Identifying the root causes of CSO spills

Published 21 February 2024

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Southern Water is tackling its worst-performing storm overflows ahead of PR24 to help shape a targeted approach. Pathfinder lead Keith Herbert updates Utility Week on the progress made so far.

Southern Water’s drive to reduce spills at its worst-performing combined sewer overflows (CSOs) has entailed optimising assets, installing water butts and becoming “absolute experts on our catchments”.

Using £35 million in accelerated funding, the company is working to fast-track the reduction in the use of storm overflows across the South East in a pathfinder project. The initiative has a target of saving 420 spills at 30 sites by March 2025.

Keith Herbert, pathfinder lead at Southern Water, says the work has already seen 30 spills avoided at some CSOs due to its interventions made to control rainwater. These include optimising assets, redirecting misconnections, and working with communities to site water butts and storage for roof run-off.

Small steps can equal big reductions

Herbert says the first difficulty the project had to overcome was choosing which CSO to target. Initially the team looked at the sites that spilled frequently and undertook connectivity work to understand the catchments and where the water was coming from. But that didn’t characterise the catchment or identify the root cause of the spill.

By installing an event duration monitor to measure the spill level in a sewer the team was able to start to chart how the CSO responded to rainfall and predict how much rainfall would result in a spill. The data showed that some of the worst performing sites had more than a 99% chance of spilling after 5mm/hr of rainfall.

“If you were a campaigner you would say that is completely unacceptable, and I would be in agreement with you. But if you were going to be optimistic then you would say that you don’t have to take a lot of stormwater out to make a big reduction on those spills.”

Early studies have shown that if 40% of the water is removed from the system then spills can be reduced by 80%. “But what the data also shows is the root cause of the spill. You can look at the calculations to see what percentage is rainfall causing that spill, and what is the background levels of infiltration, because background infiltration just eats at the capacity and is the real problem.”

Once identified, hydraulic models were used to locate the impermeable areas which were contributing to storm overflows, such as carparks, pavements, driveways and roads, with their stormwater connected into the foul sewer.

Herbert says this identification stage is critical, because the key to tackling CSO spills is designing for the area. “If the area is inundated with groundwater there is no use in putting in water butts because that’s only going to be effective if you have storm water. You need to diagnose a solution for each catchment.

“Probably the most important finding we have learnt is that we need to be absolute experts on our catchments, we need to understand our sewer networks in a way that we have not understood them before.”

For stormwater catchments a five-step process has been designed for tackling each area.

1. Optimisation

The process first considers whether Southern’s own assets can be optimised, by such actions as increasing the return, the pumping or the treatment, or by optimising the pumping. Southern has been able to reduce spills by between 30-40% just through optimisation, says Herbert.

A key finding of the project has been that while grey solutions are beneficial, storm tanks particularly should not be seen as the go-to option. Herbert says they can be effective at small rural works. “If you build a tank you would get a big reduction in spills, but if you have a big urban works with a flow of several hundred to 1,000 litres per second you are simply not going to be able to build a tank big enough, and once its full its not going to offer any benefit the next day.”

Another way Southern is exploring spill reduction using its existing assets is by prioritising less resilient sites, but this approach quickly puts water companies in breach of their permits, says Herbert. He would like to see a fast-track process for expanding on permits as currently sometimes “spill reduction and permit compliance are incompatible with each other,” he says.

“If we had area based permitting or more flexible permitting we could have an aggregate reduction in spills across a catchment, rather than all these little sites effectively batting for their average.”

2. Misconnections

Once its own sites have been optimised, Southern has looked to tackle misconnection of housing developments. “Misconnections are a big problem and are where developers have separated their stormwater, built the development, had no where to put it and so put it in the foul anyway. We find lots of these,” he says.

Herbert says that in some instances the flow from misconnections is so great that it has had to apply for flood risk assessment permits and there will be no obvious place to put the flow. “Although we haven’t really explored complete surface water separation in any detail, if someone has done it for us we might as well continue what they have started.”

3. Non-household SuDS

Southern has identified any building over 300 metres squared within a catchment area and carried out a connectivity survey on it. “If the roof drainage was found to be connected into the foul then we had a consultation with the owner to say look, we have a problem, let’s talk about how we help manage the drainage from your property, because you can get those really big numbers quickly.”

Typical buildings included industrial units, schools, supermarkets and churches, with solutions including planters or big storage tanks for industrial settings, with a view to slowing down or disconnecting the flow from Southern’s system.

“Of the 30 roofs that we managed within Gurnard, not one owner said they didn’t want to engage, but as soon as you start to work on stuff that is not our own, people are well within their rights to say they don’t want to.

“Expect to go to lots of public meetings and engage with lots of people to try and bring them on board with the problem because it is going to require water companies to be really active within the community.”

4. Highway schemes

Heat maps are used to identify problematic roads, with more connectivity surveys conducted to find out if they are connected into the sewer before local authorities are engaged. “Probably the most challenging part of the project is agreeing terms of reference and a memorandum of understanding with a local authority to manage their highways drainage for them,” says Herbert.

Southern has reached the final design stage and is currently working on agreeing details such as who is going to maintain, build and own the assets. “We have got many hectares that we can manage just by putting in things that I consider more attractive than the highways gulleys. We really want to get repeatable designs that we can saturate urban areas with.”

5. Household SuDS

In areas where the stormwater cannot be managed in any other way then the team has rolled out water butts to domestic properties on mass. These water butts have a valve drilled halfway down so the flow is slowly returned to the sewer, thereby ensuring capacity for any future stormwater.

“If we roll these out on mass we effectively get storm tanks out in the catchment and they are also a way to include and engage the public with what we are doing.”

This article first appeared in Utility Week’s Digital Weekly issue. Click here to read more.