Case Studies

Design & Build, Former Gasworks, Chertsey

Remediation & Enabling

Tamdown undertook the remediation of the former Chertsey gasworks site for Explore Living Ltd, to facilitate construction of residential apartments with communal gardens. The works required the targeted remediation of approximately 5,000 tonnes of soils contaminated with BTEX compounds, petroleum hydrocarbons (TPH), phenols and coal tars. Off-site disposal of all soils as predominantly hazardous waste would have been cost prohibitive so Tamdown segregated 500 tonnes of grossly impacted (untreatable) soils at source for landfill disposal and treated the remainder on-site by ex-situ bioremediation prior to beneficial re-use as a backfill material.

Remediation works were commenced during late September and ran through the winter months on a restricted area of land. Conventional bioremediation methods would not have met the required treatment objectives in such a short timescale and prevailing climatic conditions. Therefore, Tamdown adopted the following treatment method for enhanced bioremediation:

• Construction of a soil treatment bed with engineered leachate containment measures incorporating a geo-membrane liner and concrete leachate collection sump
• Screening soils to remove oversize materials
• Application of spent mushroom compost, a liquid bio-augmentation solution and a natural seaweed extract to increase bio-availability of complex hydrocarbons, all with the purpose of accelerating the bioremediation process. This blend of bio-stimulants had been successfully trialed by Tamdown on another remediation project
• Use of a low vacuum extraction system to continually extract air through the soil treatment ‘biopiles’ without the need for mechanical turning (this retains heat and prevents significant disturbance of the soil microbial populations)
• Biopiles enclosed in large ‘poly-tunnels’ typically utilised in horticultural applications, to promote a greenhouse effect and retain temperatures above external ambient, whilst mitigating potential dust & odour issues
• Extracted vapour / air passed through a  water trap and granular activated carbon (GAC) for VOC / odour abatement
• Outlet of the blower connected to each enclosure using buried ductwork to provide an input of recycled warm air into the poly-tunnel enclosure, thus further increasing ambient temperature to optimise the bioremediation performance
• Leachate collected in the water trap and sump recirculated via an irrigation system mounted in the roof of the poly-tunnels, to maintain optimal moisture content in biopiles and recycle biomass, nutrients etc

The use of bio-augmentation agents, poly-tunnel and warm air input resulted in temperatures within the enclosures to be 20 – 30 degree Celsius above ambient. These temperatures promoted a rapid reduction in TPH, BTEX, phenol and certain polycyclic aromatic hydrocarbon (PAH) concentrations. Remedial targets were achieved within 10 weeks of commencing treatment, with a >90% reduction in overall contaminant loading achieved during this period.

To meet the requirements of its Environmental Permit, Tamdown implemented a rigorous environmental monitoring regime including noise, dust, odour and volatile organic compounds at various locations surrounding the soil treatment bed, site boundary and excavation areas. All soil treatment works were managed by Tamdown’s in-house WAMITAB 4-TMH accredited manager.

The innovative use of a combination of bioremediation processes ensured that hazardous soils could be rapidly and effectively treated during autumn / winter months on a restricted available footprint to meet the clients overall tight programme of works. Tamdown met the clients remediation budget whilst preventing 200 loads of hazardous waste being removed off-site to landfill.  Furthermore, this enabled the treated soils to be re-used in a more sustainable manner as a backfill material, enabling an equivalent volume of recycled crushed concrete to have a more beneficial use as a piling mat / passive gas venting layer beneath the proposed building. This all resulted in an overall saving of £195,000 on the cost of remediation works compared to off-site disposal and importation of backfill material.

Our client's said; “As well as significantly reducing the volume of hazardous waste sent to landfill, Tamdown’s soil treatment system made the most of the limited footprint and timescale available to them. Environmental controls were of the utmost importance on this project due to its highly sensitive location, and the use of an enclosed system prevented emissions attracting complaints from nearby residents”. This contract also won Tamdown the 'Best use of a remediation technique' at the Brownfield Briefing Remediation Awards 2008.