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Water treatment scheme flying high

In a challenging brief, water engineering specialist Trant constructed and commissioned the groundwater improvement scheme at Guernsey Airport, Laurent Wallis, Trant M&E project manager explains

Guernsey Airport’s groundwater quality has been enhanced by UK water engineering specialist Trant Construction. Arcadis UK engaged the Southampton-based contractor to complete the project for Guernsey Airport, overseen by the island’s Public Services Department.

In essence, 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 water 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.

Teamwork

With operational completion on 19 March and handover on 30 March, the success of the project was achieved through in-house multi-disciplinary teamwork and experienced site staff, with Trant’s specialist clean water engineering team leading the way. Trant worked closely with Arcadis UK, Guernsey Airport and Guernsey Water to ensure plant operation and all necessary tests were completed to allow closure of the airport on 20-21 March. This was reportedly the first scheduled closure, 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.

Construction project manager, Arcadis UK, which carried out the environmental study and design for the scheme, said: “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 March 30 planned completion. This was to ensure that groundwater 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.”

Site works

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:

nProject flexibility – if things did go wrong at the works there was nowhere else to take the sludge for processing

nContinued 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

nContinued process support, post-contract completion

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

The height and gradient presented difficulties requiring health and safety expertise. The gradient required that a working platform be constructed for the 24t excavator prior to works taking place.

Face protection of the excavation was achieved by using close sheeting and manhole frames. The submersible pumping station (PS) consists of a wet well including two 15kW variable speed duty/standby pumps capable of the designed 20l/s.

The rising main from the PS was constructed of PE pipe electrofusion welded on-site. The gravity return pipeline was constructed of uPVC plastic.

In-house design

Design of the pressure and granulated activated carbon (GAC) filters was completed in-house by Trant’s clean water specialist design team. Duty/standby pressure filters, comprising anthracite filtration media, were installed to remove solids from the groundwater and to protect the GAC filtration media from fouling. Each pressure filter is capable of the 20l/s required to be filtered under the waste license.

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, one GAC feed water tank, two filtered water tanks, one GAC backwash waste tank and one wastewater tank.

Backwash systems

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. The 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. Backwash flows are controlled to 10l/s by a modulating valve on the output of the duty/assist/standby pumps. 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 100μ and 3μ 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.

System control

A 7m-long, 400amp motor control centre (MCC) with a standalone UPS-protected treatment control panel was installed. The MCC 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 was provided with 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.

Airport working

Trant’s design team worked closely with Arcadis UK and Guernsey Airport to ensure that 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.

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

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

Due to the largest crane on the island being 25t, and the layout of the filter plant, it was necessary to perform a tandem lift for each of the GAC filters. Throughout the project Trant remained in close contact with airport operations and the control tower to ensure that sufficient notice was given of any requirements and that any safety concerns they had were dealt with immediately.

Topic: Treatment
Tags: waste , filtration , water , pollution , surface water , Contractor

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