Contractors BDB Special Projects successfully installed SDS Aqua-Xchange™ into an existing filter drain along a 1km stretch of the M56 near Lymm. The new granular treatment media was developed by SDS to capture and retain copper and zinc, poisonous metals released through the abrasion of vehicle tyres and brake pads that dissolve in surface water during heavy rain.
Metals pollution in rivers and streams is known to be directly toxic to fish and insects. The Environment Agency has identified more than a hundred water bodies in England that are currently polluted with either copper or zinc, and some with both.
Filter drains are stone-filled trenches that provide a highly-effective and sustainable way of capturing suspended solids as they run off to the sides of the carriageway. However, they are not capable of capturing soluble pollutants. 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.
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.
As part of the continuous improvement programme for the Highways England Area 10, a study was completed of the Mag Brook outfall, a small tributary that runs through farmland to the south of Lymm. 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.
Jo Bradley, Market Development Manager and water quality specialist for SDS, worked closely on the M56 project from its earliest stages, through its first successful installation. She said: "The risk assessment revealed the stream was vulnerable to pollution because of the high traffic volumes travelling to and from Manchester and the airport. There was very little water flow in the brook to dilute the runoff from the motorway. It confirmed a treatment scheme was needed to clean the water significantly, reducing both the soluble copper and zinc levels to bring them within permitted levels.
"There was 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. So, it would have been extremely problematic to mitigate the pollution risk otherwise. Using SDS Aqua-Xchange™ also meant less excavation and less disruption. There will also be no need for maintenance during the 12 to 15 years design life."
Contractors BDB Special Projects Ltd completed the installation during ten overnight closures. 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 Mag Brook.
Ben Dobson, Director of BDB said: "Through our experience in filter drain refurbishment works, we understand that detailed planning is key to successful and safe delivery. By making a few small changes, introducing Aqua-Xchange™ into the process became seamless, leading to efficient outputs not dissimilar to traditional filter drain replacement methods.
"It was very refreshing to work closely with the SDS team and to experience their real commitment to seeing the environmental benefits of this product being realised."