Leachate treatment gets the green light

Historically, no engineering was undertaken prior to the deposition of waste in this area and since the contaminated land area was relatively level there was little potential for rainfall run-off with all effective rainfall penetrating the waste to form leachate. The low permeability of the underlying clays caused any leachate generated within the site to perch within the waste and the subsequent build up of head amounted to several metres. Leachate heads within the site were controlled by a collection ditch set at the site boundary and, with time, a dynamic equilibrium was established whereby the head of leachate at any point became generally constant, the inputs of infiltrating rainfall being balanced by release of leachate into the ditch.

Prior to any new landfilling taking place under the new planning consent a series of desk studies and site investigations was undertaken which showed that the materials contained within the site were contaminated with a wide range of organic and inorganic substances. Although the contaminants originated from the deposited materials, the build up of leachate within the site created a means by which the pollutant load could be mobilised and pose a risk to potential receptors including the local population, the River Thames and surrounding marshland and the groundwater beneath the site. Although the risk to the environment was considered low, it was expected to increase through the planned landfill activity, principally by the displacement of leachate following the consolidation of the made ground.

Scheme agreed with Agency

Before any development work could progress a scheme to reduce the risk associated with the proposed landfill activity had to be agreed with the Environment Agency. The accepted scheme was designed to allow the control of leachate heads at or below existing levels; the proposal also allowed for the eventual dewatering of the made ground. The dewatering scheme submitted allowed for the installation of approximately 10,000 linear metres of 8 metre leachate collection drains with the leachate collected from these drains being pumped to a leachate treatment facility for final discharge to foul sewer.

The deep drains were installed within three months by Lareco Nederland bv, using a Mastenbroek deep trenching machine. This technique was chosen because it prevented the need for large scale excavation using conventional trenching techniques and as such minimised the contact of ground-workers with the contaminated materials. The trenching machine was able to cut the trench and automatically install a drainage pipe complete with a gravel surround and independent vertical riser in one pass.

The chemical quality of the leachate varied significantly across the site. During the site investigation and desk study stage the site was zoned into areas of similar leachate quality. The zoning being based on the type of treatment the leachate would need.

‘Traffic light’ colour coding

A traffic light colour coding was used for this application, the leachate being described as red (leachate that contained persistent chemical species and gross concentrations of volatile organics), yellow (leachate of intermediate quality containing high levels of volatile organics) and green (leachate of quality similar to typical methanogenic leachate but containing low concentrations of volatile organics). By assigning each drain to the appropriate colour code, the leachate stream that required different leachate treatment could then be segregated, this zonation also allows for flexibility of treatment to be maintained and efficiencies maximised. A pneumatic leachate management system was subsequently installed with different leachate conveyance pipes being installed for the different leachate streams.

The green leachate is treated by aeration before direct discharge to foul sewer, whereas the yellow and red leachate is passed, in equal proportions, through a 150m³/day capacity activated sludge treatment plant, the optimum hydraulic residence time for this plant is seven days. The red leachate is pre-treated by air-stripping before being delivered to the plant; all off-gases generated by this process are vented through a woodchip biofilter before release to atmosphere.

The project provides significant environmental betterment for the area by removing and treating the pollutant load that is held within the site and by minimising the generation of new leachate by the placement of a high quality capping layer over the newly deposited waste.

The majority of the design and project management for this development has been largely completed in house by the Environmental Projects Team, part of Cleanaway’s Landfill Technical Department.