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Spring clean

Engineers in Blackpool are having a second go at meeting European Union sewage treatment standards after their first £500M attempt failed.

Blackpool's main car park is currently closed. Replacing its neat white lined bays is a mud covered site crowded with cranes, dump trucks, spoil settling lagoons and bentonite segregation plants. Centre stage is a pair of Hydrofraise diaphragm walling rigs completing the final stages of two vast circular walls sunk 45m into the ground.

They are building diaphragm walls for two enormous storm water storage tanks beneath the carport. Associated work worth £70M at six sewage treatment plants along the coast is on a fast track programme, targeted for completion by July - the start of Blackpool's main bathing season. This associated work will add more storm water storage capacity and tertiary ultra violet treatment for the town's sewage.

Blackpool and the adjoining Fylde coastline has eight beaches designed for bathing by the European Union. It is said to be Europe's largest coastal resort. As such, it is an obvious prime focus for the problematic EU bathing water directive, now fully adopted by the British Government, and aimed at ensuring the sea alongside our most popular beaches is fit to swim in.

Until three years ago virtually all the town's raw sewage - some 570M litres a day - flowed untreated into the sea from short outfalls close to town beaches. In 1994, operation Sea Change was designed as the ultimate £500M solution to bring bathing waters up to EU standards.

This involved laying a 14km sewer beneath Blackpool's sea front to intercept all the outfalls and route the raw effluent north to a new full treatment plant at Fleetwood. Secondary treatment was added at two existing plants, more new sewers laid and pumping stations enlarged.

Completion of the scheme in 1996 saw engineers enthusiastically rushing into the sea to collect water samples. The results soon removed their smiles - they faced an embarrassing failure.

Average faecal coliform levels in the water had been reduced by 50%, but half of the eight key bathing beaches still failed to reach EU standards. And they failed dramatically, with readings of more than 10,000 faecal coliforms/100ml of seawater - five times the EU limit.

'We were all totally amazed, as we had expected the scheme to solve all our problems,' recalls North West Water's bathing water project co- ordinator Julie Wakeham. 'Clearly the coastal regime was far more complex than we had thought.'

The European Commission was furious and threatened to prosecute the UK Government. Britain's politicians responded by demanding immediate face-saving schemes from engineers to show Brussels we were trying.

The current fast track works package is an engineers' 'best guess' result. Most of the schemes had to be built in less than a year to ensure completion for this summer's season.

These 10 latest projects concentrate on more ways to improve the quality of effluent leaving treatment plants. New storage tanks at five sites will increase storm water holding capacity along the coast by 166,000m3. This means that instead of untreated storm water spilling direct into discharge pipes when the plant is operating at full stretch, all the excess can be held back and treated after the main storm flow has passed through.

Blackpool's two new 36m diameter underground tanks will increase the local area's storm water storage by 60,000m3. The £10M year-long design and build contract calls on main contractor Bachy Soletanche to complete the 45m deep structures almost 50% faster than normal.

This is why the site boasts two Hydrofraise diaphragm wall piling rigs. Equipped with reverse mud circulation and cutter drums attached to a heavy guide frame lowered to form each wall panel, the sophisticated £2M machines have much greater vertical accuracy and a 20% faster excavation rate compared to standard kelly mounted grabs.

They have already formed the tanks' 1.5m thick perimeter walls which are being progressively exposed as excavation within the perimeter ring deepens.

'Bringing in a second machine from France saved us 14 weeks,' claims Bachy Soletanche project manager Bernie Kiernan.

Redesigning the tank's concrete roof, which will double as a ground slab for the reinstated car park, has slashed a further month off the timetable. Original plans to temporarily stop excavation 5m down, providing a platform from which to build an in situ concrete roof, were scrapped.

Instead the roof will be formed of large 80t precast beams positioned after excavation is complete. Digging out the first tank is 60% complete, with the second 16m down. The contract, managed for NWW by Bechtel Water Technology, remains on schedule for completion in mid July.

Along the coast from Blackpool and up the Ribble estuary, Clifton Marsh treatment plant - servicing most of Preston's sewage - is also getting more storm water storage. A £3M, 120m long concrete tank boosting the plant's storage capacity fivefold to 75,000m3 is being built by Harbour & General.

Speed of construction again rules. A high water table meant that forming the tank entirely above ground, avoiding cofferdams or extensive dewatering, provided the fastest solution.

Clifton Marsh is also one of four plants being fitted with ultra violet tertiary treatment. Destined to be the largest of its type in Britain, more than 1,100 uv bulbs will disrupt the DNA of bacteria still in the effluent after secondary treatment and prevent them from replicating after discharge into the estuary.

Whether all this additional treatment and storm water storage will meet EU demands is 'far from guaranteed,' says Wakeham. 'Everything we are doing is still based on assumptions,' she admits.

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