Thames Water is upgrading five sewage treatment works at a cost of £675M as part of a massive project to improve water quality in the Thames. Margo Cole reports on progress at the largest of the sites, at Beckton in east London.
When you arrive at a site to find that the security and logistics operations are modelled on those of the Olympic Park, you know there is probably a substantial amount of activity going on beyond the security gates. And so it is at Thames Water’s facility at Beckton, east London, a vast area of land that is home to a sewage treatment works that deals with the equivalent of 3.5M people’s waste every day, as well as the company’s innovative desalination plant - the first in mainland UK - and a heat recovery plant that turns sewage sludge into energy.
For the past two years these facilities have co-existed with an increasingly complex construction operation that is part of Thames Water’s £5bn project to reduce the amount of sewage that gets into the river Thames. Beckton is at the epicentre of work on the London Tideway Improvements programme, which is designed to massively increase storage and treatment capacity in the capital (see box). Work at the site includes excavation for the new Lee Tunnel (NCE 13 September), as well as an upgrade to the existing works that will increase treatment capacity by 60%.
The treatment regime at the works consists of screening, grit removal, primary sedimentation (to take out the “settleable organic matter”), activated sludge treatment and final settlement, before the clean effluent enters a discharge channel, from which it discharges into the Thames.
According to Thames Water head of major projects Nick Fawcett, there is enough primary sedimentation capacity to cope with the increased volumes expected, so the £190M upgrade principally consists of building six new aeration lanes for the activated sludge plant and adding 16 new circular final settlement tanks, each 45m in diameter.
The construction contract won by Tamesis - a joint venture of Laing O’Rourke and Imtech Process - also includes a new blower house to blow air into the fine bubble diffusers at the bottom of the new 90m long, 40m wide and 8.5m deep aeration tanks.
Also included in the upgrade are two new tunnels, one to bring the sewage into the new works, and a second - measuring 800m in length and 3m in diameter - to take the clean flow out to the existing effluent channel. This larger tunnel was excavated using a tunnel boring machine known as “Beckton Bettie”, the little sister of “Busy Lizzie”, which is digging the Lee Tunnel.
Most of the new facilities are being built on an area of former industrial land within the existing site boundary.
“One of the things we’re really excited about at this site is that we have been able to explore and showcase construction innovation”
Nick Fawcett, Thames Water
“We had to treat the soil before construction, because there was the potential for contamination,” explains Fawcett. The new structures all sit on piles, some of which go down to 20m through the alluvial deposits and gravels.
Where the new structures differ from anything preciously built at Beckton - and, indeed, at any treatment works - is that their walls have all been built using precast concrete panels, rather than traditional in situ concrete techniques.
“One of the things we’re really excited about at this site it that we have been able to really explore and showcase construction innovation - in particular we’ve used off-site manufactured panels to form the structures,” says Fawcett.
Tamesis is also doing the upgrade work at Crossness treatment works on the other side of the river, and the circular tanks at both sites have been built using the precast panels. “Here [Beckton] we had to opportunity to take advantage of the precast approach for the aeration tanks as well, and that’s worked brilliantly,” says Fawcett.
“We’re really proud of that. I came here one day and there was just a base. Two days later they’d done one whole tank. It’s really impressive how these things are formed.”
Laing O’Rourke has invested heavily in recent years in a precast manufacturing facility at Steetley in Nottinghamshire, and, once the firm’s joint venture had landed the Beckton and Crossness contracts, Fawcett says Thames was keen to find out how they could make use of this technology.
“The first opportunity we wanted to talk to them about was the circular tanks,” he recalls. “It’s all about how you fix them together to form the circle.”
The resulting design is a series of straight, vertical panels that are prestressed together to form the 45m diameter circular tanks. A similar design was used at Crossness, and the same erection gang moved to Beckton once they had finished on the south side of the river.
When it came to the walls for the massive aeration tanks, Fawcett says Thames Water was aware that the contractor’s precast facility was able to produce twin wall panels, consisting of two concrete sections with a void in between that can be filled with concrete in situ. “They had been used on other sites, but not for large water retaining structures,” he explains.
“This definitely feels like the future of the industry… I think this is just the start of the innovation journey”
Nick Fawcett, Thames Water
To test their suitability for this project, the contractor built a demonstration facility, and filled it with water - a test that proved so successful that Thames Water went ahead with this method for all the new tanks.
The first lift of panels fits directly on top of starter bars cast into the tank base, and were propped in place while the concrete fill was pumped in between the “biscuit” sections. At the start the contractor was doing a single lift all the way round a tank then concreting the gap, but soon discovered it was possible to do two lifts at a time.
At the lower levels the gaps between the biscuits have 12mm mesh reinforcement inside them, changing to lattice in the upper levels. An in situ horizontal reinforced joint is formed between each lift, to transfer the loads through the panels to the ground.
“It’s amazing how quickly they put them together - it’s just like Lego bricks,” says Fawcett. “It’s very safe and very controlled, and the quality of what we’re getting is fantastic.
“It’s a view of life that we want to make sure is across everything our contractors and us are thinking about for [the next investment period] AMP6. We want to make sure we spend every penny as wisely as possible, and this has given us lots of programme benefits - we’ve saved 100,000 hours - as well as the safety performance.
“This definitely feels like the future of the industry,” he concludes. “This has been retrofitted into an existing design, but if we could combine that with BIM [building information modelling] techniques, so that we’re designing with manufacture in mind, the end result could be even better. I think this is just the start of the innovation journey.”
Beckton is one of five sewage treatment works along the Thames being upgraded as part of Thames Water’s £5bn London Tideway Improvements programme to help stop sewer overflows and improve water quality in the River Thames.
London’s Victorian sewerage system, which dates back to Joseph Bazalgette, was designed as a combined system with stormwater and foul water both going into the same network, so when it rains the sewers become filled with a mix of rainwater and sewage. If this volume becomes greater than the system can accommodate, it is discharged into the River Thames, to prevent homes and streets from flooding.
A threefold rise in population since Bazalgette’s day now means that 2mm of rainfall can fill the sewers, causing around 39M.t of untreated flow to be discharged into the Thames every year.
The London Tideway Improvements programme tackles this in three ways: increasing capacity of the five major treatment works so that more of the flow is treated before it is discharged; building the Lee Tunnel to store water when the Beckton works is full; and building the 32km long Thames tunnel to collect the flow that is discharged from 34 smaller combined sewer outfalls along the river.