OVERVIEW

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 provided a highly-effective and sustainable way of capturing the suspended solids in the runoff. However, they aren’t capable of capturing soluble pollutants.

WHAT WE DID

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 geosynthetic 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.

RESULTS

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™ also meant less excavation and less disruption, as well as no need for maintenance during the system’s 15 years design life.

The UK has many hundreds of kilometres of filter drains that are close to areas known to be at risk of metals pollution from runoff. Highways England has identified 2,500 high-risk pollution locations across the UK as part of its ongoing Priority Outfall Programme and further installations of SDS Aqua-Xchange™ are planned.