Engineering natural processes to pre-treat water
Project Overview
Witches Oak Water is a series of gravel pit lakes southeast of Derby, alongside the River Trent. The 80ha site, part of a larger sand and gravel quarry, was acquired by Severn Trent Water more than 20 years ago to store water as part of a drought resilience project.
Severn Trent constructed an intake, a pumping station and a pipeline to its nearby treatment works at Church Wilne , but the quality of the raw water proved disappointing. It required more treatment to meet drinking water standards than had been anticipated, so abstraction soon ceased and the site was mothballed until a solution could be found.
Now the utility company has decided to bring Witches Oak Water back into operation by recommissioning the existing infrastructure and increasing the available raw water supply by up to 90 Ml/d. This additional water will improve Severn Trent’s resilience to hot and dry summers and secure water resources for future generations.
This project will not only see more water for our region, but the fact that we’re turning to nature to help pre-treat the water will mean we’re doing this in the most sustainable way possible and could help change how the [water] industry looks to be carbon friendly when it comes to water treatment.Green recovery delivery business lead, Severn Trent
A natural, biological solution in the form of floating wetlands to provide preliminary water treatment is being combined with refurbishing the existing abstraction infrastructure at the lakes to supply water to a new treatment facility being constructed near Church Wilne.
Our bespoke management plan initially focused on the construction and launch of three trial wetlands as well as the removal of a redundant human-made wetland from the lake. We have since installed a further 28 floating wetlands on the three northernmost lakes and have now switched our focus to upgrading the existing pumping station and river intake, and the installation of a fish recovery system and three new transfer pumps.
Helping nature to help us
Severn Trent is keen to explore the potential of low carbon, natural, biological water treatment processes in conjunction with conventional, chemical treatment to remove pollutants from the water abstracted from the River Trent. Historical data reveals nitrate levels in the Trent have occasionally exceeded the 50Mg/l concentration value prescribed by the Drinking Water Inspectorate and regularly breached Seven Trent’s own, more stringent standard.
This is the first time in the UK that wetlands have been used for operational water pre-treatment on this scale. Theoretical studies show that, on average, these floating wetlands could remove 40% to 60% of both total nitrogen (ammonia, organic and oxidised nitrogen) and total suspended solids (particles larger than two microns), considerably reducing the amount of energy and chemicals needed for the subsequent traditional treatment process.
The floating wetlands can remove up to 60% of both total nitrogen and suspended solids
Floating wetlands were selected over conventional or static wetlands because Witches Oak Water floods regularly during the winter. The pontoons can rise and fall with the water level, so are resilient to changes in load and seasonal fluctuations, whereas a static wetland would be submerged during a flood event, diminishing the processing provided by the vegetation. This was thoroughly tested over the winter of 2023/24, when the site experienced extensive one-in-100-year flood events. The floating solution also typically requires less maintenance, including no watering or fertilisation.
The Biomatrix floating ecosystems modules selected for Witches Oak mimic elements of self-sustaining, naturally occurring riparian wetlands. Water quality is improved by the root mass, which provides a large surface area for the microbial communities needed to remove pollutants. Planted with several types of sedge, hard rush and flowering species, including purple loosestrife and meadowsweet, these islands provide biological treatment to reduce the turbidity and nitrates in the raw water.
Installing the wetlands
Thirty-one floating treatment wetlands have been positioned in the natural water flow path in three lakes. The first three wetlands were launched on the middle lake (Lake 2) in December 2022, with the remaining 28 installed across the three lakes during summer 2023.
This is a true ‘One Mott MacDonald’ project: Members of our consultancy teams worked on the feasibility of using floating wetlands to pre-treat water. They then worked on their outline design and design development, through early contractor involvement. The project was delivered by our integrated design and construction business, Mott MacDonald Bentley.
The first three wetlands were a trial before we went ahead and launched the remaining twenty-eight. They were in position for the whole winter season to ensure the anchoring system could withstand the wintery conditions at Witches Oak.Project leader, Mott MacDonald Bentley
Design included measures to minimise the environmental impacts of construction itself. At the site compound and in areas trafficked areas by heavy equipment, such as the launch area, temporary track mats were used to minimise ground compaction and erosion. This allowed those areas to return rapidly to grassland when work was completed. A silt curtain was erected in the lake to prevent debris from removal of an old manmade wetland polluting the water. This was important because that wetland was colonised by invasive species, mostly Himalayan balsam on land and zebra mussels below water. The curtain ensured that sediment disturbed by the removal work did not spread.
Three trial wetlands were installed in December 2022
The remaining 28 wetlands were installed across the three lakes during summer 2023
Bringing existing infrastructure up to date
After launching and anchoring the 31 floating wetlands, we have moved on to refurbishing the abstraction pumping station built in the 1990s, to enable raw water to be supplied to the new Witches Oak water treatment works.
Located on the western shore of Lake 3, the pumping station is undergoing a complete refurbishment, including the installation of three new transfer pumps to deliver up to 90Ml/d of pre-treated water – enough for a city the size of Derby.
Barbel, bream, chub and roach are common fish species found in the River Trent, and two new screens will be fitted to filter fish – as well as debris – out of the water drawn in by the pumps at Witches Oak Water.
The fish recovery system will consist of an above ground pipe connecting the pumping station intake culverts to Aston Brook, which runs alongside the river, and from where they can eventually return to the Trent.
Our design layout for the pumping station includes a new water quality kiosk and a wash-water booster set to clean the fish screens.
We are also modifying the existing river intake in the northeast corner of Lake 1. Our design will protect both fish and the gravel lakes: Two new 2.5m x 4m concrete channels will be constructed to create a flow low enough to allow fish and eels to swim back to the river, while two large actuated intake penstocks will be installed to protect the lakes if there is a river pollution incident.
The introduction of new recirculation pipework will enable water to be pumped back to the river from the pumping station when necessary. Under our proposals, river water quality will in future be monitored upstream of the intake as well as at the intake itself.
Our plan for Witches Oak includes modernisation of the 1990s pumping station and associated structures as well as installing the floating treatment wetlands
Greater supply resilience
Witches Oak is one of seven projects that form Severn Trent’s green recovery programme. Overall, it is worth more than £700M and aims to develop low carbon water resources, improve river water quality, replace redundant lead pipes, install smart metering and make urban areas more resilient.
The additional water supply from Witches Oak will help meet demand for water from a growing population and improve the resilience of supplies during hotter, drier summers.
The 31 constructed wetlands use naturally occurring biological processes to contain and treat ammonia, suspended solids and trace metals. Coupled with installing ceramic membranes at Witches Oak water treatment works (to provide a filtration barrier, reduce turbidity and remove pathogens such as cryptosporidium), the project has substantially lower whole life carbon emissions than a conventional treatment solution.
By introducing enhanced natural functions to these gravel lake ecosystems, we are promoting biodiversity and ecosystem services. Above the water, the planted vegetation is a haven for wildlife, including nesting birds. Below the surface, the floating beds provide habitat for fish, eels and other aquatic species. The fringes of the lake support coppiced woodland that provides further wildlife habitat.
The wetlands are planted with several types of sedge, hard rush and flowering species, and are a haven for wildlife
The wetlands are planted with several types of sedge, hard rush and flowering species, and are a haven for wildlife
The wetlands are planted with several types of sedge, hard rush and flowering species, and are a haven for wildlife
The wetlands are planted with several types of sedge, hard rush and flowering species, and are a haven for wildlife
Positive social outcomes and community engagement are key elements of Severn Trent Water’s 25-year water resources management plan. Our delivery team presented to the local girl guide group to tell them about the work we’re doing, and a visitor engagement centre has been installed to showcase this exciting project to local school and community groups.
“This project will not only see more water for our region, but the fact that we’re turning to nature to help pre-treat the water will mean we’re doing this in the most sustainable way possible and could help change how the [water] industry looks to be carbon friendly when it comes to water treatment,” says Kye Ettridge, green recovery delivery business lead at Severn Trent.