Southern Water scheme to clean up Kent coast nears completion
A £70M scheme will take sewage from Margate and Broadstairs in Kent, previously discharged into the North Sea, pump it 11km, and treat it at a new sewage works near Ramsgate. Dean Stiles reports on the challenging venture.
Work to upgrade Southern Water’s Margate and Broadstairs sewage treatment, required under the Urban Waste Water Treatment and Bathing Water and Shellfish Waters directives, is almost complete.
The scheme, a joint venture between Black & Veatch and Costain for Southern Water and built for an estimated £70M, takes sewage from Margate and Broadstairs which previously was discharged to sea, for treatment at a new facility.
Planning constraints prohibited building a new treatment works near Margate, says Martin Ellis, design manager for Black & Veatch. Instead, waste is pumped from Broadstairs and Margate via an 11km pipeline to a new treatment facility built at Southern’s existing Weatherlees works serving Ramsgate. From here, treated water is pumped back to the coast via a second parallel pipe for discharge to the sea.
Planning delays also meant that the contract was awarded as late as April 2005 leaving a short window for construction and operation by October 2007 when tertiary treatment has to be in operation.
“The project was very demanding in terms of timescale,” says Ellis. “Basically, in less than two years, we have designed and built a large, complex project with planning and construction taking place almost at the same time.
“There was a huge challenge making sure outline design, design development and detailed design stayed ahead of construction.”
At the heart of the scheme are eight Grundfos 315kW pumps which transfer flows from headworks to the treatment works and back for discharge. Three pumps, at two locations, provide a transfer pumping capacity of 809l/sec at 72m head; there are two standby pumps.
Power consumption of these pumps required upgraded electrical supply and new standby generators at Margate and Weatherlees. The generators can feed the national grid, and are available when there are grid supply problems in the area not affecting the treatment works.
During the project’s research and modelling phase, four scale-models of the pumping station were built to test computer simulations. At Margate, the pumps are installed in existing channels that required modification to ensure suitable flow patterns.
Ellis says: “You can run all the computer programmes you like, but there’s nothing like building a one-tenth scale model. You physically see the flows and what’s going to happen.”
The motor control panels at the three locations are integrated by a profibus system. Although not new, profibus is rarely used in the water industry. But in this application it allows control of all three sites – from any one – with monitors at each displaying information about the entire system.
A fibre optic link, laid alongside the pipelines, transmits data between the three sites. The software for the scheme was provided by Coventry-based MSC Software in a deal worth £1.7M.
The 11km twin pipelines that convey flows to and from Weatherlees for treatment and discharge are constructed from 800mm-diameter ductile iron.
The decision to use ductile iron, rather than the carbon steel originally specified, brought about significant savings. Most of the pipe run is over agricultural land, and was laid by open-cut with work contracted to Dragtone.
Tunnels were constructed to carry the pipe through built-up areas with shafts at either end to provide access for the tunnel-boring machine and to remove spoil. A Lovat full-face boring machine was used for two separate 2.7m concrete-lined tunnels, 1.1km and 600m respectively, which run beneath Margate.
Open cut was used for most of the 2.8km, 450mm-diameter pipeline that transfers flows from Broadstairs headworks to Margate headworks. Directional drilling was used for a section running beneath a golf course. Following a re-evaluation, the original directional drilling run was significantly reduced in length, which cut costs further.
The pipeline crosses the end of the runway at Kent International Airport, Manston. Originally, tunnelling was planned for this section. But, after negotiations with the airport operator, open-cut was used. This saved an estimated £1M.
For additional protection for this section of the pipeline, the ductile iron pipes sit inside pre-cast concrete pipes, with the annulus filled by foamed concrete.
The scheme also required the construction of a 600m storm outfall at Foreness Point. The 1.8m-diameter pipeline was jacked beneath the seabed, requiring the excavation of a submarine reception pit.
The project included upgrading both headworks, the bulk of which are housed within underground structures built into the chalk cliffs. “Work at Margate and Broadstairs transfer stations is within Victorian underground structures, which form part of the cliff face.
“We had to undertake major civil engineering as well as mechanical and electrical work, within these structures and keep the existing systems running,” says Ellis. “All this made phasing very difficult with the added complication that we were also dealing with live works.”
Improvements at Foreness include the provision of 6mm screening, with enhanced washing and dewatering. Stormwater retention capacity has been increased by 13,000m3 with construction of a 24m-deep, 12.5m-diameter shaft.
As a result of the increased pumping capacity required to pass flows to Weatherlees, the power supply and electrical equipment had to be replaced.
The transfer pumps have a 908l/sec capacity with the storm pumps providing 7m3/sec lift.
The works close proximity to housing means that an odour abatement system has also been installed and silencers for air intakes and exhaust gases on the generators.
At Foreland, lift pumps with 1,100l/sec capacity and transfer pumps with 264l/sec capacity were installed to pump wastewater to Foreness. The inlet screens were upgraded with storm screens able to handle 800l/sec. The odour control equipment was improved.
Secondary and tertiary treatment takes place in the new works at Weatherlees, built alongside but totally separate from the existing treatment facility serving Ramsgate and which discharges to the River Stour.
The new works, which has a full flow to treatment of 809l/sec (120,000 people equivalent), consists of a step-feed activated sludge plant comprising four aeration lanes with fine-bubble diffusion. Following final settlement, wastewater receives UV treatment to ensure discharges comply with shellfish and bathing water directives.
A new pumping station returns the treated water for discharge at Foreness. There are no primary tanks in
the new treatment works, a construction cost saving, and relies on larger aeration lanes.
Sludges from both treatments works at Weatherlees are processed on site and moved by road for further treatment at Southern Water’s Ashford sludge treatment facility itself in the process of an upgrade.
Weatherlees is subject to odour limits so odour abatement equipment has been installed for the aeration lanes and sludge treatment plant.