By Jo Bradley, market development manager, SDS Limited
WHEN it comes to preventing surface water pollution, Scotland leads the way. Scotland’s wide-scale use of sustainable drainage systems has delivered more installations, more examples of best practice – and probably more lessons learned – than anywhere else in the UK.
Scottish regulations and guidance have prioritised surface water quality for more than 10 years, when elsewhere policymakers, particularly in England, have focused on controlling water quantity. It’s easily overlooked that although surface water causes flooding, it also causes pollution, and the principles of SuDS, quite rightly, put quality and quantity on an equal footing.
Before joining SDS, I spent 25 years at the Environment Agency getting up-close and personal with surface water pollution and highway run-off in particular. I know that SuDS are the best way to treat surface water, so it is refreshing that in Scotland it’s almost always a legal requirement they should be used to drain new developments as well as being used extensively for highway drainage schemes.
Scotland’s success reflects the fact that it has a robust legal framework, sound technical guidance and an excellent, multi-agency SuDS Working Party. Preventing surface water pollution is most important for commercial and infrastructure developments, for example where there are large numbers of vehicle movements, and for heavily trafficked urban highways, trunk roads and motorways.
When designing a SuDS scheme for such high-risk sites, the trick is to understand the pollution being treated. Surface water is contaminated by gross pollutants such as oils, grit, silt and sediment that get picked up in the water when it rains heavily.
Hazardous pollutants from brake and tyre erosion, exhaust fumes and oil spills are carried in the runoff, bind to the sediment and some get dissolved in the water itself. So, the SuDS must remove both the sediment and the soluble pollutants to properly protect the environment. The pollutants of most concern for the aquatic environment and for human health are metals, particularly copper and zinc, together with a spectrum of chemicals known as polyaromatic hydrocarbons (PAHs), including benzo[a]pyrene.
It’s likely that most contractors, developers and engineering designers in Scotland have worked on SuDS projects. However, I’d be prepared to wager they do not all share the same definition of what SuDS are; there’s still plenty of myth-busting to do.
SuDS mimic natural drainage processes by dealing with surface water as close as possible to where the rain falls and, wherever possible, incorporate vegetative devices to deliver multiple benefits to the environment. A good SuDS management train will often combine above-ground, vegetative devices with manufactured components such as below-ground storage crates, filtration and separation systems that can actually enhance and enable the most sustainable outcomes.
In Scotland, the widespread use of filter drains across the road network addresses surface water pollution particularly well. Filter drains provide hundreds of cubic metres of treatment capacity, without taking up a large land area, because they are installed along the entire length of the carriageway, instead of as an end-of-pipe treatment device. Where land is readily available, ponds and wetlands can also provide excellent water treatment; manufactured devices can help to deliver effective SuDS where space is limited.
Manufactured devices can support the function and operation of vegetative devices by enabling them to operate for much longer before they need to be cleaned out. A hydrodynamic separator, such as an SDS Aqua-Swirl (pictured above), reduces the pollutant load entering a pond or basin and prevents clogging. As a result the surface water management train operates effectively and maintenance is less frequent, well-planned and cost-effective.
SuDS construction and maintenance
There can be a perception that installing SuDS is costly. Careful use of components may offer pragmatic solutions. For example, an SDS Aqua-Swirl can replace over 40 gully pots by the side of a road, reducing construction and maintenance costs. Choosing a manufactured device made from plastic rather than concrete also avoids the use of heavy lifting machinery, making installation quicker and cheaper.
All SuDS are precision-engineered and employing experienced contractors is vital to ensure high quality construction and proper maintenance to avoid pollution from the site during the construction phase. Establishing a lifelong maintenance schedule is also essential so SuDS continue to operate to their design performance. Manufacturers should be expected to provide independent certifications to verify operating performance and to advise on maintenance schedules appropriate to the site.