Collaboration helps Severn Trent Water deliver high-quality drinking water in Derbyshire

We helped Severn Trent Water to overcome technology and project challenges and successfully complete a £2.4M water treatment works upgrade.

Severn Trent Water has completed a £2.4 million upgrade to its Little Eaton Water Treatment Works (WTW), helping to ensure high-quality drinking water provision for the Derbyshire area in a project that required technology and teamwork to overcome significant project challenges - and close collaboration between Severn Trent Water, design and build contractor MWH Treatment Ltd and ourselves dramatically reduced project timescales by more than six weeks and achieved significant construction, equipment and operating cost savings.

The project

Located North West of Derby, UK, Little Eaton WTW treats raw water from the River Derwent to supply the strategic grid with between 70 and 90 million litres per day (mld). Improvements to the clarification stage at the works were needed to increase the design capacity to 120 mld and replace life-expired plant that was presenting an increasing maintenance burden. At the same time, Severn Trent had to meet stringent Drinking Water Inspectorate (DWI) standards for cryptosporidium content in the treated water.

In a contract worth £1 million, we provided 16 specially modified Lamella Plate Pack Separators and four ZickertTM Reciprocating Sludge Scrapers to deliver the solids separation and removal required.

"The project’s first major challenge was to achieve the increased capacity within the plant’s existing reinforced concrete tank structures. The river turbidity at the inlet of the treatment works is usually under 5 NTU, but it can go as high as 50 NTU during heavy rainfall.  With such a broad range of turbidity, Lamellas were considered a favoured technology, however other clarification technologies were also seriously considered. We approached Hydro International and they came up with an innovative scheme to modify the Lamella plates to ensure increase separation capacity that could be incorporated within the same footprint."

- Stephen James, Programme Engineer, Severn Trent Water

In order to meet the requirement we developed new manufacturing and production techniques to increase the length of the Lamella plates to three metres, extending the conventional design by half a metre, and thereby increasing the total projected settling surface area to 1,856 m2.

"The longer plates offer the advantage of a greater settling surface within the same footprint.  A longer plate-to-width ratio also improves the clarification performance because there is more laminar flow between the plates, meaning smaller particles are able to settle. Extending lamella plates to three metres is very unusual indeed, and a first for the UK.  As part of the project we secured Regulation 31 approval for the design, which is a significant advance on the only other installation in the world that we’re aware of, which was in the United States more than 15 years ago."

- Paul Barter, Principal Process Engineer, Hydro International

Without the new design, to achieve the additional 30 mld output with a conventional Lamella design would have meant extending the existing tanks, with consequent civil engineering costs and time implications that would have been very difficult to accommodate within the project deadlines.

An installation challenge

Completing the works within the required timescales to meet DWI consent deadlines was also a critical target. The MWH team worked with our experts, conquering the first challenge by removing the legacy tube-type lamella plant and pipework then cleaning the tanks of residual sludge. 

"To achieve the project timescales, we came up with a solution to remove the roof from the existing tank structure.  Although this entailed some risk of weather-related delays, it enabled us to remove the existing plant by overhead crane and then build the Lamellas onsite in sight of the crane pad, before lifting them into place. With good collaboration, the solution knocked 10 weeks off the programme, reducing the initial timescales by more than half. Hydro International were very helpful in supporting this plan by making the required modifications to the Lamella and Zickert designs to enable a quick and smooth installation.  Their team also attended onsite to optimise the clarification process to ensure successful commissioning."

- Kelvin Smith, Project Manager, MWH

Assembly of Lamella plate packs entails sliding each plate into a framework.  At Little Eaton, the uniquely long plates made it even more cost-effective to deliver the Lamella packs ‘flat packed’, saving on transportation costs, and assemble them on-site. To accommodate the extra length and weight of the plates, we also made modifications to the Lamella framework, including fabricating a guiding V-notch frame in the bottom beam to reduce the sliding force. This made it quicker and easier to assemble the plates smoothly on site.  

Our team also modified the ZickertTM Sludge Scraper design for potable water application, ensuring that the hydraulic rams were positioned vertically above the water level to avoid any risk of contamination, using a pivot joint to transfer the motion smoothly to the horizontal scrapers on the tank bottom.

Teamwork

Close teamwork and collaboration between Severn Trent Water, MWH and our project and site services teams resulted in successful outcome from a challenging set of circumstances. The solution optimised the clarification process within the existing footprint, with a best value operating cost outcome.  The modified Lamella and ZickertTM equipment also offered a low maintenance, Totex-friendly solution.

Installation and commissioning of both tanks was completed well within project timeframes. The project met regulatory deadlines three months ahead of time with the first tank gaining "plant in use" status on 22nd December 2015.

"Hydro International were very happy to engage with us, listen to our ideas and support the development of the design and construction solution. The project was completed on time and within budget overcoming some significant difficulties.  Good teamwork and dedication between MWH, Hydro International and ourselves was critical to the project outcome."

- Stephen James, Programme Engineer, Severn Trent Water

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