CASE STUDY |
where else to take the sludge for processing • Continued equipment sup- plier
support, post-contract
completion – the States of Guernsey expects and re- quires the same level of re- sponse from suppliers as if on the UK mainland • Continued process support, post-contract completion
The two main areas of
Flows up to 20l/s are diverted up to the filtration plant; the 7m- long, 400amp MCC is fed from a new transformer dedicated to airport equipment
works comprised the Lovers Leap area and the process slab area.
A weired reinforced con-
crete diversion chamber 3m deep was constructed to divert up to 20l/s to the submersible pumping station. This was 7m deep and both were con- structed in a small valley with gradients up to 1:4. The gradient required that
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 deliv- ery were providing Guernsey Airport with: • Project flexibility – if things did go wrong there was no-
a working platform be con- structed for the 24-tonne ex- cavator prior to works taking place. Face protection of the excavation was achieved by using close sheeting and man- hole 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 spe-
cialist design team designed the pressure and GAC fil- ters. Duty/standby pressure filters were installed to re- move 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 vari- able speed pumps. This con- tact time enables the removal of PFOS from the groundwa- ter. 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 back- wash waste and wastewater. Backwashing of the pressure filters occurs on a differential pressure and time expired ba- sis. Backwash flows are con- trolled to 10l/s by a modu- lating valve on the output of duty / assist / standby pumps. Waste from backwashing is passed to the waste storage tank.
Backwash of the GAC fil- ters also occurs on a differ- ential pressure and time ex- pired 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 analy- sis prior to disposal. The fil-
“Trant relished the opportunity to be involved in this interesting and challenging project”
tered 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 contami- nation. A 7m-long, 400amp MCC
with a standalone UPS-pro- tected treatment control panel was installed, and is fed from a new transformer dedicated to airport equipment. The (form 4 type 2) MCC was lo-
cated in a purpose-built elec- trical kiosk. The MCC has the latest generation of Siemens touchscreen HMI and was se- lected for its superb graphical displays of site status. An eth- ernet card was included to al- low the future installation of the Guernsey Water SCADA system used for island-wide monitoring and control of all pumping stations and treat- ment works. Trant’s design team worked
closely with ARCADIS UK and Guernsey Airport to en- sure the project remained on schedule and that the par- ticular health and safety and security aspects involved with working on an international airport were adhered to. Advanced notice was re-
quired 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 designat- ed 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 assis- tance of airport operations.
www.trant.co.uk
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