with Entrust as an environmental body, allowing waste management firms to recoup 90% of their contributions from the public purse through the landfill tax credits scheme. Environmental consultants MJ Carter Associates are currently drafting the body's research programme.
However, a consensus on research priorities will prove tricky. Some major landfill companies with interests in exploiting alternative waste management opportunities remain opposed to the bioreactor concept.
"If research produces practical results to help degradation processes in landfills then that's fine," said David Savory, Environmental and Technical Manager at Biffa Waste Services, a major landfill operator. But he described plans to develop full-blown flushing bioreactors as "cloud cuckoo land".
"The last place you want to process waste [to achieve stabilisation] is in a landfill site - you need a process plant," according to Mr Savory. "There has to be a counter to the extremely enthusiastic body that has formed around this concept."
Alternatives to the full-blown flushing bioreactor have been floated in the IWM working group. One member, Derek Greedy of Greenways Waste Management, believes that composting could be used as a post-landfill treatment. Waste would be dug up after some years of accelerated anaerobic degradation in the landfill cell, and composted in windrows before final disposal.
Another idea is for a two-stage anaerobic process. UK Waste's Chris Parry says that the bioreactor approach poses fewest problems on a small scale. He proposes that materials could be treated in a relatively small accelerated stabilisation cell before being transferred, after some years, to their final resting place.
An advantage of these two-stage landfill approaches is that they would allow deep quarries to continue to be used for landfill. Currently, wastes are so heavily compacted at the bottom of deep sites that leachate can scarcely flow, making degradation very slow. But the approach may be unpopular with planning authorities because of delays in site restoration.
Finally, Mr Parry - whose company favours pre-treatment before landfill - is taking the discussion full circle. He suggests that a "sustainable landfill" system could include a pre-treatment plant which would be an anaerobic digester. "The more I look at it the more I think we're looking at a reactor as a piece of process plant outside the landfill site," he told ENDS. Mr Parry noted that pre-treatment by digestion could halve landfill tax costs by reducing the waste mass.
In the Landfill 2000 project in West Yorkshire, researchers deposited 1,000 tonnes of household waste mixed with sewage sludge in a small cell up to five metres deep. They added sewage effluent and recirculated the leachate through the waste mass.
After four years, 58% of the "readily available" landfill gas had been generated. The researchers predict that such a site would take only seven years to reach "stabilisation", but this forecast only takes into account "readily degradable" organic wastes - principally putrescible materials. Paper and textiles would not degrade in this period, possibly because of the role of lignin as a barrier to microbial attack. Waste characteristics changed "little" during the four-year study, they reported.