THE public are invited next week to hear about plans to dig a giant sewage and surface water storage tank in a field bordering the Airport, close to the Coop service station.
The ‘attenuation tank’ – which generated strong complaints from parishioners but ultimately received planning permission in June – is part of an Islandwide programme which could, potentially, see up to nine of them dug at various sewerage network pinch-points around the Island.
On Wednesday, a drop-in session will be held at St Peter’s Parish Hall between 5.30 pm and 7.30 pm, where officers from the Infrastructure and Environment Department will share details of what will be built.
There is no start date yet because a number of conditions that were attached to the approval, given in June, first need to be met. These include submitting a full assessment of the nature and extent of potential contamination, including PFAS.
Samples taken by Government show trace levels of PFAS at the site – but I&E is waiting to hear back from the Regulation team in the Environment Department.
PFAS is a family of chemicals, which has been linked to causing cancer and other conditions, that was an ingredient of firefighting foam sprayed at the Airport.
Tom Le Gallais, who is lead engineer at I&E, told the JEP that the tank was designed to store foul and surface run-off water at peak times, such as after heavy rainfall.
It will be 25m in diameter, 20m deep and be able to store 6,100 m3 of liquid. By comparison, the cavern under Snow Hill car park has a capacity of 25,000 m3.
He said: “The tank is part of a range of measures identified in the Bridging Liquid Waste Strategy to deal with, among other things, the new affordable housing sites, other development, population changes and the impact of climate change.”
Currently, all sewage from most of St Ouen, St Mary and St Peter flows through a 300 mm pipe which runs along Grande Route de St Pierre, then goes under the runway and down the Pont Marquet valley towards St Aubin.
The drainage pipe is at capacity so is currently being upgraded to 450 mm, which is why there are currently a series of road closures near St Peter’s Village.
Like the town cavern, the tanks will store sewage and any surface water before it being fed into the main treatment works at Bellozanne.
On a dry day, Bellozanne receives around 35,000 m3 of sewage but this can increase to more than 70,000 m3 on a wet day.
When it comes to constructing the tank, before excavation a ring of 910 cm-diameter overlapping ‘secant’ reenforced concrete piles will be driven into the ground. This will form a circular wall which will provide the structural strength to allow digging to take place within.
Most of the spoil will be recycled at La Collette.
There are height restrictions during construction due to the site being so close to the Airport so a specialist rig is coming over from Germany. Work will take place during normal working hours. Because of the geology, piles can driven to the required depth without the need for blasting.
Rok-Regal Construction is the principal contractor, and other local sub-contractors will be used. Structural engineering firm T&G has designed the tank.
An application to build a storage tank near Maufant Village, behind the large greenhouse site, was approved in May but is the subject of a third-party appeal.
