Canadian Consulting Engineer
BIOSLURPING AT CNEngineering
Recovering the diesel oil that contaminates a CN rail site in Montreal has yielded 600,000 litres for re-use so far.Diesel must have been seeping into the ground at a Canadian National Railways site i...
Recovering the diesel oil that contaminates a CN rail site in Montreal has yielded 600,000 litres for re-use so far.
Diesel must have been seeping into the ground at a Canadian National Railways site in downtown Montreal at an astonishing rate. So far, clean-up operations on the Butler Spur along the St. Lawrence River have yielded almost 600,000 litres of the black stuff. Helene Richer, ing. of Golder Associates, consulting engineer on the project, laughs ruefully about the fact that they are still sucking up the oil plume: “We thought we would be finished by now,” she says, “but there is still no end in sight.”
The railyard was used for repairing and refuelling diesel trains for almost 30 years between the 1950s and 1980s. During that time a broken fuel pipeline was pouring diesel into the water table below. To complicate matters, the site was a municipal landfill for 100 years starting in the middle of the 19th century. The soils are permeable and the water table fluctuates between 7 and 8.5 metres below the surface.
By 1991, CN had discovered three diesel plumes totalling about 780 metres long below the rail track, and realized they were leaking downhill to the edge of the property. The site was too large to contain the plume by slurry walls or piles physically. Conventional “pump and treat” methods were also not too promising as they would have involved removing immense amounts of water.
CN and Golder decided to use “Bioslurping.” The method is also known by the more scientific but much less picturesque term, Vacuum Enhanced Recovery, or VER.”
Recovery and Recycling
Bioslurping is an in situ remediation treatment that evolved from vacuum dewatering techniques used in construction. Slots in the end of the slurp tube in the recovery wells enable the air and groundwater to be extracted simultaneously with the diesel. This product is technically known as free-phase, light non-aqueous phase liquid (NAPL). The mixing reduces the overall density of the oil mixture and makes the vacuum action much more efficient. And since the vacuum process creates a lot of air movement around the extraction tube, it encourages aerobic biodegradation of the hydrocarbons that remain in the soil.
The main advantage of the VER system over conventional pump and treatment methods is that much less water has to be extracted and treated. Another advantage of the system is that it operates in a cycle, switching between groups of wells, which reduces the number of pumps necessary to remove the oil product efficiently.
Once extracted, the vapour and liquid mixture goes to a pump and treatment unit on the surface with an air/liquid separator and oil/water separator. The oil is recovered into a holding tank, while the groundwater and vapour are reinjected into the ground. In some sites, the discharged water is sent through biofilters of peat moss and wood chips to further clean it, although this was not deemed necessary here. The recovered oil from the Butler Spur is shipped to a nearby cement plant to be used for fuel.
Each of the three diesel plumes at the Butler Spur has its own pump and treatment unit on the surface. These units service 122 extraction wells, and are spaced approximately 7.5 metres apart, spread in a long line over the plumes. Uphill, about 10 metres, are the reinjection wells consisting of 2-inch diameter PVC pipes straddling the water table. The system pumps at an average oil to water ratio ranging from 14:1 to 127:1, and with an operating efficiency of up to 85 per cent.
The Butler Spur system cost around $1 million to build and hundreds of thousands every year to maintain — roughly $6 per litre of recovered diesel product. However, Richer points out that it extracts much more fuel than a conventional pump and treat system could do in the same time, and uses less manual labour. A programmable logic controller (PLC) automatically controls and operates the three pump and treatment stations, and they are monitored remotely by staff.
Golder Associates’ Montreal office was heavily involved in all aspects of the project, from concept design to testing and construction and operation. At the time the project started, bioslurping was often used in the United States, where the Ground-water Remediation Technologies Analysis Centre, a government agency, supports it. In Canada, however, the method was basically an “R & D” project, says Richer, especially because the site was so large.
Today, CN has about 12 of these bioslurping recovery projects in the Montreal area alone. The Butler Spur, however, is still probably the largest treatment site in North America. And even though the engineers thought about 60 per cent of the plume was depleted by March last year, as Richer says, the diesel still keeps coming. — BL
Client: Canadian National; Normand Pellerin
Consulting engineer: Golder Associates;
Hlen S. Richer, P.Eng., Pierre Beaudry, Martin Poulin
System manufacturer: SCG CCE