UK’s first installation of SDS Aqua-Xchange™.
Contractors BDB Special Projects have installed SDS Aqua-Xchange™, a new granular treatment media, into an existing filter drain along the M56 to prevent pollution of a vulnerable Cheshire stream, which receives runoff from a 1km stretch of the motorway via a single outfall. SDS Aqua-Xchange™ has been developed to capture and retain copper and zinc, poisonous metals released through the abrasion of vehicle tyres and brake pads, which dissolve in surface water during heavy rain.
As part of their Highways England Area 10 maintenance contract, designers from the Balfour Beatty Mott MacDonald (BBMM) Joint Venture first investigated the potential of SDS Aqua-Xchange™ to deal with pollution entering Mag Brook, a small stream that runs through farmland to the south of Lymm. Their risk assessment revealed the need to reduce the soluble zinc content in the runoff by a minimum of 7% and the soluble copper by 52%. Water from Mag Brook flows via another stream, Bradley Brook, into a small lake known as Lymm Dam, a popular local beauty spot and nature reserve; Mag Brook is a small tributary with limited water flow to dilute the runoff from the motorway.
The filter drains, that line the sides of the carriageways, are stone-filled trenches that provide a highly-effective and sustainable way of capturing the suspended solids in the runoff; they are, however, not capable of capturing soluble pollutants.
How it works
When added to the filter drain, a layer of SDS Aqua-Xchange™ uses the processes of adsorption and ionic exchange to capture the metals dissolved in the runoff as they filter through the material. The chemical process forms unbreakable bonds, so the pollutants are retained even in heavy storms and during winter road-salt applications.
BBMM appointed contractors BDB Special Projects Ltd, to complete the installation in November 2018. A total of 184 one cubic metre bags of Aqua-Xchange™ were delivered to site by SDS, which BDB set out at seven metre intervals along each side of the carriageway so that the correct volume of material could be applied evenly.
As there were cabled services in the existing filter drain, a vacuum excavator was used to remove the existing stone to the desired 400mm depth. A team followed directly behind to line the trench with a geotextile membrane, then a grab wagon was used to lift and discharge each cubic metre bag of Aqua-Xchange™ material into the trench and it was raked level. The old stone was taken away for cleaning and reuse.
The 250mm layer of Aqua-Xchange™ was covered with a geosynthentic grid for surface stabilisation, then with a final layer of clean stone, designed to capture initial suspended solids and gross pollutants before the water is treated by the Aqua-Xchange™ beneath. The water then continues to percolate through the non-woven geotextile wrap and filters through a further layer of stone before entering the perforated pipe at the bottom of the trench. The cleaned water is discharged into the Mag Brook.
BBMM’s risk assessment showed that Mag Brook was vulnerable to pollution because of the high traffic volumes travelling to and from Manchester and the airport. With no space to build SuDS ponds or other vegetative features behind the carriageway to treat the metals, or to install a manufactured stormwater filter to capture the pollutants, it would have been extremely problematic to mitigate the pollution risk otherwise. Using SDS Aqua-Xchange™ has also meant less excavation and less disruption, and there will be no need for maintenance during the 15 years design life.”
SDS Aqua-Xchange™ could now be installed into many hundreds of kilometres of existing filter drains that flank motorways and trunk roads across the country. Highways England has identified 2,500 high-risk pollution locations as part of its ongoing Priority Outfall Programme and is looking for practical and affordable treatment solutions.
The first installation of SDS Aqua-Xchange™ has been successfully completed on a busy stretch of the M56 motorway.
There are many hundreds of kilometres of filter drains throughout the UK, close to areas known to be at risk of metals pollution from runoff.
Further installations of SDS Aqua-Xchange™ are already in the design stages for locations across the UK.