Wessex ploughs ahead with CSO treatment strategy despite regulation stalemate

Wessex Water is risking non-compliance to prove treatment with nature-based solutions is a viable option for storm overflows. The company’s director of environmental solutions, Ruth Barden, explains why it is worth the risk to “solve the problem rather than putting on a sticking plaster”.

Wessex Water has chosen to forge ahead with its preferred strategy of treating combined sewer overflows (CSOs) with nature-based solutions (NBS) even though the treated flows will still be considered spills by regulators.

Within its CSO investment for the PR24 price control the company has committed to delivering 28 schemes by the end of 2025, in the hope of providing the evidence base regulators say is needed to approve the option in the future.

Currently, the PR24 water industry national environment programme (WINEP) guidance does not allow the treatment of storm overflows, but the Department for Environment, Food and Rural Affairs (Defra) is prepared to consider the option if the industry can provide more data on the effectiveness of nature-based solutions.

Wessex has therefore chosen to invest in nature-based solutions during the next price control period, even though that approach will not meet current regulations to break the stalemate.

Ruth Barden, director of environmental solutions at Wessex Water, says that although the move is a risk, the company is convinced it is the right solution and has a decade of data to back up its beliefs.

“Defra have said they are willing to consider this but they want more data and evidence, so there is a bit of a circular argument here saying if you want more data and evidence then you have to have more sites and get the data and evidence from those sites.

“They will still be treated as storm overflows in that intervening period so essentially what we are saying is we are spending money on doing something that we think is right but ultimately it is not going to improve those sites in the way that it is currently assessed.”

Although many water companies would like the option to treat CSOs rather than just preventing them, Wessex Water has pushed particularly hard on the topic due to the topography of its operating area.

Groundwater inundation of foul sewers in its region is a problem due to the chalk and mudstone geology in the south east and north west, and the fluvial inundation of the Somerset Levels and Moors during wet winters.

Although Wessex proposed to spend £10 million in its drainage and wastewater management plan to reduce groundwater infiltration, it is not practical nor cost effective to seal the network to the level required.

Instead, treating dilute storm discharges with nature-based solutions to a similar or higher level than would be achieved through a wastewater treatment is a better solution, it argues.

“We have been incredibly persistent and we are convinced that this is the right solution and this should be the direction of travel that we are moving into, so let’s take the risk and demonstrate that it works.”

Wessex has plenty of data to back up its standpoint, having operated a reedbed at its Hanging Langford pumping station for over a decade. Like much of Wessex Water’s operating area, Hanging Langford suffers from water ingress into the public sewer due to it being at the bottom of a chalk valley with a high water table.

The three villages served are quite spaced out, says Barden, meaning there is a significant area of private drainage network that Wessex doesn’t own through which the high water table can enter the sewer.

Added to this, during the winter residents are forced to direct surface water down manholes to protect their properties from flooding.

“We used to have real problems with flooding people’s properties and gardens, and with permission from the EA we could pump out sewage from the pumping station into the water course, but that wasn’t satisfactory for anyone,” says Barden.

“What we agreed to do was construct a reed bed and instead of pumping out into the river we would treat the excess flows through this reed bed and it would be discharged, treated, into the river.

“What that would do is, certainly if not resolve the flooding, ease the flooding and loss of service for the properties and ensure there was a level of treatment on this discharge prior to it going into the river.”

The reedbed was constructed next to one of the lakes in the Langford Lakes nature reserve in 2011 and has been monitored for over a decade. Results show that the reedbed removes 72% of the suspended solids and 88% of the E-Coli bacteria.

“We’ve got really good data that shows that we have no environmental impact from this treated discharge and that’s partly because it is quite dilute due to the amount of groundwater going into it but also the reedbed does offer a reasonable level of treatment,” says Barden.

“It would easily comply with the permits of both the upstream and downstream sewage treatment works,” she adds.