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Where beauty lies in the form not the function

Working in partnership with three contractors has helped Severn Trent Water deliver big savings on its Minworth sewage plant

Project Minworth sewage treatment works
Project value £148 million
ClientSevern Trent Water
Civil and hydraulic designPick Everard
Main contractor BNM Alliance – a Biwater Treatment/North Midland Construction/Nomenca alliance
Concrete contractorStam Construction
Bulk earthworks contractorHawk Contracts
Piling contractorsVan Elle and Simplex

For large images click here

Building a sewage treatment plant can hardly be described as the sexy end of construction.
Worthwhile? Yes. Technically challenging? Yes. Sexy? Never.

But a 162 ha site near Minworth on the north east outskirts of Birmingham is challenging those notions.

Paul Fisher, project delivery manager for client Severn Trent Water and the rest of the site team may never be surrounded by hordes of adoring fans, but this is a project vital to the delivery of a 21st-century sewage treatment system for Birmingham and the wider West Midlands conurbation.

Minworth sewage treatment plant has a direct population catchment area of almost two million people.
It also treats effluent sludge brought in directly by the tanker load. This brings its total sewage treatment population to two-and-a-half million. That’s a lot of sludge.

But while the squeamish may be turning their noses up at the thought of dealing with all that waste, for Mr Fisher revamping a sewage treatment works for most of the West Midlands conurbation is an exciting challenge.

“This stuff is my bread and butter,” he deadpans before adding, “It is a huge £148 million project. Of course it is a challenge but it is one which we can and will rise to.”

New sewage tanks

The new build element involves the construction of 22 reinforced concrete primary settlement tanks (PSTs) 30 m in diameter, a huge 112 m long, 91 m wide and 6 m deep four-lane activated sludge plant (ASP) and eight 35.5 m diameter final settle-ment tanks (FSTs).

New preliminary sewerage treatment works including new effluent channels, inlet works, effluent screening, screens handling and grit removal plant are also being built.

But those are just the main facets of the project. A host of refurbishment work including improvements to both the primary and secondary sludge treatment process and extensive refurbishment to add to the life of the existing FSTs is also programmed under the lengthy list of work the delivery partnership, the BNM Alliance, has to carry out.

The plant is designed to be able to handle a full flow to treatment of 12.6 cu m/s and peak storm flows of 30 cu m/s. The new secondary treatment process will cater for some 16 per cent of these flows while the refurbished existing treatment works will manage the rest.

Essential to the delivery of the project under the water industry’s current asset management plan programme, AMP4, is reducing the level of ammonia in the fluid discharged from the plant into the River Tame which borders the site.

Currently ammonia levels of 5 mg/l are permitted but this is being reduced to 3 mg/l - a target which is one of the key quality drivers set by the Environment Agency in a bid to improve the river’s water quality.

Another driver is boosting the available space for stormwater storage, helping to decrease the likelihood of ‘unsatisfactory intermediate discharge’ (UID), or overflows of effluent into the River Tame during storms.

Coping with storms

Currently, any excess effluent during storm periods is stored in the existing stormwater holding tanks for treatment once the flow has dropped.

But when the new PSTs have been brought on-line, the existing PSTs will be converted to supplement the available stormwater storage volume by 61,000 cu m, more than doubling overall capacity at the plant.

This additional capacity should help significantly reduce the number of UIDs says Mr Fisher.

“It is difficult, if not impossible, to quantify exactly how many UIDs we will avoid. But certainly an extra 61,000 cu m of capacity will result in less frequent and smaller discharges into the river.”

Another key quality driver is the Integrated Pollution Prevention Control-licensed hazardous waste disposal tip on the south east corner of the site.

Currently a dumping ground for incinerator bottom ash from one of Severn Trent’s other sites, a 2 km-long, 700 mm thick bentonite clay perimeter wall is being built around the dump in order to prevent leachate from finding its way into the watercourse.

“There is a bentonite cut-off wall all the way around the tip,” says BNM Alliance project manager Mark Lee. “It is cut straight into the Mercia mudstone bedrock at various depths between 6 m and 26 m deep.”

But it is back on the western side of the huge site that most of the work is currently going on.
The project team has made rapid progress since its start on site during the summer of 2006 and already great lengths of reinforced concrete effluent channels have sprung up ready to take screened material from the preliminary treatment works to the settlement tanks.

A forest of piles

These canals cut through a site which, although vast, seems crammed full of recently poured concrete and are supported on a forest of piles.

The ASP and FSTs are supported by 400-450 mm -diameter continuous flight auger piles at depths of between nine and 15 m.

Although initially it was considered that none of the 22 -primary settlement tanks would need piling, a chance discovery during the initial site strip from the inlet works made piles for one of the tanks more likely, explains Adrian Brown, civils project manager.

“We found an old beamed structure down to a depth of 6 m which meant it was more viable to use concrete piles. There are 118,400 mm diameter piles under that tank. Four of the other PSTs are ground bearing and the other 17 are sitting on stone vibro columns,” he says.

Slab thickness for the inlet varies as it crosses the site but is on average around 500 mm.

The slab for the activated sludge plant stands at 750 mm thick around its perimeter before slimming down to 550 mm toward the centre.

“The biggest fixed moment is around the bottom of the walls before falling away toward the centre,” Mr Lee explains.

Slabs within the PSTs are 500 mm thick. The sloping base slab in the tanks is cast in three separate pours.

“Initially the hopper walls and the first sloping 1.5 m of the 30 m diameter tank is poured, then the toe and a further 1.5 m section of slope is cast.

Finally a four-hour pour of 240 cu m C30/35 structural -concrete makes up the infill.

Huge strides have been made in pinning back the project costs and shaving valuable time from the overall programme. It is after all a delivery schedule that cannot be broken.

The new plant must be delivered by the time the next asset management plan, AMP5, is due to begin in April 2010. Any failure could adversely affect Severn Trent’s funding and that will not happen says Paul Fisher firmly.

“If we do not complete the work on time then we are at risk of failing the ammonia consent.

“If this works becomes a ‘failing works’ then ultimately that could affect our funding. I am confident the project team will not let that happen.”

How the BNM Alliance was formed

The contractural relationship between delivery partners at Minworth is essentially a tie-up between two of Severn Trent’s 19 asset management plan contractors: Biwater Treatment and North Midland Construction (NMC).

Normally on other AMP4 associated works Biwater work in isolation while NMC operates in partnership with its mechanical and electrical arm Nomenca.

But because of the scale of the Minworth works and Severn Trent’s drive to balance workload, it decided it would apportion the inlet works and primary settlement tanks to NMC and Nomenca with the activated sludge plant and final settlement tanks allocated to Biwater.

Sundry works would be shared equally between each company.

But Biwater, NMC and Nomenca decided that because of the links between and timing of all the work packages, the most sensible way of delivering the £148 million project would be to form a partnership across all three organisations – the BNM Alliance.

Severn Trent uses an amended ICE 7th term contract for its projects and features a pain/ gain share mechanism on a target price, cost reimbursable basis.

Partnering saved £10 million

One of the benefits of having such a close contractual relationship across the delivery team is the ability to iron out waste through the early contractor involvement.

This has helped snip an estimated £10 million off the total project cost according to Paul Fisher: “I think £148 million could easily have become £158 million if we hadn’t taken on board some of the suggestions from the supply chain,” he admits.

Mr Fisher highlights the suggestion to build 22 PSTs instead of 23, a decision which saved £1 million.

Building eight larger capacity FSTs rather than the 12 initially proposed also shaved a further £1 million.

But it was the choice of and purchase of materials that really helped trim costs.

“We have used all manner of plastics instead of ductile ironwork pipes on the project. For years we have used plastic pipes on the network, why we have not used them on the treatment works beats me,” says Mr Fisher of a decision that he estimates saved £1.8 million.

And by buying unbent steel reinforcement well ahead of programme the team saved another £400,000 according to Mark Lee.

“In the first phase of works there is about 3,500 tonnes of unbent 12-20 mm bars. By buying in advance we saved approximately £150,000 and on the second phase probably £250,000,” he says.

Further savings included using limestone aggregates within the concrete to help cut down on the amount of rebar and translating formwork shuttering sizes into the final design.

For large images clickhere

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