Storage tanks are not a silver bullet for CSOs

Grey solutions such as storage tanks will not get water companies to the target for reducing combined storm overflow events but nature-based solutions such as drainage basins are “no-brainer” options for the industry.

That’s according to water companies that have carried out work to inform their strategies for reducing combined sewer overflow (CSO) spills during the upcoming AMP8 period.

Southern Water’s pathfinder lead, Keith Herbert, says: “Getting to the government target with storm tanks alone is unlikely to be successful.”

Herbert says 10-15 million litres of capacity was brought online at one of its key works as part of the company’s efforts to optimise its own assets.

“We had surveyed endless amounts of storage and we eagerly awaited a rainfall event,” he says. “It was full in about two to three hours; the flow went from 100 litres per second to 1,000 litres per second.

“The day after we had another spill because it simply hadn’t emptied in time. Storage can work in some instances. But as a general default rule, it shouldn’t be seen as the go-to option.

“We are not going to be able to build tanks big enough and I think we need to ask ourselves the question, is it really worth treating what can be 95% rainwater? Is it going to do our carbon footprint any good? And wrapped up with that is the ongoing operational costs of pumping cubic metres round and round in circles. That’s no way forward, it’s not going to be progressive.”

Wessex Water has also found storm tanks to be an expensive option at some of their sites, given their limitations, prompting it to look at other solutions.

At its water recycling centre at Shrewton, which suffers with groundwater induced flooding for several months of the year, 130,000 metres cubed of storage would have cost £80 million and would only have reduced spills by 10 events per year, according to the company’s chief operating officer John Thompson.

Instead, he said the “right option” has been for the company to explore nature-based solutions, with it eventually opting for two reed beds at Shrewton rather than traditional tanks.

“Nature-based solutions are not suitable for everywhere, but they have their place,” he says.

Steve Kaye, chief executive of UKWIR, agrees that water companies do not have the funds to be able to rely entirely on traditional grey solutions for reducing CSO spills.

“To engineer a solution in the traditional way will probably cost £300 billion. We haven’t got that so we are going to have to do it in a different way. We are going to have to talk to customers, look at customer behaviour, look at the infrastructure we have already got and look at things like nature-based solutions as opposed to concrete and steel,” he says.

Detention basins are a ‘massive’ opportunity

One nature-based approach which is providing a cost viable and effective solution to reducing CSO spills is the creation of detention basins by Severn Trent as part of its Mansfield Green Recovery Project.

The project is pumping £76 million into nature-based solutions to try to protect communities from flooding, while also reducing storm overflow activations.

It has a target of producing the network equivalent storage of 58 million litres of surface water. Detention basins are proving to be a very effective part of its strategy says Adam Boucher, operations lead – Green Recovery at Severn Trent.

“Detention basins are a massive opportunity for us as an industry, they are really cost effective and really simple, its just about having the available land and the technicalities of getting the water in and out,” he says.

Boucher says most water companies will have the right sites for detention basins within their own operating areas but warns that “these are the kind of opportunities that engineers will just walk past if you are not careful”.

The basins need planning permission, which involves establishing that the flood risk of the area will not change. To this end, Severn Trent is sitting in the offices at Mansfield District Council to complete the process.

Creating basins while also complying with the new biodiversity net gain rules will also be a challenge the industry will need to work on with Natural England, he says. However, they are quick to build and commission.

All the designs for Severn Trent’s detention basins have been scored as being low risk by the Royal Society for the Prevention of Accidents, as the 1:4 slope with a flat base allows for self-rescue, and even in heavy rainfall events water depths are restricted to a maximum of 1.2 metres.

Boucher says the company is particularly turning to detention basins to manage misconnections to its network. At the beginning of the project the team identified 50 sites where the surface water was dropping into the foul, but this has now been reduced to 10 sites, while also creating 10 million litres of storage for critical storm events.

This article first appeared in Utility Week‘s Digital Weekly edition. Click here to read the issue in full.