Team effort saves time
How innovation and collaboration knocked weeks off timescales on Severn Trent's Little Eaton WTW upgrade
Severn Trent Water’s £2.4M upgrade to its Little Eaton Water Treatment Works (WTW) has been completed, 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.
An innovative approach and a close collaboration between Severn Trent, the design and build contractor MWH Treatment and Hydro International dramatically reduced project timescales by more than six weeks and achieved significant construction, equipment and operating cost savings.
Located north-west of Derby, Little Eaton WTW treats raw water from the River Derwent to supply the strategic grid with between 70mld and 90mld. Improvements to the clarification stage at the works were needed to increase the design capacity to 120mld 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 £1M, Hydro International provided 16 specially modified Lamella Plate Pack Separators and four Zickert Reciprocating Sludge Scrapers to deliver the solids separation and removal required.
Both the Lamella Plate Separators and Zickert Sludge Scrapers are suitable for use with drinking water – the lamellas have full regulation 31.4(a) approval, and the Zickert scrapers have regulation 31.4(b) approval via WRAS (Water Regulations Advisory Scheme) approved materials or dedicated BS6920 testing.
“The project’s first major challenge was to achieve the increased capacity within the plant’s existing reinforced concrete tank structures,” says Severn Trent programme engineer Stephen James. “The river turbidity at the inlet of the treatment works is usually under 5NTU, but it can go as high as 50NTU 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.”
Hydro developed new manufacturing and production techniques to increase the length of the lamella plates to 3m, extending the conventional design by half a metre, and thereby increasing the total projected settling surface area to 1,856m2.
Paul Barter, principal process engineer for Hydro International, says: “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 3m 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 US more than 15 years ago.”
Without the new design, to achieve the additional 30mld 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.
Stephen James adds: “Extending the tanks would have been challenging and we also looked at incorporating other clarification methods in the footprint such as a micro-sand process. Neither option was feasible on a cost-benefit basis; operating costs would also have been significantly higher than a lamella plate setup.”
Completing the works within the required timescales to meet DWI consent deadlines was a critical target. The MWH team worked with Hydro’s team, conquering the first challenge by removing the legacy tube-type lamella plant and pipework then cleaning the tanks of residual sludge.
Kelvin Smith, project manager for MWH, says: “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 ten 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.”
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 onsite.
To accommodate the extra length and weight of the plates, Hydro 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 onsite.
The Hydro team also modified the Zickert Sludge Scraper design for potable water application, ensuring 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.
Further design modifications were made to enable an additional walkway to be constructed between each tank allowing better access for clean down and maintenance.
At Little Eaton, the raw inlet water is dosed with ferric sulphate and polyacrylamide for flocculation and coagulation of suspended solids. The water is then gravity fed through the lamellas to separate out the solids. Two 13.6m x 6m Zickert scrapers are installed at the bottom of each tank to provide transportation and collection of the settled sludge. The sludge is transferred to the outlet with minimal disturbance thanks to the reciprocating action of Zickert, then pumped to a sludge thickener for onward treatment and disposal.
Meanwhile, the clarified water is passed through rapid gravity filters, granular activated carbon, then through a contact tank for chlorination before being held in service reservoirs before distribution to the grid.
Close teamwork and collaboration between Severn Trent, MWH and the Hydro 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 Zickert equipment also offered a low maintenance, totex-friendly solution.
Installation and com-missioning 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 December 22, 2015.
Stephen James concludes: “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.”
This article first appeared in the December 2016 issue of WET News.
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