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Blackburn and Darwen WwTWs (2022)

Completion of the £160m Water Framework Directive (Blackburn Batch) project to construct Europe’s largest purpose-built Nereda® process plant

Aerial view of the Blackburn Nereda plant and tertiary treatment plant at Blackburn WwTW - Courtesy of United Utilities

Both Blackburn WwTW and Darwen WwTW discharge final and storm effluent to watercourses that are tributaries of the River Darwen, feeding into the River Ribble estuary and the bathing waters of the Fylde coast in the Irish Sea. At Blackburn WwTW, which provides treatment for a population equivalent (PE) of approximately 400,000, construction of the new works is now complete and the focus of United Utilities, in partnership with Laing O’Rouke and Atkins, has moved on to optimising and commissioning the largest Nereda® plant in Europe. This case study is the sixth in a series of articles published by UK Water Projects detailing the development of the United Utilities’ sites at Blackburn, Darwen and Nabs Head.


LiMA, (Laing O’Rourke), one of United Utilities AMP6 Construction Delivery Partners, was appointed in December 2017 as main contractor for the WPD Blackburn Batch which covers:

  1. Blackburn Wastewater Treatment Works.
  2. Darwen Wastewater Treatment Works.
  3. Darwen Pipeline.
  4. Nabs Head Inlet Works.

Significant improvement works at the above four sites have already addressed the AMP6 and AMP7 requirements to improve the river and bathing water quality and the project is reaching the later stages of delivery in 2022.

Nabs Head - storm storage and pumping station - Courtesy of United Utilities

Nabs Head – storm storage and pumping station – Courtesy of United Utilities


Built in the 1960s, the works treats principally domestic crude sewage from the Blackburn catchment, with two major additional contributors; a high strength trade effluent from the InBev brewery and an ammonia rich return liquor flow from the sludge treatment plant. The key river water quality/water framework directive parameters within the revised environmental permit require the need to achieve a BOD of 7 mg/l and ammonia of 1 mg/l both on a 95%ile basis.

Blackburn & Darwen WwTWs: Supply chain – key participants

Progress summary

The solution is taking advantage of the natural topography of the site and avoiding the need for the ongoing costs of interstage pumping resulting in lower power consumption and operating costs linked in with United Utilities’ targets for net zero carbon.

Blackburn WwTW import screening facility - Courtesy of United Utilities

Blackburn WwTW import screening facility – Courtesy of United Utilities

In the last 12 months the project team have:

  • Blackburn WwTW: Continued to optimise the 72,000m3 Nereda® plant, the permanent Mecana cloth filtration, the inlet works, the new ferric dosing and UV plant.
  • Darwen WwTW: Converted the existing settlement tanks into stormwater storage and connectivity to allow gravity flow to Blackburn WwTW.
  • Nabs Head Inlet Works: Continued to operate the new 23,100m3 stormwater storage tank and pumping station, refurbished four Longwood escalator screens, four Kuhn wash presses, two detritors, grit pumps and classifiers.
  • Darwen Pipeline: Completed the 1.68km transfer pipeline, which included culvert crossings on Branch Road, a section on Cross Street, tunnelling under the M65 and a bifurcation chamber.

Blackburn WwTW

The new inlet channel to feed the 72,000m3 in situ concrete Nereda® tanks is complete and the final stage of the transfer of full flows to the new treatment process has now occurred.

The Nereda® system is now treating full flows of Blackburn WwTW’s crude sewage and has been developed to treat peak load at the works – i.e. a complete replacement and expansion of the existing wastewater treatment plant with a future flow to full treatment (FtFT) of approximately 3,135 l/s, (including flows from Darwen WwTW).

The works are now taking the forward full flows of 517 l/s from Darwen WwTW via the completed transfer main from Darwen WwTW to Nabs Head and onwards to Blackburn WwTW.

The permanent ferric dosing rig is now complete and was one of the last assets to be constructed on site, following the demolition of one of the original second stage filters after transfer of flows. The permanent Mecana filter block has been constructed with all ten filters in operation at the back end of the Nereda® plant.

Blackburn WwTW: New UV plant - Courtesy of United Utilities

Blackburn WwTW: New UV plant – Courtesy of United Utilities

The new UV disinfection system has been commissioned by Xylem Water Solutions, and was designed to treat the final effluent prior to discharge meeting the EA requirements for compliance with the Bathing Water and Shellfish Directives.

Migration work has been extremely complex, however, the use of a staged controlled ‘interim Authority to Operate’ (iATO) procedure has assisted with a controlled change over plan. Parameters were set at each stage of the migration to maintain compliance at every level.

The plant is currently in the final stages of being optimised after the introduction of the Darwen flows. The old works have been decommissioned and made safe with staff re-trained to operate the new process.

Darwen WwTW

The works was built in the 1960s and comprised an inlet works, primary settlement tanks, trickling filters and humus tanks.

A new energy intensive treatment process would have been required to achieve the treatment element of the Water Framework Directive (WFD) driver and the lowest cost solution identified was to abandon the works and transfer flows under gravity into the adjacent Blackburn drainage catchment; ultimately receiving treatment at the new Blackburn WwTW.

Re-purposed PSTs and MCC kiosk at Darwen - Courtesy of United Utilities

Re-purposed PSTs and MCC kiosk at Darwen – Courtesy of United Utilities

A ferric dosing system operating upstream of the primary settlement tanks was installed in November 2018 and successfully achieved the new phosphorous permit of 2mg/l, (now decommissioned after turn of flows).

The second WFD element at Darwen WwTW required the reduction of intermittent discharges by increasing the volume of stormwater storage to meet the required river water quality (FIS/RE 99%ile) standards. To achieve both regulatory requirements the flows to Darwen WwTW have been diverted to Blackburn WwTW via a newly commissioned transfer main. The Darwen works were re-purposed and existing primary tanks converted into 6,000m3 of stormwater storage. Redundant plant has now been decommissioned and made safe and the process site is being transferred to the network teams.

Darwen Pipeline

The pipeline has been completed by VJ Donegan Civil Engineering Ltd. This incorporated a 1.5m diameter tunnel beneath the M65 motorway before reverting back to a traditional gravity sewer construction as it passed alongside residential and business developments, a river and along Branch Road. The solution submitted at the tender stage for a 1.6km pipeline of 700mm diameter remained substantially unchanged, although some route realignment had to be made due to significant service congestion on the route.

Network modelling work was undertaken to confirm the stormwater storage requirements and ensure that flows diverted from Darwen into the Blackburn catchment cause no ‘level of service’ issues within the existing sewer network.

Darwen WwTW continuity pipeline - Courtesy of United Utilities

Darwen WwTW continuity pipeline – Courtesy of United Utilities

A Section 61 agreement was enabled with the support of Highways England for the pipeline to traverse under the M65 motorway, (16m underneath) to facilitate a key section of the pipeline route.

Further to this, a bifurcation chamber has been completed north-west of Ewood Park, connecting a high level transfer main to a low level transfer main to pass forward the full flows to Nabs Head and onwards to Blackburn WwTW.

Progress/current status

At the time of writing (July 2022), the Nereda® plant is now fully operational, with further optimisation work underway to fine tune its performance.

The migration of all the conventional process flows at Blackburn WwTW and new flows from Darwen WwTW have been successfully completed. The tertiary and UV plant build is built and operational. The build has substantially completed and focus is now on further enhancing plant performance, completion of final snagging/defects and seasonal reinstatement. These project enhancements are delivering the benefits required by our regulators to significantly enhance the local rivers, bathing waters and shellfish leading to the Fylde Coast significantly benefiting the environment.

Darwen WwTW: re-purposed PSTs with storm return pipework and pumps - Courtesy of United Utilities

Darwen WwTW: re-purposed PSTs with storm return pipework and pumps – Courtesy of United Utilities

United Utilities are already well progressed with works on the new AMP7 requirements to further enhance the performance of the works. This project is due to complete in 2022.

The editor and publishers would like to thank Kevin Moody, Whole Life Programme Manager, and Tony Elliott, Senior Project Manager, both with United Utilities, for providing the above article for publication.