20 September 2019
Cognition is delighted to announce that Meridian One at Willoughby Lane has won Project of the Year at the 2019 Brownfield Briefing Awards.
Cognition Land and Water was appointed by Enfield Council to carry out remediation works at Willoughby Lane which is part of the Meridian Water regeneration project that will create 10,000 homes and thousands of jobs in Enfield, North London.
The site was formally part of a large gas works which was operational for over 100 years, resulting in heavy metal, hydrocarbon, cyanide and asbestos contamination in soils and NAPL in shallow groundwater. The site is in a sensitive location bounded by residential housing, busy roads, commercial premises and a railway line. Several utilities on the site needed diverting, including HP gas, HV electricity, water and sewers, requiring careful consideration when planning the remediation works.
Innovative and sustainable remediation
Cognition considered the most sustainable and cost-effective means of remediating the site to deliver the works to meet the redevelopment programme. The team focused on delivering innovative remedial solutions to address contamination in both groundwater and soil, whilst meeting the socio-economic objectives underpinning the scheme.
A targeted NAPL recovery system was successfully used to address free product in the groundwater whilst an advanced bioremediation additive proved to be an effective and rapid means of addressing heavy TPH and naphthalene contaminated soil. The innovative solutions enabled the project to be delivered within client’s overall budget, fulfilling the remediation criteria thereby enabling future redevelopment; this was something that eluded the previous landowner.
Social and economic benefits are key drivers for Enfield Council; the works will aesthetically improve the area whilst providing jobs and housing for the local community. As well as enabling the council to keep within their financial model, the remediation technologies helped treat soil and groundwater contaminants more quickly than previously known techniques or products. Council-lead community engagement days, supported with newsletters throughout the project, ensured local residents and key stakeholders were kept informed of remediation plans, progress and delivery of Meridian Water project milestones.
Groundwater in the Kempton Park Gravels was contaminated with NAPL (dense and light) and was a major constraint for the previous landowner. Despite twelve years of investigation and negotiation, regulatory approval was never obtained for a remediation strategy or piling solution for the new development. However, with a new and innovative remediation solution, together with further assessment of SI data, the project team was able to demonstrate low NAPL transmissivity to the underlying aquifer and show that pockets of NAPL were recoverable, and once removed, eliminated residual risk.
Cognition was appointed by Enfield Council to conduct the NAPL recovery using a NAPL Fabric Extraction System.
The works included:
NAPL Fabric Extraction System
This treatment method is significantly more cost-effective than conventional techniques for the remediation of NAPL; by targeting NAPL it eliminates the need to remove vast quantities of groundwater. It works by drawing NAPL from the groundwater using a hydrophobic/oleophilic fabric on a continuous motorised loop (pulley system). The NAPL is then desorbed from the fabric by squeeze rollers and drained into containers prior to off-site disposal.
The benefits over conventional systems are:
• Eliminates the need to pump & treat groundwater;
• Recovers LNAPL & DNAPL from contaminated groundwater direct from wells;
• Can be used to perform recovery potential tests to demonstrate if NAPL is recoverable or not;
• Significantly cheaper than conventional remediation techniques on a project lifecycle basis;
• Delivers endpoint quicker than conventional techniques;
• Unaffected by fluctuating groundwater conditions;
• Small footprint, allowing it to be deployed on operational sites;
• Quick installation and low power/maintenance costs;
• Proven technology used worldwide;
• Solution supported by the Environment Agency.
Cognition initially performed recovery potential tests to determine the mobility and recoverability of the NAPL and demonstrate if NAPL recovery is sustainable. If sustainable, a secondary objective of the test is to determine the optimal NAPL recovery rate from the geological formation.
If NAPL recoverability proved to be limited or unachievable, COGNITION used the results to demonstrate to the Environment Agency that recovery is not warranted thus enabling sign off.
The NAPL Fabric Extraction System proved to be highly effective, recovering NAPL with almost no water (between 93-99% NAPL). The remedial target was achieved across all fourteen plots at the Willoughby Lane site over a two- and half-year period, with average sign-off time ranging from 3-12 months for each plot.
During the soil remediation and enabling works, grossly contaminated soil was excavated and stockpiled. Much of the soil was recovered from within and around the former gasworks infrastructure. It was originally anticipated that the material was too contaminated for treatment and re-use on site and would require disposal. Laboratory analysis showed very high concentrations of PAHs, including naphthalene.
A previous contractor who had carried out the slab/foundation breakout had commenced bioremediation trials but none of the additives appeared to show the required level of improvement. Cognition was engaged by Enfield Council to continue the trials but test further additives. The trials were expanded and larger test biopiles constructed to simulate more realistic conditions for bioremediation. Six different bioremediation additives/combinations of additives were assessed, including the more conventional approaches using indigenous bacteria, nutrients, soil conditioners and surfactants. Cognition has had considerable experience in using bioremediation additives for treatment of complex gasworks wastes and has employed a wide range of available specialist proprietary products including bacterial and fungal based hydrocarbon degraders, supplementary oxygen release products, micro- and macro-nutrients and vitamins. Many of these products have proved successful but on occasion are unable to deliver improvements across the full spectrum of contaminants; naphthalene has often proved difficult to bioremediate. This is because PAHs generally have low water solubility, high hydrophobicity and low bioavailability; PAHs are also very stable and highly toxic. An unfortunate property of naphthalene is its propensity to volatilise causing odour issues which are typically associated with gasworks sites.
Cognition worked closely with a suppliers to trial a newly engineered additive with bacterial strains formulated to effectively degrade both aromatic and aliphatic hydrocarbons whilst thriving in harsh conditions. This advanced formulation not only breaks down the concentrated hydrocarbon polluted soil, but also activates the indigenous bacteria to supplement the bioremediation process.
Prior to commencing the trials, baseline sampling was undertaken to confirm the levels of contaminants of concern; samples were taken fortnightly to allow accurate degradation curves to be plotted specific to each trial. Notes were also taken on the visual appearance of the material and the odour score to allow comparison with the regulatory approved non‐analytical acceptability criteria for bulk fill materials for the site. Each biopile was monitored daily during the working week for temperature. A landfill gas analyser was used to monitor for carbon dioxide, oxygen and methane to assess the level of aerobic microbial activity within each biopile.
The results of the trials showed that the new additive delivered significant reductions in TPH and PAHs when compared with the conventional additives. As well as reducing contaminant concentrations, the soil was visibly improved and had significantly lower odour. This particular product had not been used on a remediation project in the UK at full scale previously.
Full scale bioremediation treatment
Following the success of the bioremediation trials, Cognition was contracted by Enfield Council to treat the stockpiled contaminated soil. The work was carried out under Cognition’s Mobile Plant Permit with a treatment area prepared away from site boundaries and sensitive receptors, including neighbouring residential properties and a school. Odour suppression units were deployed together with a contingency boundary odour management system in the event that odour thresholds were exceeded. An environmental monitoring plan put in place to ensure no adverse impact, particularly from dust/odours. The stockpiled material was then processed using screeners, excavators and allu mixing buckets to remove the concrete, metal and other deleterious waste and ensure the soil was broken up to enable the additives to sufficiently penetrate the material. A new bunded biopile was constructed immediately adjacent to the stockpiled material to minimise transportation and potential odour/dust mobilisation. The biopile, formed using a plastic across the base and lapped over the bund walls, had a capacity of 4,600m3. Material was placed over a 0.1m sacrificial layer of clean material and filled to a depth of approximately 1m.
A tractor-mounted specialist soil stabilisation spreader and mixer manufactured by Stehr was used to spread and mix the treatment additives into the contaminated soils, treating 0.5m layers at a time. The Stehr soil stabilisation grinder was used to ensure that the additives were applied uniformly and to break up and loosen the soil, improving aeration and encouraging aerobic activity.
For the purpose of verification, the biopile was divided into a grid with samples taken from each of the 20 grid squares to verify against the re-use criteria. Prior to commencing bioremediation baseline samples were taken and showed TPH concentrations at 49,000mg/kg, PAH at 6,500mg/kg and naphthalene at 1,100mg/kg. During treatment samples were taken fortnightly to allow degradation curves to be plotted. Visual appearance and odour were also assessed/scored to allow comparison with the non‐analytical acceptability criteria for bulk fill.
Cognition successfully met the bioremediation criteria in all grid squares within 8 weeks which was significantly quicker than achieved with similarly contaminated soil using conventional additives. Naphthalene, which was the risk driver, was reduced by 99% from an initial concentration of 1,108 mg/kg to just 9 mg/kg, which was x10 lower than the remedial criteria providing added comfort.
Early achievement of targets enabled the commencement of the new railway station construction and ensuring that the programme for the Meridian Water scheme remained on track.
The NAPL fabric extraction system has proved very effective at removing NAPL. A conventional pump-and-treat option would have been considerably more expensive to operate, highly energy demanding and resulted in thousands of cubic meters of water having to be pumped and treated and disposed of to the sewer. Often pump-and-treat fails to recover NAPL and simply ‘pulls’ the water from above/below the NAPL. These solutions also interfere with the groundwater regime and can had an adverse impact on groundwater hydrodynamics beyond the site boundary, changing the shape and extent of contaminant plumes.
Initial assessment of the contaminated soil on site suggested that it would likely need to be disposed of to a hazardous landfill site; bioremediation of the soil enabled it to be re-used on site saving move than a thousand lorry movements (export and import) in an already very congested area of London. The treatment additives worked quickly with minimum odour and nuisance to local residents.
Both technologies are being used at other remediation projects; the NAPL extraction system has been trialed at a nuclear site, where minimal impact on groundwater flow was critical, as well as at a number of former gasworks sites. The bio-additives are being trialed on a UN funded remediation project in Kuwait for treating the polluted oil fields, a legacy of the first Gulf War.
NAPL Fabric Extraction System
Mixing bioremediation additives
Spreading the additives into the soil