Supply Chain - Water Pumping Stations - Abstraction

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. Historic data revealed that nitrate levels in the River Trent have occasionally exceeded the 50 Mg/l concentration value prescribed by the DWI, and regularly breached Seven Trent’s own, more stringent standard.

As a result, 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 (called the Raw Project) to supply water to the new water treatment works being constructed in Church Wilne.

This is the first time in the UK that wetlands have been used for operational water pre-treatment on this scale and in the ‘real world’, but theoretical studies show that, on average, these floating wetlands could remove in the region of 40-60% of both total nitrogen (ammonia, organic and oxidised nitrogen) and total suspended solids (particles >2 microns), helping to considerably reduce the amount of energy and chemicals used in the traditional water treatment process.

Floating wetlands were selected over conventional or static wetlands because Witches Oak 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 process benefits provided by the vegetation. This has been thoroughly tested in winter 2023/24 when the site experienced extensive 1 in 100-year flood events. The floating solution also typically requires less maintenance, including no watering or fertilisation.

The Biomatrix floating ecosystems 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 meadow sweet, these islands provide biological treatment to reduce the turbidity and nitrates in the raw water.

In December 2022, Mott MacDonald Bentley initially focused on the construction and launch of three trial wetlands in Lake 2, as well as the removal of a redundant man-made wetland from the lake. Since then, a further 28 floating wetlands have been installed on the three northern most lakes during summer 2023.

The 31 floating treatment wetlands were positioned in the natural water flow path in Lakes 1, 2 and 3.

Work is also being undertaken to upgrade of the existing pumping station and river intake, along with the installation of a new fish recovery system and three new transfer pumps.

This is a true ‘One Mott MacDonald’ project with members of the consultancy teams having worked on the feasibility of the wetlands’ abilities to pre-treat water and then their outline design, before handing over to colleagues at Mott MacDonald Bentley through early contractor involvement and then design and build delivering the detailed design, pre-construction and construction activities.

To minimise the environmental impact, temporary track mats were used for the base of the works’ compound and launch area at Lake 2, allowing the site to return to managed grassland when the mats were lifted.

A silt curtain was erected in the lake to prevent debris from the old wetland polluting the water. This was important because the wetland was colonised by invasive species, mostly Himalayan balsam on land and zebra mussels below water. The curtain also ensured that sediment disturbed by the removal work did not spread.

The design has also had to accommodate regular site flooding; in particular for the temporary works around the river intake where, as an example, the cofferdam was designed specifically to account for controlled and safe flooding of the excavation to manage the high river levels experienced on-site.

Launching and anchoring the 31 floating wetlands was the first phase of the Raw Project. Phase two is refurbishing the abstraction pumping station built in the 1990s to enable raw water to be supplied to Witches Oak WTW.

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 96 Ml/d of pre-treated water; enough for a city the size of Derby.

The design layout for the station also includes a new water quality kiosk and a wash-water booster set to clean the fish screens.

The river intake in the north-east corner of Lake 1 is also more than 25 years old and is being modified. The design will help to 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 the proposals, river water quality will in future be monitored upstream of the intake as well as at the intake itself.

Barbel, bream, chub and roach are common fish species found in the River Trent and two new screens will be fitted to filter fish and 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 fish can eventually return to the Trent.

Witches Oak is one of seven projects that form Severn Trent’s Green Recovery Programme, worth more than £700m, which 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.

By restoring natural functions to these gravel lake ecosystems, the project is promoting biodiversity recovery and contributing to enhanced ecosystem services. The wetlands provide amazing biodiversity benefits including:

In addition, positive social outcomes and community engagement are key elements of Severn Trent Water’s 25-year water resources management plan. The 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.

The existing screening has coarse (9mm aperture) and fine (5mm aperture) screens within the pump house, which discharge debris and eels back to the river via a chute through the side wall.

The current screening will not meet 2025 statutory requirements and South West Water are addressing the issue now to ensure future compliance.

This project will deliver improved screening at the Pynes WTW intake with Eel Regulations compliance, which is required by the end of AMP7, ensuring continued operational abstraction at the site. There is a need to improve the exclusion of the freshwater stages of the critically endangered European eel from river intakes and abstractions allowing lifecycle completion at sea. Additionally, there is need for protection of downstream migrating salmonids. This includes both Atlantic salmon and brown (sea) trout.

The proposal was to install two 2.352m wide Hydrolox S1800 rotating band screens within the existing bankside infrastructure to replace the existing screening. This will reduce fish and eel entrapment at the intake and ensure compliance for continued operational transfer to Pynes Water Treatment Works. These screens also offer health and safety benefits, as well as OPEX and TOTEX cost benefits.

To allow the planned work to be undertaken, the inlet lanes had to be closed one at a time for the new screens to be installed and the old screens removed whilst still allowing for the 46,000m3/day abstraction. High water demands during the summer months had to be considered along with other consents and approval conditions, so the earliest date restrictions could be imposed was early September 2023. The work was therefore initially planned for a 4-month duration between September 2023 and January 2024.

During the installation period, South West Water had to allow for ‘recharge periods’ where both lanes could be open to allow for the 60,000m3/day abstraction. This would be over a weekend (Friday-Monday), and this allowed for a total of 50% of the installation period where recharge could be undertaken.

To be able to produce a feasible design, various surveys and reports had to be undertaken. These included dive surveys, ecological surveys, protected species surveys, GPR surveys, existing infrastructure surveys, 3D scans of the pump house building and underwater intake bays, and crane lift surveys.

The results of the surveys found the existing intake structure was structurally sound to be re-used for the new Hydrolox screens, which removed the need for using ‘wet’ concrete to construct the work, which would of entailed using a cofferdam. This removed a hazard from the new work within the design stage, benefitting both programme and cost savings.

Hydrolox S1800 rotating band screens were chosen for this scheme as they are considered an industry best practice solution by the Environment Agency. Benefits included:

The work site was approximately 330m from an unclassified single vehicle access road, and having to navigate down this road and track with oversize vehicles, along with having to contend with a section of Japanese knotweed was an early challenge. The rutted access track had to be regraded to accommodate the large vehicles and crane attendances along with protection of the knotweed to prevent cross contamination of the area across the area.

The site was extremely restricted and a 90t crane was required and had to be set up in a small area constrained by the existing building, the Network Rail mainline, 11Kva overhead power cables, outbuildings and mature trees.

Because the location was restricted and unsuitable for larger vehicles, the contractor had to change their methodology and bring the new structural elements in as individual components. These components were transferred onto smaller flatbed vehicles away from the site and reassembled in the small working area.

One of the first tasks was to remove the existing course trash screens and decking area over the River Exe intake in a safe and controlled manner, utilising a dive team to prop open the existing penstocks which remained in place until the scheme was complete and the equipment removed.

Upon successful removal of the existing equipment, the galvanised steel platforms (as required under DWI Regulation 31) were assembled on site and lowered into position within the existing reinforced concrete inlet structures and fixed with galvanised mechanical fixings. The steel platforms would house the two Hydrolox screens.

Other work streams involved mechanical and electrical interfaces, where decommissioning and removal of existing redundant equipment including the two sets of fine and coarse screens, wash water booster set, pipework and obsolete electrical equipment and cabling, all of which had to be phased in agreement with the Operations team.

Electrical scope included the installation of the instrumentation required to control the new screens and four ultrasonics, including a temperature sensor, and temporary isolation and installation of actuators for the external penstocks so that the new steel decking could be installed. This required good communication and agreement with the South West Water Operations team and the control centre.

The contractor, Fishtek Ltd, worked efficiently, continually keeping the project team informed with progress and any issues that arose to agree a solution to keep the project momentum. During the time that the contractor was on site, value engineering principals were applied. Examples of this include the reuse of existing equipment such as the launder chute where a partial section was manufactured and installed which reduced the material usage by some 80%.

Another example of the value engineering approach was to take most of the removed equipment to Pynes WTW, which would be available to be re-used for spares and parts replacement.

This benefitted South West Water from disposal costs and the costs of new parts.

Another positive outcome of the project was that Fishtek Ltd, engaged South West Water’s O&T team to undertake the instrumentation and controls scope. This worked extremely well to the benefit of all parties, setting a precedent for future collaborative working.

Good engagement with all stakeholders (EA, Network Rail, National Grid, landowners and South West Water’s Asset & Operational team) was maintained throughout the programme and there were zero complaints.

To get equipment and cranes onto site, the team required access via a landowner’s property so good relationship was developed and maintained throughout the project. By way of thanks, a new reinforced concrete slab was constructed at the field entrance as a good gesture for the landowner’s cooperation. Upon project completion, a final re-grading of the 330m track was undertaken, and the Japanese knotweed protection was kept in place.

Though the project was completed in an agreed revised programme due to several issues outside of the contractor’s control, the scheme was still completed almost one year ahead of the regulatory compliance date (March 2025).

The contractor demobilised from site in April 2024, with all that remains to be undertaken being the removal of one fine screen which was left in place at the request of the South West Water Operations team until the new screens had sufficient bedding in time, and after five river spate conditions of the River Exe had been experienced.

While on site, the contractor adopted a ‘see it, sort it’ ethos and several site operational safety improvements were identified and acted upon. Despite this additional work, the project scheme still came in 9% under budget.

Good health and safety was managed throughout the programme and the project was delivered with zero incidents or accidents.

The overall objective of the project was achieved and exceeded, gaining valuable lessons learnt for all involved so future schemes can be optimised throughout the project life cycle from conception through to construction and handover. This will allow for improved programme durations and activities, increased risk identification, improved tender scope and work information, as well as logistical restrictions and operational interface requirements.