Site Report: Galliford Try Meica JV's Kings Lynn wastewater treatment plant

The landscape around Kings Lynn in Norfolk is flat farming country sliced into a patchwork of fields by long, straight drainage ditches, but with little to catch the eye above ground level.

A little way outside the town, however, the skyline is broken by two large concrete tanks. These 25m-high digester tanks mark the site of a new £25m biosolids treatment centre being built for Anglian Water (AW) by the Galliford Try Meica (GTM) joint venture.

GTM is part of Anglian Water's Special Projects framework for AMP 4, but the team still had to undergo a measure of competition to win the Kings Lynn project. An integrated project team approach is being used and this extends to key subcontractors and materials suppliers. But, in what may well be a first for the water industry, AW has incentivised the project by asking GTM to maintain the plant and optimise its efficiency for two years after completion.

Ian Turner, AW project manager, explains: "Normally the contractor would hand over the plant on completion and walk away, leaving the client to get to grips with running it. But we want to retain the interest of the contractor after completion, so we get the best whole-life cost solution.

"We want to make the plant as efficient as possible, not just during construction, but also in operation, so we've incentivised the contract to get GTM to help us operate the plant more efficiently. It marks a major shift in behaviour for both of us."

The incentivisation works like this: the more gas produced (a byproduct of the treatment process) and the lower the consumption of resources used in the process, such as electricity and chemicals, the higher the financial reward for GTM.

That encourages GTM to create a design solution that provides the most efficient and economical operating and maintenance regime. And that can mean spending more up front on construction techniques and solutions, as well as on the process machinery to provide a more efficient and more reliable operation.

GTM was appointed preferred bidder in December 2005 and soon began making its influence felt. It persuaded the client to buy an extra 50m strip of land next to the site. This provided more space for the works, better and safer storage of materials, and allowed different aspects of the work to be undertaken in parallel, thereby saving time and cost. GTM also influenced the layout of the plant to provide a more linear arrangement, which made pipe runs much simpler.

Hitting the deadline

The team was ready to go by May 2006, but the scheme was hit by planning delays and land purchase issues, which threatened to put the programme back by at least two months. Chris Hastings, GTM contracts delivery manager, says: "Ian asked us, 'how are you going to ensure the completion date is going to stay the same?' We went to the Inland Drainage Board and asked if we could relocate a very large drainage ditch while we waited for the planning to be sorted out, so that was a start."

Once planning permission was granted, work began immediately on the piling. The ground conditions were very poor - a 12m-deep band of alluvial silts. Hastings likens them to a "thin crust on top of toothpaste - it oozes".

Galliford Try's sister company Rock & Aluvium was brought in between August and September 2006 to install 500 CFA piles up to 27m deep.

"We were originally going to use only one piling rig, but to help mitigate the planning delays, we brought in a second," says Hastings. "It meant higher mobilisation costs, but it was a project team decision."

Spoil from the excavation works was reused on site to create a bund around the works thereby avoiding landfill tax. Work then began on the two 360m3, 25m-high digestion tanks. Steel is normally used for tanks containing gas, but Hastings says the team was not convinced steel would provide the design life needed for such large containers.

The flat land around Kings Lynn is subject to 65mph winds, which get particularly vicious when funnelling through the gap between the two tanks. So steps were taken to mitigate the risks of working at height by forming the rebar at ground level.

Temporary concrete ramps matching the interior and exterior radii of the digestion tanks were used as moulds. The rebar was then hooked onto a curved steel lifting beam and hoisted into position.

The walls of the digestion tanks were poured in four 5.4m-high bands. Peri's climbing shutter system was used, whereby the shutters are clamped and cantilevered to the walls as they are built up.

The concrete was poured using two 52m-long booms moving in opposite directions over 180o each. The continuous 180m3 pours took between eight and 10 hours.

The tanks hold gas so it is vital that they do not crack. Concrete supplier Cemex was asked to come up with a slow-cure, high replacement mix able to resist chemical corrosion.

The mix had to be changed slightly for the second pour, as a spell of cold weather looked like delaying it. Hastings says: "We had to get the pour done before Christmas, so we asked Cemex for a solution to allow us to keep working. We added hot water to the mix - a solution everyone agreed to, although it was at GTM's risk."

The pour was successful, saving a three-week delay.

Instant solution

In 2007 work began on the Monsal process tanks and some of the M&E, including installation of the aquabelts in the sludge thickening building, along with the construction of a 12.5m-deep, 7.5m diameter return liquors pumping station shaft, which was sunk using the wet caisson technique.

But the high winds again threatened progress on the digestion tanks. "The wind funnelling through the tanks was causing the shutters to spin," recalls Hastings, "so we invested £100,000 in an extra set of climbing shutters, which saved three months of delay."

Turner adds: "It was a safer solution. Instead of moving the shutters from one tank to the next, which was causing delays, it was safer and quicker to invest in another set."

As the tanks rose, an access platform, designed to resist 65mph winds, was erected between them. But the winds were so bad at the top of the tanks in particular, that it had to be strengthened to resist 90mph winds.

The digestion tank walls were finished in March 2007. Work then started on the construction of the roofs. Falsework was installed inside the tanks to support the roof, rebar fixed in place and the concrete poured. Staircases have been fixed to the side of the tanks and a steel bridge between the two is due to be installed shortly.

At the time of CJ's visit in June the steam heat boiler had been delivered and the floor slabs for the skips and gas holder completed.

Hastings says the work is on schedule for completion by the end of this year. AW is clearly pleased with the way the project has gone as GTM has not only been awarded a similar project in Northamptonshire, but has also been awarded a £50m clean water treatment works at Wing in the same county.

Project Fact file

Project: Kings Lynn Biosolids Treatment Centre

Cost: £25m

Integrated project team

Client: Anglian Water Operations

Programme manager: Mott MacDonald

Delivery partner: Galliford Try Meica JV

Civil engineering designer: Mott MacDonald

Process designer: Monsal

How the biosolids treatment plant works

In the old days raw sewage was disposed of at sea. Now of course it has to be treated before it is disposed of. Sewage sludge is essentially dirty water and an extreme method of dealing with it is to dry it completely by removing 95% of the liquid. But this is very energy-intensive and the costs are huge. The most sustainable solution is to put it on the land, but this can only be done once the pathogens are removed.

The new biosolids treatment centre at Anglian Water (AW)'s existing Kings Lynn wastewater treatment plant is designed to do just that, by processing the sludge to turn it into sludge cake, which can then be sold as a fertiliser for agricultural use.

Ian Turner, AW project manager for the scheme, says: "Our strategy is to maintain a sludge-to-land policy, but the product must be as good as possible with as many of the pathogens removed as possible."

One solution is to use lime, but this ends up with AW removing more product from the plant than went in.

The existing treatment process at Kings Lynn reduces the amount of product by 20%, but the new plant, which has the additional Monsal treatment process, will reduce the sludge by 40%.

So how does it work? The sludge will be taken from the existing Kings Lynn waste treatment works and combined with a further 40% delivered by road. This will be pumped in to the north end of the new biosolids treatment site where it will be screened so any plastic or other non-treatable waste can be removed.

It will then be transferred to a thickening plant and fed through four aquabelts, which will remove a high volume of the water content.

The sludge will then be sent to the Monsal tanks, where it is heated to 55oC for four hours, which reduces the bacteria level from 100,000 parts per million to just one - safe for use on the land. The product then moves on to the digestion process. A byproduct of this process is methane gas, which will be drawn off and used to power two CHP engines, which in turn produce electricity.

As Tim Speakman, the GTM design manager from Meica (now Imtec Process), points out, this is a very sustainable solution. "We expect the CHP engines to produce nearly 1MW. We can use that power to make the existing plant and the new one self-sufficient for energy, and create surplus 'green' energy to sell to the National Grid."

The CHP engines create exhaust gas, which in turn is used to create steam heat. This is captured and diverted directly into the pasturisation process, so that all the energy associated with the CHP system is reused. The sludge then undergoes a final dewatering process, before its conversion into sludge cake fertiliser.

The aim of the design produced by AW, GTM and its partners is to invest more on the capital expenditure to produce a more efficient and economic operating expenditure. AW's Turner sums it up: "The sludge cake produced at this new plant will be half the cost per cubic metre than at the existing plant. So we get a better quality product at a lower operating cost. Anglian Water's energy bill is huge, so anything that reduces that is good for us."