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No dig. No dump. Just a huge vacuum cleaner

EARTHMOVING

Faced with the task of clearing up a former Yorkshire gasworks, Secondsite called on the services of remediation specialist Celtic. Alasdair Reisner discovered that suction was the answer to the contamination problem

THIS site sucks. Literally. When remediation contractor Celtic was asked by Secondsite, formerly British Gas Properties, to clean up a former gas works site in West Yorkshire the firm decided the best solution was to get its vacuum cleaner out.

But this is no ordinary vacuum cleaner. When you are talking about clearing out decades' wor th of toxins compr ising polyaromat ic hyd rocarbons and tars produced when coal was heated up to produce gas, your average Dyson or Hoover is not going to cut the mustard.

Instead the firm is employing high-vacuum extraction, a heavy-duty process that literally sucks contamination out of the soil.

Celtic's Robin Edwards says: 'The contamination is essentially in three phases. You have soil contamination, the physical, solid contamination.

You then have liquid contamination ? light nonaqueous phase liquid or LNAPL. There is also dense, non-aqueous phase or DNAPL. DNAPL sinks while LNAPL stays at the surface.' While this sounds like a pretty nasty chemical soup, it is not a real hazard to the future use of the site. The main risk was to the groundwater that runs 5 m below the site. Celtic's role was to carry out a betterment exercise that would reduce the site's total loading to groundwater.

'Following a site investigation and risk analysis a 2 m cut was carried out across the site. This removed all the pipework and tanks f rom the old facility, ' says Mr Edwards.

By doing this the source of any further contamination was removed. Where possible any contaminated material was bioremediated using windrows on site to avoid having to remove it by lor ry to landf ill. Celt ic then set about sorting out the LNAPL and DNAPL that had seeped into the ground over the years.

This involved installing 86 wells across the site, centred on areas where the site investigation had indicated contamination. The 50 mm wells were sunk using an auger to depths of between 10 m and 20 m based on the depths the contamination was understood to have reached.

'Within the wells is our extraction system, ' explains Mr Edwards. 'There are two vacuum extraction systems that will pull liquid phase and vapou r phase. Each well is then at tached to a man ifold that can take six wells. These manifolds then go to the extraction system, which can be programmed to pull from different wells at different rates.' The ability to pull at different rates is vital to get the best performance out of the system. It is controlled by a bank of computers that gives Mr Edwards real-time feedback about what is going on across the site. While clearly Celtic will be keen to pull as much of the contam inat ion out as quickly as possible, there is a danger that, if you try to pull too fast, the viscous nature of the contamination may make 'strings' of contaminants snap, making further extraction more difficult.

'On one system I could be pulling for an hour on each manifold or I could pull 15 minutes on, 15 minutes off. It all depends on what the contamination is because the LNAPL will change. Some will have more naphthalene, others more benzene.

The stuff with more benzene in is the greater risk but I can target that, ' says Mr Edwards.

The HVE system can extract 760 cu m each hour, giving the Celtic team a formidable weapon in its fight to clear up the site. Most of that volume will be vapour, with a small amount of liquid. Once the material is pulled out of the wells the vapour and liquid are separated. The vapour goes through a catox or cataly tic oxidiser that oxidises the hydrocarbon contamination in the vapour in a self-sustaining reaction at 300 deg C. This process releases nothing but water vapour and CO 2 to the atmosphere.

The liquid phase of the contaminants is diver ted to Celtic's treatment plant. First an oil/water separator takes off the gross oils which are skimmed as they rise to the surface. A second oil/water separator has a clarifier to remove further oils. The remaining material then goes through a sand filter and a granulated activated carbon filter. The liquid that comes out of the GAC should have no more f ree product with less than 25 ppm of total petroleum hydrocarbon, as it and the sand filter remove any dissolved phases. This allows the Celt ic team to meet its consents agreed with the local water authority for discharge to the sewer system. Mr Edwards says the system easily beats that standard.

Mr Edwards is even trialling a bioremediation system on the products of the sand f ilter in order to reduce the amount of material going through the GAC, offering a cheaper and more environmentally friendly solution.

So were there any alternatives that could have been used to carry out the job or was HVE the firm favourite all the way through the process?

'We chose the HVE system because it's an intelligent, robust method. It has saved the client a lot of money and has produced an extremely effective outcome. It pulls both liquid and vapour phases whereas if we were just to sink wells and install down-well skimmers we would only be treating the liquid. There is vapour down there and we want to pull that out too.' One alternative would have been to just dig out all the contamination and dump it to landfill. Mr Edwards says this was not really a viable option, even without the increased costs and difficulties associated with the introduction of new regulations following the EU Landfill Directive last year.

'We could have done dig-and-dump but people don't really do that any more because of the number of lorry movements. And it would require major excavations, which can be very dangerous.' Mr Edwards believes such in-situ treatments are the way forward for the sector.

He says: 'There is a move towards in-situ remediation because I think that is what the Environment Agency wants to see. It is less hazardous, involves fewer vehicle movements and you don't have the costs of landfill.'

But Celtic is aware that it can only compete in a marketplace dictated by the clients: 'Celtic has a lot of experience of this kind of job.

If we are not the leading player in the market we are close to the top. There are other firms that do the job we do but I feel we are the firm that makes the most use of advanced technology. We are having to work against people that just br ing a sk id un it onto site and claim they can do the same job. They can't.' The effectiveness of Celtic's progress on the site is demonstrated by a set of diagrams showing current contamination across the site compared with earlier in the year, demonstrating a significant reduct ion in contamination. So will the team be able to completely rid the site of contamination?

'You will never be able to get rid of every thing but you are reducing the risk the site poses so it can be reused. There may still be some contaminants but they are not going to be mobile. Everything is based on the risk assessment. It's like crossing the road. You wouldn't cross a motorway because the risk is too high. But if it was a singlecarriage road you could because you have been able to assess the r isk and know it is safe.' The team hopes to be finished by December, leaving behind a clean site ready for redevelopment ? all thanks to a massive vacuum cleaner.