Eastern Daily Press - September 24, 2011
The vast mountains of decaying rubbish buried beneath Norfolk's landscape may be hidden from view - but their potential environmental consequences are not so easily masked. Waste dumped by generations of families and businesses is still decomposing, oozing toxic chemicals and greenhouse gases which could threaten their natural surroundings.
But the challenge of managing our landfill legacy is being met through an innovative engineering effort, while policy-makers continue battling to find more sustainable ways to dispose of the refuse left by a growing population.
Norfolk County Council is responsible for more than 150 closed landfills, of which six are the larger "permitted" sites which were still operating by the time new environmental regulations came into force in the 1990s. Teams of technicians work to a double remit: to monitor and minimise the ecological impact of gas and leachate generated by so much buried waste, and to find ways to maximise income from its useful by-product - methane.
Turning the greenhouse gas into energy at four sites is currently worth about £280,000 per year to the county council, 22pc of the total costs for managing the authority's closed landfills.
Meanwhile, water percolates through rotting rubbish, collecting chemical contaminants as it goes, and the resulting leachate must be routinely monitored, abstracted and sent away for treatment. In all, keeping permitted sites in compliance with environmental regulations costs taxpayers about £1.4m per year.
Bill Borrett, Norfolk's cabinet member for environment and waste, said: "I think people have got to understand more about landfill, because once you understand the amount it costs, the environmental hazards and the amount of space it takes up, people become more aware of the legacy of their waste and the importance of recycling.
"It is not simply put into a hole and forgotten about - there is years of work in maintaining the land after that." The site at Mayton Wood, near Coltishall, accepted 1.5m tonnes of commercial, industrial and household waste between 1971 and 2003. The waste is grouped into huge domed "cells" contained by a liner and capping system.
The base of the landfill consists of a clay base, followed by a waterproof plastic membrane which is protected by a shingle or sand drainage layer above it. At the top, a similar set of layers encase the waste - along with the gas and liquids - in an impermeable bowl, with the angled sides draining any leachate to a collection sump at the bottom. Meanwhile, rainwater is prevented from getting into the landfill, and is instead channelled to the perimeter across the domed surface.
Hydrogeologist Tim Wilkins regularly samples, monitors and controls leachate levels with the help of a computerised system of wells and pumps. "Leachate is what you get when water percolates through the waste and picks up contaminants," he said. "Some of it comes out looking like coffee and smells quite nasty. The main constituent is ammoniacal nitrogen, and it can be quite toxic to the aquatic environment.
"We have got 11 wells on this site and each has a pump and a level sensor which sense the level of the liquid. The pumps are operated automatically when the leachate reaches a level of one metre above the base layer. "All the permitted sites are now within compliance, but when we took them over (in 2008) they generally had 14-15 metres of leachate. They were all saturated, which meant there was more chance of the leachate escaping."
About 70 gas wells are also sunk into the Mayton Wood waste to harness landfill emissions which are typically 45pc methane, 25pc carbon dioxide and the remainder mostly nitrogen. The gas is drawn into perforated pipes which reach 10 to 16m underground to the base of the cell, and are connected through a network of sealed pipes above ground to a powerful fan at the on-site energy plant. The generator draws in 450 cubic metres of gas per hour and converts it to electricity which is sold on to the National Grid, with the profits shared by the council and the private plant operator.
Des Holmes, the council's landfill gas project officer, said: "We're talking about the equivalent of 45 double-decker buses full of gas
being sucked out of the ground every hour. The whole site is under negative suction. "The machinery is basically like a large diesel engine that has been converted to run on landfill gas.
It produces 600kW/h, enough for 500 homes, and it is constant. The only time it stops is when it breaks down, but it can keep going at that rate 24 hours a day, seven days a week. "The gas will diminish over the years, but I reckon we have got about 15 years until it stops becoming economic to produce electricity here." Gas production at closed landfills is estimated to reduce by 10pc each year as the waste degrades, prompting efforts to find more innovative ways of squeezing every last pound out of the resource. The council's strategic waste manager Paul Borrett said there was a "fine balance" between preventing rainwater seeping into the site, and keeping the waste damp enough to maintain chemical reactions.
"The amount of rain water that collects on the site is more than we can cope with, but we want the waste to stay wet so it continues to produce gas," he said. "In trying to reduce the 'tail' of the gas production, we are looking into recycling some of the leachate we have collected back through the waste. Once it gets saturated or if it dried up, we have got no gas production and no income - so it is a fine balance." The generating plant at Mayton Wood also contains a gas flare, to safely burn off any potentially dangerous pressure build-ups of methane, which is estimated to be 21 times more dangerous as a greenhouse gas than carbon dioxide.
Work on the landfill is continuous as the waste compacts, changing the shape of the cap and the effectiveness of the wells. But Paul Borrett said although it could take several decades for landfills to settle, there were plenty of examples of where they had been replanted as woodland, public amenities or even a pitch and putt course.
"A lot of people's view of landfill is a big pit where people throw rubbish away - an eyesore," he said. "But we have moved on from that. We manage our landfills here in Norfolk and you can absolutely get to the point where they can be turned into something that's more useable - but we do have ongoing respon-sibilities and liabilities."
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