By Chris Sheppard
The main capital headline for Guernsey Water in 2012/13 was the development of a new wastewater inlet works and storm storage facility at its Belle Greve site. Work on the project began in March 2012 and was completed in October 2013.
The Belle Greve Wastewater Centre is located on the east coast of Guernsey, and receives over 99% of sewage flows from pumping stations around the Island. Historically, the flow was macerated here before being discharged to sea through a long-sea outfall. The outfall stretches out a mile into a fast-moving body of water known as the Little Russel.
The existing plant was 40 years old, with obsolete equipment making spare parts very difficult to obtain. There had been issues with odour nuisance to neighbours as a result of the inlet chamber being open, and the plant being unable to cope with storm events which lead to discharges being released to sea via the less effective short-sea outfall.
In 2012, following results of a scientific study of the receiving waters by global experts Intertek Metoc (www.metoc.co.uk), the States of Guernsey agreed that the Island’s wastewater could continue to be discharged into the Little Russel provided that non-biodegradable material was removed. The Metoc study provided firm evidence that the discharge from an improved outfall pipe at Belle Greve would:
protect the surrounding water from the impact of wastewater discharges
not pose a risk to the immediate benthic environment.
This could all be achieved without the provision of further sewage treatment due to the unique characteristics of the Little Russel.
Now constructed, the new plant is housed inside a building with odour control, and consists of modern industry-standard screens and grit removal equipment. The state-of-the-art mechanical screens remove grit and non-biodegradable material larger than 6mm in any two dimensions. Washed and compacted screenings and grit are transported to local landfill sites for disposal. The remaining biodegradable elements are quickly broken down through natural processes in the receiving waters without detriment to the natural environment. Washwater for the mechanical screens is taken from a nearby raw water source at Marais Stream.
In addition to the above, new stormwater storage tanks have been built to increase the capacity of wastewater that can be held during peak flows (when heavy rainfall combines with high tides), reducing the need to discharge untreated overflows into the sea via the short-sea outfall. The new storage capacity measures 4,000 cubic metres and has been created to cope with a 1 in 30 year storm of one hour duration.
Local construction and engineering company Trant (Guernsey) Ltd was awarded the contract for the project, and work was overseen by the management of Guernsey Water and their appointed consultants, Atkins Ltd. The budgeted cost for the whole project was a little over £11m, and the final expenditure figure came in under that figure.
The project was a challenge for a number of reasons. From a design point of view, Atkins had to factor in the use of existing assets on a very tight footprint while maintaining existing flow through the site. Another issue was the close proximity of residential areas around three sides of the site, which lead to Guernsey Water, Atkins staff and Trant holding two public drop-in sessions primarily for nearby residents and businesses, and issuing a number of information leaflets during the project’s timeframe. This allowed the public to learn more about the project and air any concerns that they might have. Project staff maintained a close relationship with the public to make sure that any issues were identified and rectified quickly, thus minimising inconvenience in what was a substantial capital project for Guernsey.
As with all of its operations, Guernsey Water had to also carefully consider the environmental impact of the Belle Greve Wastewater Centre development. As part of the preliminary project work, a land swap was organised with the Environment Department in order to protect a recognised ecologically important reed bed. Also, the new storm tank structure was built fully below ground so as not to impinge on the views of the Chateau des Marais, a historic castle in St Peter Port.
The official opening for the Wastewater Centre was carried out on 1st October 2013, where those involved could reflect on a successful project which will form the foundation of Guernsey’s wastewater infrastructure for future generations.