Mike Wilyman, remediation director, Carl Bro Engineering Consultants and Guy Pomphrey, manager, DEC Environmental Contractors, outline the benefits of soil washing
The technique is best suited for soils with a high granular content, typically below 30% fines is ideal. It can also be used as a process to treat hazardous waste, making it suitable for local landfill disposal. How it works
First the contaminated soil is screened to remove oversize material which is treated separately for reuse. It is then passed to the first stage of the process, a soil scrubbing unit. A water-wash is fed through it, which may contain detergents to alter pH and remove organics and heavy chemicals. Some of the pollution may dissolve or float to the top. The dirty water is recycled and either reused in the scrubbing unit or discharged.
Coarser fraction gravels are separated at this stage and are usually reusable. The remaining mixture of water, sand and fines is pumped through sieves, mixing blades, a hydrocyclone and water sprays. This gravity separation and washing technique separates particles with high or low specific gravity, such as compounds containing heavy metals.
Depending on the nature of the contamination, it is sometimes necessary to carry out sequential washing using different wash formulations or concentrations in order to remove all of the different types of contaminants. Eventually, the soil is separated into clean sand and a sludge that contains the contaminants. This material is then fed through a filter press to reduce the water content and produce a cake which can be handled and is suitable for transportation.
The cake material is normally removed to secure landfill. However, depending on the scale of the project, alternative treatment options for this material are also available. The cleaned sands and gravels are usually re-used on site or sold as an aggregate.What are the benefits?
Soil washing provides a cost-effective alternative to disposal, with typical processing costs of £7-15/tonne. Soil washing can be undertaken on relatively fixed programme schedules, regardless of weather conditions, reducing overall risk and potential delays. Also, it can be undertaken on relatively small sites and in areas with high disposal costs.
There are also significant environmental benefits, particularly a huge reduction in the volume of heavy traffic required. The volume of contaminated material that needs to go to landfill is also greatly reduced. In addition, as all cleaned soils are suitable for reuse, there is usually a reduced need to import materials to replace landfilled material. Also, his material is often sourced from quarries, identified in themselves to be a non-renewable resource. Transport requirements to and from a quarry is also significantly reduced.
Soil washing is suitable for use in sensitive developed areas and can often reduce the overall impact associated with remediation of a site such as noise and dust issues.Soil washing in the UK
Carl Bro is currently undertaking soil washing at a gasworks site in Elgin, Scotland, on behalf of SecondSite Property. This is the first soil washing remediation in Scotland and one of only a handful UK-wide.
"At this site we have effectively saved over 1000 truck movements through Elgin and the Scottish Highlands to the nearest landfill that can accept this waste, which is over 200 miles away near Glasgow," says Carl Bro remediation director UK Mike Wilyman.
"As well as a reduction in traffic, the solution has resulted in a reduction in disposal to landfill space and a significant reduction in the quarried materials that would have been required to replace any landfilled soils.
"As a result the process demonstrates a sustainable and viable alternative to the traditional 'dig and dump' solution and, contrary to current industry perception, can be applied to small, environmentally sensitive sites for the treatment of relatively small volumes of waste.
Nicola Paton, contaminated land officer at Moray Council, says: "The council is always keen to explore sustainable technologies and believes that alternatives to landfilling should be encouraged. There has been a lot of interest in the soil washing equipment applied in Moray, not only locally, but also from representatives of many other Scottish local authorities. Hopefully this level of interest in sustainable solutions will lead to their increased use in the future."