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AMP6 Oswestry Water Treatment Works - Quality Improvements Scheme

Value £54m

Client: United Utilities

Completion: Summer 2017

The AMP6 Oswestry Water Treatment Works (WTW) treats raw water from Lake Vyrnwy Reservoir; providing drinking quality water to one million domestic, commercial and industrial customers in Liverpool and parts of Cheshire. The raw water is fed 18 miles by gravity aqueducts from Lake Vyrnwy Reservoir, to the WTW from where treated water is gravity fed 50 miles by aqueduct pipelines to the supply service reservoirs in Merseyside and Cheshire. Once the upgrade project is completed the works will be able to provide up to 210Ml/d of drinking water to the latest standards, well into the future.

The technical stuff:

  • Improving manganese removal
  • Improving iron compliance
  • Reducing occurrences of discoloured water in the areas supplied by the works
  • Reducing the risk of bacteriological failures at the WTW
  • Restoring the WTW production output capacity to 210Ml/d
  • Improving overall control optimisation and efficiency

The on-site construction works started early summer 2015 and including commissioning and performance trials, the project will be completed in summer 2017.

Process design

The process design of the new plant includes:

  • Provision of a new clarification process stage (comprising coagulation, flocculation and settlement) to improve colour removal from the water with pre-oxidation using Chlorine Dioxide (ClO2) to help remove manganese
  • Modification and upgrade to the existing Rapid Gravity Filters (RGF’s) from direct filtration to second stage filtration
  • Disinfection within a new contact tank with pH optimisation using lime water
  • A new clearwater tank provides on-site storage of treated water for security of supply

Once the Oswestry WTW upgrade project is complete, the existing direct filtration will have been superseded and the existing slow sand filters retired.

Chlorine dioxide process

The process includes oxidation of manganese in the raw water using chlorine dioxide, a powerful oxidant which will facilitate manganese removal. Chlorine dioxide is generated on site using a reaction of PurateTM (a stabilised solution of sodium chlorate and hydrogen peroxide) and sulphuric acid.

On site hydro-generation

The Kaplan type turbine is being provided to supply energy for the plant and off-site export. This together with the ecological precautions being taken to minimise the effect on local flora/fauna reflects the intentions for an environmentally friendly project.


The use of Design for Manufacture and Assembly (DfMA) has been extensively used in the project. From concept stage the works have been developed to minimise effort on-site and seek production of all products that can be pre-made off-site. This means that less people are working on-site providing efficiencies and improvements in health, safety, quality and drive the programme. Overall this creates safer and more efficient construction.

Over 11,000T of concrete has been precast off site for the project, with more than 700T of steel products fabricated and assembled before delivery. As a result of DfMA the amount of man hours not done on-site totals at 7,000hrs.

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