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UK-first in water treatment

Oswestry Preview

Hidden behind the town of Oswestry, near the Welsh border, is a £54M water treatment works overhaul that includes a UK-first.

The 29,000m² Oswestry Water Treatment Works is easily the largest industrial structure in the town of 17,000 residents.

It is a critical piece of UK infrastructure for almost 1M tap users in Liverpool and parts of Cheshire.

The overhaul involves constructing two rectangular tanks –  clear well and a flocculation basin – using about 2,000 precast concrete sections. These are subdivided into smaller sections which will house new and innovative treatment processes.

The UK-first treatment process harnesses chlorine dioxide, a powerful oxidant which will facilitate manganese removal. Using this, and a few other features, C2V+ – a VolkerStevin and CH2M joint venture – is partnering with Scotmas Group to deliver on behalf of United Utilities.

Kd 16660 jpg

Kd 16660 jpg

As construction takes place on the flocculation sedimentation facility (bottom left) 20 of the remaining slow sand filters can be seen surrounding the operations centre.

First on the project to-do list is retire 23 Victorian-era slow sand filters which make up the majority of the site’s footprint. These structures could be demolished and turned into solar schemes, but United Utilities says it has no plans for the land yet.

“It’s Victorian technology that’s been used and used, sometimes improved, over the years. But at the heart of it is that process installed in 1890,” says VolkerStevin contract manager James Worth.

Each slow sand basin measures 3600m² (about half a football pitch) and contains a 1m depth of sand, with 1m depth of water on top.

“With slow sand, as you filter through, and it clogs [the water flow slows] and to recover the rate of filtration you have to drain the filter, skim the top layer off the sand,” says United Utilities asset manager Ian Skilling. “It’s labour intensive, and you end up doing that every two weeks. And every so often you have to change all the sand.”

Slow sand basins have largely been supplanted by the newer and smaller rapid gravity filters which measure 200m², including a 1m depth of media – a mix of sand and anthracite – and 3.5m depth of water. The crucial “rapid” part of the process arrives from periodically back-filling the filter with water to flush out dirt.

Source: C2V+


The project falls under the AMP6 Framework (2015-2020), which sees Ofwat calling for improved efficiency through replacing ageing structures and systems, towards a “totex” (total expenditure) investment where design work anticipates future maintenance and repair during an asset’s life. The regulator sees efficiencies, not to mention cheaper water bills, resulting from upgrades that take on a long-term perspective.

Key points in the Oswestry tender were: capacity increase, quality increase, improve manganese removal, maintain bacterial compliance, maximise any hydroelectric capability and manage security issues.

Worth says these key demands were placed alongside requirements for more standard efficiencies in operating expenditure: less manpower, chemicals, electricity and maintenance.

“We’re also looking at enhancing the instrumentation – how it can be run, controlled remotely, say by computer screen rather than manual labour,” says Worth.

Collaboration was also a driver, United Utilities and C2V+ working for months before the start of construction.

Official start to construction was in January 2015, and the first major part of the challenge so far has been constructing, installing and testing new equipment, all while maintaining an uninterrupted water supply.

“The project’s complexity, on its own, it’s a quite challenging scheme, but particularly so in the middle of live works, to keep that [water] service running 24/7, every day of the year,” says Worth.

And as the seasons change, water quality changes. “And that changes every year as we go forward,” says Worth. “So we’ve got the flexibility here to adjust and optimise that – almost every week basically. So they’ve never had that facility before.”

At the same time, capacity will increase about 25%. “It’s restricted to about 170M.litres/day. One of the goals of the project is to restore that to where it should be, which is 210M.litres/day,” says United Utilities’ Skilling. For comparison, that is equivalent to moving from 68 Olympic-size swimming pools per day to 84.

To create the new large concrete vessels, the project is taking full advantage of design for manufacture and assembly, which allows for cost savings, time savings and increased accuracy of build. The clear well and flocculation sedimentation basin are both primarily made up of the precast concrete sections, imported from Irish company Carlow, which is also responsible for delivery, assembly and installation. “The consistency and quality of the finish of the concrete is very important for us in water treatment,” says Worth.

The precast solution also means fewer truck movements. Individual loads can be up to 40t at a time, rather than your average 16t, 6m³ concrete mixer.

“The flocculation basin is quite a complicated ’Meccano set’ shall we say,” says Worth. “It’s a jigsaw of many many different size shapes and units – a very different approach to the clear well, where there’s only about a dozen different types of mould.”

The precast units are stitched together insitu, with pre-cast columns and beams installed first, followed by pre-stressed roof slabs.

The precast wall units are brought to site with reinforcing steel “starter Bars” protruding to the sides. Adjacent units are placed about 500mm apart and have reinforcing steel fixed to the starter steel to form a continuous wall. The 500mm wide “stitch” is then shuttered and poured with concrete to infill the stich section of the wall.

A0 pic for printing

A0 pic for printing

Construction continues, including on the flocculation sedimentation tank (bottom right).

But long before construction began, a miniature version of the proposed Oswestry plant was assembled inside a shipping container by a specialist in the United States. This pilot plant was then shipped to the UK, installed in Oswestry and connected to the raw water supply. This was run for several months, mimicking the proposed plant process.

“[On the testing, there have been some] minor tweaks for efficiencies, but importantly it allowed us to present bulk data. There are a lot of things that changed as a result of that research, which created a lot of front-end value,” says Worth.

Under construction in the site’s northeast corner is the flocculation sedimentation basin, a new 73m by 34m by 7m open-top structure. This is the new “hero” of the operation, performing the lion’s share of treatment.

Inside the new tanks the objective is to de-stabilise suspended particles in water. Chemicals are added, particles collide, form “flocs”, which because of their larger size are more easily removed via sedimentation.

Mixers slowly combine water and chemicals before they head towards lamella packs: inclined steel plates designed to collect particulates. As the particles become heavier, they sink and fall to the bottom of the tank. That sludge/dirt gets swept away out of the tank and is later pressed into a cake and shipped off for use in agriculture as a fertiliser.

Kd 04711 jpg

Kd 04711 jpg

Pre-cast concrete panels are stood up and later ‘stitched’ in situ, providing for great accuracy and build speed.

The flocculation sedimentation tank is also where the chlorine dioxide process is added. So what is so special about this process, that is already used in the US and other parts of Europe?

Chlorine dioxide is generated on site using a reaction of Purate (a stabilised solution of sodium chlorate and hydrogen peroxide) and sulphuric acid, which are delivered in bulk to site. Chlorine dioxide generators mix the Purate and sulphuric acid under water, producing an aqueous solution which is piped to the point of application.

The chemical compound has a range of advantages over tradtional chlorine, but it is used here primarily for the removal of iron and manganese. Chlorine dioxide oxidizes the floating particles of iron and manganese – this is not harmful to health but affects the water’s colour and taste – aiding the coagulation process and the removal of turbidity from the water.

After this treatment, and filtration through the rapid gravity filters, the water heads to the clear well structure: 80m by 100m, about 6m deep, and with capacity of 35M.litres, it is a holding point for water before it goes out to customers, ensuring regularity and security of supply. In a power failure, flow continues to the clear well under gravity.

It is more than three times the volume of the former clear well of 10.6M.litres.

The old clear well was also a single tank, with no way to isolate it from supply when it required maintenance or cleaning. The new clear well structure incorporates a contact tank, where the water is disinfected, and both are split down the middle to enable sections to be isolated for maintenance without supply disruption.

To make sure the Oswestry project comes into operation smoothly, a series of automatic monitors – including for temperature, turbidity and acidity– are constantly optimising chemical doses. Regular manual samples are also taken for laboratory tests for bacteria and organic compounds. C2V+ will also have staff on site for the first year of full operations, to ensure a smooth handover.

Worth says the project is on schedule and under budget for completion summer 2017.

Key stats

  • 2,500t steel reinforcement in situ placed
  • 8,200m³ of in situ poured concrete
  • Just under 4000m³ of concrete in precast panels
  • Around 200 deliveries of 40t loads
  • A hydro-electricity turbine powers the site, exporting any extra power back to the grid. An old turbine is being replaced with a new, more efficient model, with a maximum power output of about 450kW.

Related files

Readers' comments (1)

  • It would be interesting to know how the precast units are to be "stitched together" (using the writer's non-engineering terminology). It will hopefully be a method that does not risk the creation of a large number of potential leakage pathways through variable workmanship on the "stitching".

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