Kent coast standards soar in £66M cleanup project

The first flows for the Margate & Broadstairs Urban Wastewater Treatment Scheme were pumped along 11km of new pipeline to a new wastewater works at Weatherlees, Kent, in March. Natasha Wiseman visited the site.

In common with all coastal towns, treatment of sewage for the towns of Margate and Broadstairs in Kent had involved simple screening before being pumped to sea. In April 2005, Southern Water contracted joint venture partners Black & Veatch and Costain (BVC) to bring full treatment to the region to enhance the quality of coastal waters off the Isle of Thanet and ensure compliance with the Urban Waste Water Treatment Directive, Bathing Water Directive and Shellfish Waters Directive.

The £66M Margate & Broadstairs Urban Wastewater Treatment Scheme will bring full treatment, including screening, bio-treatment, settlement and UV treatment, before discharge into the English Channel at this sensitive marine location. Following screening and grit removal at existing headworks, wastewater will be discharged via long sea outfalls. New sludge treatment will thicken the sludge before transport to Ashford, where the facilities are also being upgraded.

Planning restrictions meant that it was not possible to build a new wastewater treatment works (WwTW) at either of existing the headworks - Foreness Point for Margate or North Foreland for Broadstairs. Instead, the design team decided to connect the works by a new pipeline and pump the combined flows from both headworks a distance of 11km for treatment at Weatherlees WwTW.

The new WwTW at Weatherless has been constructed on the site of the existing works serving Ramsgate. Treated flows are returned, via another new pipeline, for discharge at Foreness Point.

One of the biggest challenges facing the project team was ensuring robust performance from the eight 330kW pumps that transfer flows from headworks to WwTW and back again for discharge. Only a couple of suppliers could deliver the specification and Grundfos high-head pumps were selected to pump to an 80m head.

According to project manager Martin Ellis: "These pumps are the heart of the whole process. If you can't pump the wastewater from Margate and Broadstairs to Weatherlees and back it all goes into the sea, and you can forget about the millions being spent."

To ensure the correct specification and siting of this mission-critical element of the scheme, four scale models of the pumping station were built during the project's research and modelling phase. This allowed computer simulations to be physically tested. The team needed to avoid air pockets in the flows entering and potentially damaging the pumps.

"You can run all the computer programs you like, but there's nothing like building a one-tenth scale model. You physically see the flows and what's going to happen," Ellis says.

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 cost savings.

As most of the pipe run is over agricultural land, it could be laid by open-cut techniques. But the pipeline runs through areas of archaeological interest, so a survey was carried out prior to work starting. This helped recover Roman skeletons and Bronze-age implements around the site of Richborough Fort. All artefacts were donated to the Kent Archaelogical Trust.

The pipeline also crossed the end of the runway at Kent International Airport, Manston. Originally tunnelling was planned for this section but, following negotiations with the airport operator, the team was allowed to use open-cut techniques, saving an estimated £1M. For additional protection for this section of the pipeline, the ductile-iron pipes sit inside precast concrete pipes, with the annulus filled by foamed concrete.

Pipejacks were needed under two railway lines. Each of the two mains (for transfer and final effluent) were installed in separate tunnels at each crossing. The crossings were about 50m and 75m long, and the 800mm diameter mains were installed within 1,200mm-diameter jacked pipes.

The pipeline passes through two urban areas, and two 2.87m internal-diameter tunnels were needed to house the pipes. A Lovat full-face boring machine was used for the concrete-lined tunnels, which were 1,380m and 600m long and constructed within the chalk beneath Margate.

Open cut was used for most of the 2.8km, 450mm-diameter pipeline, which transfers flows from Broadstairs' headworks to Margate's 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, engendering further savings.

The scheme also required the construction of a 600m storm water outfall at Foreness Point, designed to handle two spills a year. The 1,800mm-diameter pipeline was jacked beneath the seabed, requiring the excavation of a submarine reception pit.

Full flow to treatment is 809l/s, but the existing 2km long sea outfall (LSO) is consented for 1,100l/s. For storm water to flow out of the new outfall, the new storm storage has to be at full capacity with 291l/s flowing through the LSO.

The scope of the project also includes upgrading both headworks, the bulk of which are housed within underground structures built into the chalk cliffs.

Project director Martin Charlton explains. "Work at Margate and Broadstairs' transfer stations is within Victorian underground structures which form part of the cliff face. Undertaking major civil and MEICA [Mechanical, Electrical, Instrumentation, Control and Automation Engineering] modification to these structures, whilst retaining operation 24/7 is what sets this project apart."

Improvements at Foreness Point include the provision of 6mm screening, with enhanced washing and dewatering. And the site's storm water-retention capacity has been increased by 13,000m3 following construction of a 24m-deep, 12.5m-diameter shaft. The increased pumping capacity needed to pass flows to Weatherlees meant that the power supply and electrical equipment had to be replaced.

At North Foreland, lift and transfer pumps were installed to pump wastewater to Foreness Point, inlet screens were upgraded and the odour control equipment was improved.

Secondary and tertiary treatment will take place in new works at Weatherlees WwTW, consisting of a step-feed activated sludge plant comprising four aeration lanes with fine-bubble diffusion. After final settlement, the wastewater is treated with UV to ensure that discharges comply with shellfish and bathing water directives. A new pumping station will return the treated water for discharge at Foreness Point.

Sludges from the new and existing treatment trains at Weatherlees will be processed by a new on-site facility. Alfa Laval centrifuges dewater the sludge to 24% dry solids, and the sludge will be transported to Ashford, once the new sludge centre is commissioned.

As a result of strict odour limits, two types of odour-abatement system have also been installed on the Weatherlees site: a dry system consisting of two carbon filters on the aeration lanes, and a wet chemical scrubbing method on the sludge area.

Black & Veatch Costain contracted Auma to supply more than 100 actuators to the Margate and Weatherlees sites during a transition to fieldbus technology.

Commenting on the selection of the actuators, a spokesman for Black & Veatch Costain says: "As we are managing a transition from traditional wiring to a Profibus solution at Margate, it was highly advantageous

- from both a cost- and time-saving perspective - to adopt Auma's dual control system. It was the adaptability afforded by Auma's products that led to their selection for the Weatherlees WwTW. They are the only company to provide a plug and socket connection, which means that, if an actuator is taken out of service, the Profibus network will continue to function."

The Margate and Broadstairs scheme has been very demanding in terms of timescale.

"Basically, in less than two years, we have designed and built a large complex project," says Charlton. "To achieve this programme, the design was completely overlapped with construction. There was a huge challenge in making sure outline design, design development and detailed design stayed ahead of construction."

Commenting on the announcement that wastewater was successfully being pumped to the new works via the new pipeline, Bruce Ainsworth, managing director of Black & Veatch Water Europe, says: "This is a great accomplishment and a credit to everyone involved. It truly is a measure of what can be done by a strong team working together."

Savings of around £2M have been achieved to date, and the project looks likely to be completed ahead of the October 2007 deadline.

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