Northamptonshire County Council invites quotations for the provision of the Operation and Maintenance (O&M) Services of the leachate treatment plant (LTP) at Gayton Closed Landfill Site.

The authority has the liability for Gayton Closed Landfill located at National Grid Reference SP715552 situated approximately 1 km north east of the village of Gayton, to the south west of Northampton. The site is roughly triangular in shape and occupies an area of approximately 12 hectares. Access is from Milton Road to the east of the site. The Grand Union Canal forms the northern boundary and a cutting for the main West Coast rail line forms the south western boundary. Milton Road forms the third boundary on the south eastern side of the landfill.

The surface of the landfill is domed to 16 m above the level of the adjacent canal. Surrounding land use is agricultural. The nearest dwelling is some 100 m from the landfill, on the other side of the railway cutting.

The eastern part of the site was purchased in 1938 by Northampton Rural District Council for use as a refuse tip. The site was taken over by the authority in 1974. The western part was the last part of the site to be landfilled and the site closed in 1988 when it was returned to pasture. The western part of the site is not owned by the authority and was operated under a lease. There is an estimated 800 000 m3 of mainly domestic and commercial waste within the site. There is no waste management licence in force for the site. Landfilling was granted under a resolution passed by the authority in 1982.

Landfill leachate is collected by counterfort drains installed within the northern slope of the site above the canal. These connect into a toe drain falling to a pumping chamber located to the east of the LTP. The pumping chamber also has feeds from a former tertiary treatment lagoon and from some leachate extraction wells equipped with pneumatic pumps, not currently in operation.

The LTP was constructed in 2001 and originally comprised a single-tank sequencing batch reactor (SBR) followed by the lagoon for tertiary polishing. Due to an issue with excessive solids in the discharge, the plant was converted in the summer of 2007 into a membrane bioreactor (MBR). This was achieved by the inclusion of a membrane filtration unit within the SBR aeration tank and the mothballing of SBR decanting facilities and the tertiary lagoon. The membrane unit has worked well with minimum solids in the discharge and has also improved ammonia treatment efficiency as nitrifying bacteria are effectively ‘trapped’ within the aeration tank. In a further upgrade in 2011 the membrane filtration unit was relocated outside the aeration tank and a second membrane filtration unit (operating in parallel with the first one) has been added also outside the aeration tank.

Treated landfill leachate (max: 100 m3/day) is discharged under gravity to an un-named tributary of the Gayton Arm of the Wootton Brook under consent to discharge No PRNNF12093 from the Environment Agency. It is expected that the service provider, in day to day operational management of the LTP will maximise discharge of treated leachate in compliance with this consent.

On rare occasions the flow of leachate exceeds the capacity of the leachate treatment plant and ‘excess’ landfill leachate is recirculated to three infilled soakaway lagoons, however this is purely a contingency measure and recirculation to the lagoons is only carried out when absolutely necessary.