We have constructed two new reedbeds at Shrewton’s water recycling centre in Wiltshire to help naturally treat the increasing amount of stormwater arriving at the site.
Using nature-based methods such as the reedbeds are part of our drive to reduce the number of hours of discharges from storm overflows by 25% by 2025 and to eliminate any harm to the receiving environment.
Why it was needed
The storm overflow at Shrewton water recycling centre often operated for long periods of time, due to the level of groundwater entering the public and private sewer system in the village, often through gaps in the system caused by tree roots or at the pipe joints.
Over the winter and spring, the water table is higher than the public sewerage system which we operate, and the private drainage systems which are the responsibility of the homeowners. This means that the system is dominated by groundwater, which mixes with foul water, and is pumped to the sewage system for treatment.
Analysis showed that the overflow discharges into the River Till were often cleaner than fully treated sewage works effluent and made up of over 99 per cent groundwater, which made it a suitable candidate for an additional natural treatment method, such as a reedbed.
Identifying the problem
The chalk geology around the village is good for holding rainwater and drains quickly. But once it becomes saturated, particularly in the winter and early spring months, the water table often rises above both our public sewer system network and the systems of private homes in the village.
This was causing significantly more groundwater to mix with the foul water (from homes and businesses) in Shrewton and flow to the water recycling centre via the pipes.
Our investigations showed that during the winter months, sewer flows to the water recycling centre at Shrewton were sometimes more than four times higher than normal.
To prevent properties from flooding and diluted sewage from overflowing into streets and homes locally a storm overflow automatically released the excess diluted storm water into the Winterbourne section of the nearby River Till.
High groundwater levels are responsible for half of storm overflow discharges in our region.
Tackling the problem
We have has spent hundreds of thousands of pounds sealing pipes and renovating manholes on the public sewer system in Shrewton since 2016. By the end of 2024, nearly a mile of sewers will have been sealed to try and prevent surface water from entering the pipes.
A new rising main (pumped sewer) had also previously been laid to the water recycling centre to screen water flows before they passed to a storm tank.
But without the power to seal the privately owned pipework, which makes up the majority of the drainage pipes in Shrewton, a large amount of groundwater was still getting into the combined sewer system causing the overflow to operate.
Adding a natural solution
Two new reedbeds were built at the water recycling centre in Shrewton but, due to the very dry summer in 2022, it took much longer than usual for increased flows to arrive at the water recycling centre. The storm water arriving at the centre is screened as normal before being settled in a storage tank.
While this is called ‘raw sewage’ by campaigners, it is already cleaner than typical treated sewage, needs little additional treatment and already complies with the standards that are set out for water being discharged from the Shrewton site by the Environment Agency.
However, the water flows into the two new reedbeds which naturally clean the water further before it is discharged into the River Till.
Protecting the environment
Twice-monthly monitoring of the river upstream and downstream of the overflow has shown that there is absolutely no adverse impact on the river resulting from using the reedbeds.
Further downstream, the variety of both pollution sensitive and rare species, including invertebrates and beetle and fly larvae, recorded at the site also indicates that the outfall is not causing a threat to the aquatic life of the River Till.
Rolling out the solution
We’re working with our regulators to design nature-based solutions, such as reedbeds, to provide further treatment to storm overflows operating in areas where there is lots of groundwater infiltration into the system, such as Shrewton.
We have a plan to deliver 28 of such nature-based treatment solutions before 2025. Since these solutions have significant biodiversity benefits, we are planning to work in partnership with local groups such as wildlife and river trusts to ensure we maximise the benefits.
We are committed to completely eliminating the discharge of untreated sewage, investing more than £150 million between 2020 and 2025 to tackle the issue.
