The project required the delivery of a replacement gas abstraction system, leachate management and sustainable drainage scheme as part of the £10m restoration works to a large closed landfill.
Sundon Landfill ceased operating in 1996 and was formerly used for tipping of mixed waste including large volumes of domestic refuse. The plan area of the landfill has a plan area in excess of 350,000sq.m and is a land raise with a maximum waste depth of 40m. To meet planning and licensing requirements the landfill required comprehensive restoration works that included the re-contouring and capping of the whole site, partly with clay and predominantly with a geosynthetic membrane and the subsequent placement of 1m thickness of inert restoration soil above the capping layer.
EPGClear’s remit was to design a landfill gas abstraction system to sustain power generation from the site, a leachate extraction and outbreak management system to be connected to a recently constructed treatment plant and on-site SUDS surface water management system to prevent overloading of the existing drainage infrastructure.
With all design elements, the key aims were to minimise the environmental impact to the surrounding villages, sustainability and cost effectiveness.
Design constraints included restricted discharge consents for both the leachate (3 l/s) and surface water outflows (150 l/s), a 1 in 100 year return storm event requirement, limited land availability for surface water balancing and the requirement to decommission an existing ‘temporary’ gas extraction system, whilst maintaining power generation. A further complication was the integration of the complex network of services with the re-contouring works.
To achieve maximum flexibility the in the extraction system 160 combined gas and leachate extraction boreholes were drilled to the base of the landfill and lined with 200mm diameter slotted wells. These were large enough to accommodate a variety of borehole pumps. 22 electric submersible pumps were installed in two ‘rings’. These discharged via ducts to a pumping station that ‘batched’ the leachate prior to discharge to the treatment plant. An emergency back-up system was also designed to re-circulate leachate in the event of a problem at the treatment plant.
The gas extraction was achieved via a network of 63 – 315mm dia hdpe pressure pipes that included valved manifolds and condensate traps. The anticipated average gas yield was 1500 cu.m/hr.
The surface water management comprised an arrangement of contour drains in the form of shallow ditches. The flow within the base of the ditches was controlled using geocellular ducts that effectively attenuated the peak flows en-route. A 20,000 cum balancing pond was also incorporated and the total attenuated run-off including the contour ditch drains was 30,000 cu.m