Bam Nuttall is completing a sewage treatment works for a cluster of North Cornwall towns, significantly improving water quality. Report by Paul Thompson.
When 180mm of rain fell on Boscastle during a downpour in August 2004 the eyes of the world focused on the North Cornwall fishing village and the damage done to it by the swirling torrent of floodwater that surged down the River Valency.
Five years later Boscastle has become a focus for hordes of geography students intent on studying the hydraulics behind the flooding.
The village has also been the focus of attention for engineers at South West Water for many years and not because of the likelihood of a catastrophic flood event.
In fact as part of its programme to rid its area of untreated raw sewage discharges to the sea South West Water had targeted the town as a potential site for a sewage treatment works well before that fateful summer’s day.
A treatment scheme was due to be delivered as part of its Asset Management Period 3 investment programme by the end of 200, but the 2004 flood forced a rethink.
“The initial plan was to put the treatment plant in the valley near the car park. After the flood we had to rethink our strategy because of that location,” says South West Water project manager Mike Court. “The site got squeezed and because of Boscastle’s topography there is not that much room anyway.”
Other sites were considered and the company got as far as applying for planning permission on several occasions but it is mainly thanks to a population reclassification that the sewage treatment scheme is getting built at all.
The area is heavily dependent on tourism and during the peak season, population numbers can swell markedly from those during low season. Under the European Union Urban Waste Water Treatment Directive the sewage from a catchment with a population of more than 2,000 must be treated at a secondary treatment plant.
A decision that the nearby catchment areas of Bossiney and Tintagel qualified for secondary schemes under that directive meant that those as well as Boscastle would need the same grade of treatment. This moved the goalposts. It meant that sewage treatment for the three villages could be combined under one larger capacity facility.
“There was a debate about the population density of Bossiney and Tintagel and it was determined that those catchments needed a higher level of treatment. It meant that all three were now at the same level,” says Pell Frischmann project manager Chris Rogers.
By amalgamating the three catchments, a larger treatment plant somewhere between the eastern end of the scheme at Boscastle and the western outfall at Tintagel could deal with raw sewage from all the villages. But this is prime tourist country and finding a suitable site for the scheme has been difficult.
Eventually the project team settled on two possible sites, one at the hamlet of Trevalga and one at south Bossiney, and submitted planning applications. Eventually in September 2008 the Trevalga site was cleared by the planning committee.
“There is an area of outstanding natural beauty. We were being squeezed. People wanted us to move away from the path.”
Chris Rogers, Pell Frischmann
Sitting halfway between the villages of Boscastle and Tintagel, the greenfield site at Trevalga is sandwiched between the main B3263 road that links them, areas of outstanding natural beauty and cliffs which plunge to the Atlantic Ocean.
“There are lots of constraints around here. The south west coastal path runs along the cliff edge so people wanted us to go up the hill but there is an area of outstanding natural beauty. We were being squeezed. People wanted us to move up the hill away from the path but that would make it expensive to pump the sewage,” says Rogers.
With a 120m hydraulic lift out of Boscastle the decision to place the treatment plant at Trevalga means that the project team of South West Water, consultant Pell Frischmann and contractor Bam Nuttall is constructing five new pumping stations to pull the sewage up from the three settlements to the treatment plant.
- Project Boscastle, Bossiney and Tintagel Clean Sweep
- Contract type IChemE Green Book Target Cost
- Contract Value £7M total − £5.4M construction
- Utilities supplier South West Water
- Consultant Pell Frischmann
- Contractor Bam Nuttall
In Boscastle itself the existing pumping station at Boscastle Harbour caters for the lower village, another pumping station at New Road caters for the higher part of the village, and a new terminal pumping station further uphill at Boscastle Forrabury will pump sewage directly to the Trevalga wastewater treatment works.
At the western, Tintagel, end of the scheme new pumping stations at Tintagel Haven and Castle Road will handle material from English Heritage’s Tintagel Castle, famous for its association with the legendary King Arthur, and from the village itself. A new pumping station at Bossiney Back Lane is the uphill terminal station. it will push material on to the Trevalga works.
“It’s being cast as a ring which means the stresses are shared through the concrete as it comes up.”
Stephen Moore, Bam Nuttall
The works will include a 20m diameter, 8m deep slip-formed concrete concentric activated sludge tank and associated collection tanks and pumping stations. It will be capable of processing 40l per second of sewage waste, roughly half from Boscastle and half from Tintagel.
Barnsley-based specialist subcontractor Galglass is installing the tank in 1m lifts using its steel faced slipform system which enables the tank to be built to a slimmer section because the hoop stresses are being shared around the concrete as it is cast.
“It’s being cast as a ring which means the stresses are shared through the concrete as it comes up,” says Bam Nuttall project manager Stephen Moore, “It is actually a very economic and slender design.
“If it was cast segmentally then effectively we would be building sections of retaining wall and that would mean larger reinforcement. As it is the concrete is self supporting and the reinforcement is minimal. We are using about half the amount of concrete and a third of the rebar we would normally expect to use,” he adds.
The walls and base of the tank are just 250mm thick and the base is cast directly onto the underlying fractured slate bedrock with no piled foundations and minimal steel mesh reinforcement. It’s fast too. The whole structure will be cast in just nine weeks and is programmed to be water tested a fortnight later.
The concrete comes from a batching plant 10km away and is a C28/35 blended mix. The tank walls are pin jointed to the base slab using 16mm diameter dowel bars at 250mm centres. “The formwork can be jacked within a day of casting,” says Moore. “The base is cast flat with bending sloped at 7.5° into the central hopper placed later.”
This central hopper will collect all the surplus sewage sludge that settles out of the effluent after the aerated sewage has been treated in the activated sludge plant. The activated sewage plant is being constructed next to the tank. Here, in a structure built to look like a barn, the raw sewage has air blown through it before recycled activated sludge is added. This contains millions of bacteria that will break down the sewage. Eventually the surplus sludge will be shipped off-site in tankers to one of South West Water’s specialist treatment facilities. The clean effluent will then be pumped back out to sea through an existing outfall at Bossiney (see box below).
Hamstrung by a string of archaeological finds along the 10km length of pipelines that are being installed, the site team is still clawing its way toward a completion date of spring 2010.
That may seem a long way off to the residents of this part of Cornwall but as Rogers says, this is probably the biggest civil engineering project on this part of the coast since King Arthur’s castle was built.
In the Pipeline
The 8km of pipe that will transfer sewage to the treatment plant will largely run in 180mm or 225mm diameter polypropylene pipes.
The short section between Boscastle New Road and Boscastle Forrabury pumping stations will the in 150mm diameter ductile iron piping. The BAM Nuttall site team is using aggregate sourced as waste from the local china clay industry to bed the pipeline but also had to carry out some roped access work through its sister company Bam Ritchies when carrying out much needed repairs to the existing 200mm diameter outfall 85m down the cliff face at Bossiney.
The team replaced the 35m long outfall which had been shattered by storms and encased it in a fibre reinforced concrete shroud with a stainless steel mesh despite working time at the extreme outfall end being limited to just two possible low spring tides.