Massive Investment in high-tech treatment provides the solution to leachate problem
The following article as published in “Local Authority Water & Environment” (LAWE), October 2001.
When the Waste Recycling Group (WRG) took over Arpley Landfill in Spring 1999, it inherited a site where leachate was up to 8 metres above consented levels. After tankering over 140,000 tonnes of leachate for treatment off-site, and investing nearly £3 million in leachate extraction infrastructure and a state-of-the-art leachate treatment plant, the site is undergoing a major transformation.
Howard Robinson, of consultant Enviros Consulting, describes how the scheme has been designed and implemented.
Arpley Landfill is located on the south bank of the River Mersey, west of Warrington, on 130 hectares of land originally used for disposal of dredgings from the Mersey and from the Manchester Ship Canal. The site has received more than 10 million tonnes of domestic, commercial and industrial wastes, emplaced in depths of up to 30 metres, and continues to receive about 800,000 tonnes per year of primarily domestic wastes. Wastes have been deposited in three phases, Birchwood, Lapwing and Walton, with increasing degrees of containment.
Initial works concentrated on improved control of landfill gas omissions – the £8 million Arpley generation scheme is the largest landfill gas power station in the UK, and began to generate 16MW of electricity in October 2000, enough to supply the needs of 16,000 households. Gas is extracted from over 200 wells, via more than 10km of pipelines. Also important in control of landfill gas, and of leachate generation, have been works to provide high specification capping to completed areas of filling.
Installation of a network of leachate extraction wells, pumps, and pipework, with a “tank farm” to feed a fleet of tankers taking it off-site for treatment has cost £1 million. Nevertheless, it has allowed over 140,000 tonnes of leachate to be removed from the site during the last two years, including 60,000 tonnes in the first eight months of 2001.
On-Site Leachate Treatment
It was always recognised, that a long-term, Best Practical Environmental Option solution would incorporate an on-site biological leachate treatment plant. Once leachate extraction was under way, and levels across the site began to reduce, extensive chemical characterisation could be undertaken.
Leachate from Arpley is remarkably strong – although COD values rarely exceed 8,000 mg/l, the active decomposition of wastes within the site (which feeds the landfill gas engines) also generates very high concentrations of ammoniacal-N, often in excess of 2,500 mg/l. On-site treatment at proposed rates of 450 m3/day, will make this the largest leachate treatment plant in Britain, in terms of the load of contaminants being treated every day.
Extensive treatability trials on a selection of Arpley Leachates began in Spring 2000, to allow optimised detailed designs for the full-scale plant to be prepared by Enviros Aspinwall, working in partnership with civil engineer May Gurney, and staff of WRG.Discharge of effluent to the sewage works on the opposite bank of the River Mersey initially appeared the best option, and a pipeline was thrust-bored under the river to allow this. Unfortunately, following extensive negotiations, the works decided it could not accept even pre-treated effluent. After liaison with the Environment Agency, a discharge into the tidal Mersey was negotiated, for effluent that would be treated to much higher standards.
The £2 million on-site leachate treatment plant begins biological treatment during October, and in November will achieve design flow rates of up to 450 m3/day. Process design of the plant is based upon Enviros Aspinwall’s well-proven sequencing batch reactor (SBR) technology, now incorporated into more than 50 full-scale plants around the world.
Treatment is entirely automated, in three large circular SBR tanks with fibreglass roofs. Delivery of leachate involves the existing tank farm, to blend leachates from various parts of the site, for optimum treatment.
Effluent from the SBR units is “polished” by passage through a series of terraced reed beds – a wholly natural process in which the reed’s rhizomes provide additional treatment to high standards, as part of Best Available Technology.
It may take time before leachate levels can be brought into compliance across all areas of the site, but as levels continue to fall, and risks reduce, the treatment scheme certainly represents a state-of-the-art solution.
Photo (Top): Bunded leachate feed tanks with one treatment reactor in the background.
Photo (Bottom): Planting Phragmites Australis in the reed beds with the main aeration tank SBR Reactors in the background.
We are pleased to report that since the article was written, the plant, has consistently achieved its design flow as required, and has continued operating extremely well, in the care by WRGs highly experienced operational staff. Compliant leachate levels were successfully achieved after this article was written, and have been maintained at these low levels ever since.
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