Better by design: Creating ever-smarter SuDS

Jo Bradley knows good SuDS. And she can’t help but wonder what manufactured solutions we might add to these schemes to give nature a helping hand

When sustainable drainage systems (SuDS) were first introduced in the UK, many people working in water and environment questioned whether sub-surface manufactured devices could be a part of a truly sustainable scheme – because they were manufactured from plastic or concrete and didn’t mimic natural processes. I disagreed with that position then and I disagree with it now.

Designing drainage systems that rely entirely on natural processes would be fine if urban drainage only contained natural pollutants, such as leaf matter and soils. But urban runoff can contain a cocktail of toxic chemicals – from vehicles, factories, homes and commercial buildings, and infrastructure – so sometimes we need to include high-spec pollution-treatment devices to manage those chemicals.

We should be pleased that we can include the innovative, tested and proven treatment devices that are now available, many of which are manufactured in the UK and have been designed alongside SuDS practitioners.

The sediment suspended in urban runoff typically contains an array of pollutants, many of which will be toxic. That means a good management train will always include a sediment-removal device.

Sometimes, nature-based SuDS devices, such as a sediment forebay in a pond or a sediment-deposition area in a rain garden, will suffice, but when the sediment load is high and the chemicals within the sediment are toxic and bio-accumulative, it’s better to capture and remove them upstream of any nature-based SuDS devices. That way, pollutants can be isolated from the environment and from wildlife, and removed safely by a licenced waste-disposal contractor.

This serves two purposes: it allows the owners of the device to budget for maintenance costs using a known, regular figure that they can control and it protects the downstream devices from unnecessary build-up of sediment and from contamination with toxic substances.

These sediment-removal devices can include vortex grit separators and oil/water separators, which are installed below ground and can be emptied using a vacuum tanker.

Chemical help
In sites where the runoff is heavily contaminated, or where space is very limited, using flocculants to speed up the settlement may be a good idea. Flocculation involves the addition of chemical ingredients that cause particles to collect together into larger clusters. It’s a new process to the world of stormwater, but one that some SuDS experts have started to consider.

If the runoff is highly polluted, or if the receiving environment is particularly precious, it may also be necessary to include a manufactured device to capture dissolved pollutants. These can include stormwater filters and stormwater-treatment media.

There are numerous devices available that have been developed in partnership with specialists, some of which can target specific pollutants of concern. The best will have undergone extensive laboratory testing and had their manufacturing processes certified, which creates an excellent resource to support SuDS designers as they grapple with difficult pollution challenges.

Their manufacturers will thus have detailed, comprehensive knowledge of their devices and what they can offer you, and it’s worth inviting them in to deliver a continuing professional development session for you and your team. In my experience, the sales teams at manufacturing companies know a great deal about their products’ functions and capabilities, as well as the best ways to deploy them, which makes it worth enduring the sales pitch to be able to learn from – and question – the experts.

Despite the fabulous range of products already out there, there’s always room for more innovation. There are so many SuDS problems that we have yet to crack – so many opportunities to make the normal better.

I think flocculation may prove to be our new-found friend, allowing us to settle sediment in polluted runoff more quickly. But what about centrifugal forces? Or flotation? Can these processes also help us out?

For highly polluted urban runoff – especially the cocktail of chemicals that washes off our busiest highways – it’s vital that we capture and remove most of the toxic substances and keep them away from our ponds and wetlands.

An installation of treatment media in a filter drain alongside the M56 in Cheshire was specified to capture dissolved metals in the motorway runoff to protect a local nature reserve. Discussions among the client, the contractor and the manufacturer led to a bespoke design and the lessons learnt were shared to make future installations easier.

Two large vortex grit separators were retrofitted to a highway outfall that was believed to be a source of pollution in the local river. Due to the size of the devices, installation is no mean feat, requiring extensive excavation, but once in place, they capture roughly half of the suspended solids in the runoff, preventing it from entering the river. Like all pollution-treatment devices, they need to be emptied every few years, but that’s a straightforward process.

Drought-friendlier SuDS
And how about smart-irrigation systems to keep SuDS watered and vigorous during the long, hot, dry summers that we’re expecting climate change to bring? Is there a way to store water and pump it back to the surface with solar pumps when the sun shines? Or is it more cost-effective to wick the water up to the plants?

The best examples of this are living green walls that incorporate irrigation systems that use captured rainwater. These create dense, cool, damp environments in which birds and insects can thrive while also reducing the ambient temperature, making life more comfortable for people working and living nearby.

And perhaps we can find ways to keep leaves and grass cuttings out of the drains in nutrient-sensitive areas to reduce pollution. If we could find a way to use this unwanted plant matter – to create a value for it – then the collection equipment and systems could pay for themselves.

We could also consider temporary roofing that captures rainwater for re-use. We could design automated umbrellas that pop up when it rains and direct rainwater into storage systems then retract when the sun comes out again. There are lots of possible solutions to think about – and lots of people are thinking about them.

But a note of caution. We must be careful not to get carried away – not to lose sight of what SuDS stands for. Our focus must always be on environmental protection and creating meaningful habitats for wildlife.

We’re part of the urban ecosystem and must stand together with all of our neighbour species, creating SuDS schemes that also keep them safe.

This post was originally published in the Spring 2024 issue of The Environment.