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Planes, drains and pollution controls

A high-profile groundwater plant scheme has been successfully completed at Guernsey Airport.

Flows up to 20l/s are diverted up to the filtration plant Flows up to 20l/s are diverted up to the filtration plant

Guernsey Airport’s groundwater quality has been enhanced by water engineering specialist Trant Construction, reflecting a challenging brief met by genuine working partnerships, trademark expertise and proven technology. ARCADIS UK contracted Southampton-based Trant to complete the project for Guernsey Airport, overseen by the island’s Public Services Department.

The groundwater plant scheme involves a new drainage system and additional pollution control measures to afford a greater level of protection for the island’s water supply. The new plant will treat surface and groundwater from the international airfield, before entering the Beau Valley stream to the north of the airport boundary, and help filter out traces of a toxic chemical, previously used in firefighting foam and called perfluorooctane sulfonate (PFOS).

An environmental requirement of the Airport 2040 development, which will equip the airfield to service the island for the next 30 years, the work dovetailed with an £80M project to upgrade the airport’s surfaces.

The operation of the treatment plant was required under waste license to allow removal of 7,000m3 of soil contaminated with PFOS; affected soil was sealed inside a container and covered with a mound of earth.

Production of PFOS began to be phased out in 2000; now concentrations in finished and semi-finished products must not exceed 10mg/kg.

With operational completion and handover in March, the success of the project was achieved through in-house multidisciplinary teamwork and experienced site staff, with Trant’s specialist clean water engineering team leading the way.

Trant worked with ARCADIS, Guernsey Airport and Guernsey Water to ensure plant operation and tests were completed to allow closure of the airport on March 20-21, reportedly the first scheduled one, other than Christmas Day, since 1974.

The scheme is designed to filter flows of up to 20l/s by diverting these from the Lovers Leap outfall up to the filtration plant where it passes first through the anthracite filters and then through the granulated activated carbon filters before returning by gravity back to Lovers Leap and discharged to stream.

ARCADIS carried out the project management and overall design, while Trant provided detailed design in-house; anthracite pressure filters, granulated activated carbon, process, electrical and ICA. Trant was responsible for the civil, mechanical, electrical installation and commissioning. Trant Systems Electrical (TSE) designed, manufactured, installed and commissioned the new motor control centre (MCC) and Treatment Control Panel (TCP), and developed the functional design specification and process software.

Laurent Wallis, M&E project manager for Trant, says: “Trant Construction relished the opportunity to be involved in this interesting and challenging project which was subject to close media scrutiny. We have again been able to demonstrate our capabilities in providing complete civil, mechanical, electrical, design, construction and commissioning within programme.”

ARCADIS UK, which carried out the environmental study and design for the scheme and project-managed construction, says: “Because of the client’s compressed time schedule, the specification was performance-based rather than prescriptive. Procurement of the bespoke process elements was always going to be extremely challenging as this was where the critical path lay.

“Part-way through the nine-month build, the client requested to have the plant functional two weeks earlier than the March 30 planned completion. This was to ensure that ground water would be treated when excavation started on March 20 for the ensuing runway improvement contract. The challenge was met by Trant, who made great efforts to successfully achieve this.”

A site compound was first established encompassing Lovers Leap and a section of the Airport Fire Training Ground, which was assigned landside status for ease of construction activities.

Key factors in project delivery were providing Guernsey Airport with:

• Project flexibility – if things did go wrong there was nowhere else to take the sludge for processing

• Continued equipment supplier support, post-contract completion – the States of Guernsey expects and requires the same level of response from suppliers as if on the UK mainland

• Continued process support, post-contract completion

The two main areas of works comprised the Lovers Leap area and the process slab area.

A weired reinforced concrete diversion chamber 3m deep was constructed to divert up to 20l/s to the submersible pumping station. This was 7m deep and both were constructed in a small valley with gradients up to 1:4.

The gradient required that a working platform be constructed for the 24-tonne excavator prior to works taking place. Face protection of the excavation was achieved by using close sheeting and manhole frames.

The submersible pumping station consists of wet well including 2no;15kW variable speed duty/standby pumps capable of the designed 20l/s, while the rising main from the PS was constructed of PE pipe electrofusion weld on site by Trant. The gravity return pipeline was constructed of uPVC plastic.

Trant’s clean water specialist design team designed the pressure and GAC filters. Duty/standby pressure filters were installed to remove solids from the ground water and protect the GAC (granulated activated carbon filtration media) filters from fouling. Each pressure filter is capable of the 20l/s required to be filtered under the waste licence. Four GAC filters were installed in two parallel trains filtering 10l/s per train over a 45-minute contact time and are fed by duty standby variable speed pumps. This contact time enables the removal of PFOS from the groundwater. The two trains operated in tandem gives a maximum filtration of 20l/s.

Five GRP storage tanks were installed on the process slab for GAC feed water, two filtered water, a GAC backwash waste and wastewater. Backwashing of the pressure filters occurs on a differential pressure and time expired basis. Backwash flows are controlled to 10l/s by a modulating valve on the output of duty / assist / standby pumps. Waste from backwashing is passed to the waste storage tank.

Backwash of the GAC filters also occurs on a differential pressure and time expired basis. As the waste from these backwashes will contain PFOS this is passed to the bag filter feed tank.

Due to the PFOS removed by the GAC Filters when a backwash of the GAC filters is performed the bag filters in series with 100 micron and 3 micron filtration remove the carbon and therefore the PFOS from the backwash waste. Differential pressure switches indicate when a filter requires removal. The waste from these bags is placed in a covered-lined skip for analysis prior to disposal. The filtered backwash can then be pumped to foul sewer.

A bunded area is provided for washdown, and water from this area is pumped back into the bag filter feed tank to prevent any possible contamination.

A 7m-long, 400amp MCC with a standalone UPS-protected treatment control panel was installed, and is fed from a new transformer dedicated to airport equipment. The (form 4 type 2) MCC was located in a purpose-built electrical kiosk. The MCC has the latest generation of Siemens touchscreen HMI and was selected for its superb graphical displays of site status. An ethernet card was included to allow the future installation of the Guernsey Water SCADA system used for island-wide monitoring and control of all pumping stations and treatment works.

Trant’s design team worked closely with ARCADIS UK and Guernsey Airport to ensure the project remained on schedule and that the particular health and safety and security aspects involved with working on an international airport were adhered to.

Advanced notice was required for lifting operations were required. Low visibility periods are common during the first quarter of the year on Guernsey and due to safety concerns it was necessary to suspend all works in designated areas and postpone lifting operations.

Also, Guernsey’s small and busy road network meant it was necessary for larger items of plant and equipment to be brought in along the island’s designated heavy goods route before 6am and across the airport runway with the assistance of airport operations. 

www.trant.co.uk

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