Firsts for second city
The Main Drainage Scheme gives Cork its first WwTW and city centre sewerage system
Cork, Ireland’s second city, has no sewage treatment and an ageing sewerage system which discharges 49.2ml/d of raw sewage directly into the River Lee and Lough Mahon. Compliance with the EU Urban Waste Water Directive dictated the construction of a new WwTW and extensive upgrading of the collection system, including laying an entirely new sewerage system for Central Island, on which the city centre lies.
The city’s geography had a major influence on the system design. Cork’s suburbs lie on the northern and southern banks of the River Lee, either side of Central Island. Suburban growth flanks the river downstream until it widens to form Lough Mahon. Bordering Lough Mahon to the north is the Little Island area which includes some industrial sites. Beyond this is the Great Island, Cork Harbour and finally the sea.
The final scheme – designed by EG Pettit – will create a system in which sewage will flow under gravity from existing main outfall points on the north and south side of the city, through a large trunk main to a pumping station at Atlantic Pond on the Lee’s south bank, down river from the city centre. From there it will be pumped via a header chamber to a new WwTW at Carrigrenan, on a Little Island peninsular projecting into Lough Mahon. The WwTW will discharge into deep water at Marion Point, just upstream of Great Island.
Getting sewage into the trunk main leading from the city to Atlantic Pond required the construction of a siphon to carry wastewater from the northern suburbs and Central Island to the south bank of the Lee. The northern end of the siphon was sunk at Horgan’s Quay, the outfall of the existing main northern bank sewer. As the siphon passes under the Lee it is joined by a second shaft carrying sewage from Central Island. The siphon ends on the southern bank at Kennedy Quay where it connects to the new main trunk sewer. The 2m-diameter siphon runs at a depth of 30m.
The 3m-diameter main trunk sewer runs for 2.7km to the new pumping station at Atlantic Pond. During the laying of the sewer, contractor Ascon Nuttall laid up to 50m of pipe in single 24h shifts. New mains were also constructed to allow the pumping station to receive flows from outer northern and southern suburbs as well as areas beyond the city’s boundaries which fall naturally into the catchment. Flows to the pumping station are under gravity, but at Atlantic Pond pipe depth is such that pumping is required to direct the flow via twin 1.5m rising mains to a central collection/header chamber at Mahon. Flows from outlying southern districts are also pumped to Mahon via a 1.2km rising main from the refurbished Tramore Valley Pumphouse at Ronayne’s Court.
According to Denis Duggan, the header chamber is key to the whole scheme. It is sited on the highest point in Mahon and housed in a tall structure. The header permits gravity feed from the collection chamber via twin 1.2m-diameter pipes, under Lough Mahon’s shipping channel, to the Carrigrenan WwTW. Flows pass through the WwTW to the outffall without any further pumping.
The tender to lay the 5km siphon from the header to the WwTW was won by Dutch contractor Van Oord. The marine pipes were manufactured in Norway and towed to Ireland in 450m sections. Van Oord dredged a channel, floated the pipes into position, then sunk them and re-filled the channel. The land work was sub-contracted to a local firm. Denis Duggan believes the Lough Mahon crossing represented the “biggest project of its kind in Europe.”
The WwTW site at Carrigrenan will occupy half of a 33ha triangular peninsular. Duggan describes the site as “tailor made” because it has an excellent natural inlet route for the siphon and the topography offers nearby homes some natural screening from the works and good opportunities for further concealment during the excavation phase. Once the works is completed the remainder of the peninsular will be landscaped into a park, “so there’s a chance to give something back here,” said Denis Duggan.
The WwTW is designed to handle a current population equivalent (PE) of 374,722 with capacity to treat a projected 413,201PE by 2020. Treatment plant loadings are:
- domestic 21,821m³/d,
- commercial 3,491m³/d,
- infiltration 8,082m³/d,
- industrial 15,720m³/d. Industrial wastewater is generated mainly from pharmaceutical and food processing factories as well as two breweries.
Providing Central Island with a modern sewerage system presented Cork City Council with a particularly difficult challenge. Central Island was made up of several small islands in the Lee Estuary. As the area on which the current city centre stands was developed, during the 18th and 19th centuries, the waterways between the islands were walled and covered and the arched culverts used to convey most of the city’s sewage into the Lee. Prior to the CMDS the ancient system of culverts remained the city centre’s principal method of wastewater disposal. “To solve that problem was difficult,” said Denis, “in that we didn’t actually have a drainage system. The only way of doing it was to put in a complete, new drainage system from scratch.” It was decided that the existing system would be used to handle surface water runoff and new foul sewers would be laid, with new connections to each building.
The result of this was that every street in the city centre had to be dug up and, because a new connection was being made to each building, the work would not be confined to the centre of the road. A decision was made to co-ordinate work scheduled by other utilities with the sewerage scheme. Utilities supplied their works programmes for the next five-ten years and when the CMDS team began work on a street any new or replacement water or gas pipes required were also laid. The intention according to Denis Duggan was that: “When we go into a street, hopefully everything will be renewed. There could obviously be an emergency where something has to be dug up, but there shouldn’t be any planned work.” In all 17km of new sewers was laid.
The city centre was divided into three sections and a separate contract was drawn-up for each so the work could be staggered. Work in the historic city centre was undertaken by the city corporation rather than a private contractor so it would be easier, and cheaper, to undertake any archeological investigations, which according to Denis Duggan were necessary “virtually every time you dig.” The council concluded that if the work was contracted-out it would end up having to pay contractors during the idle periods while archeological work was underway. “You have no control over the archaeology, it may take a week or it may take months,” commented Denis.
Another major task facing the CMDS team was minimising the considerable impact the work would have upon the city and helping its people and businesses cope with the inevitable disruption. At first it proved difficult to impress upon people the scale of the project, the fact that every area of the city would be affected, and that it would take at least four years to complete. Initially, public concerns were focussed upon the location of the WwTW, but as Duggan and his team had to go to great lengths to point out: “The treatment plant was only about a third of the whole scheme. You have to get the stuff down there and you have to tackle the city centre as well. To get that message across was difficult.”
The council put a lot of work into explaining why the work was necessary, what it would entail, how it was planned and what the impacts would be. Much time and effort was also invested in keeping people informed of how the work was progressing. The CMDS team organised a series of presentations to the city council, chamber of commerce, business organisations and Garda as well as talking to individual businesses. A steady flow of publications and progress notices have also been used to help keep the city’s inhabitants abreast of the work. “A lot of work was put into explaining and making contact with these people, and it really was appreciated. If people have a problem, they know where to come, they know me and my staff,” said Denis Duggan. The council hired a PR company, Casey Communications, to help with its public information strategy. Duggan meets PR team on a weekly basis to discuss current and future developments.
With so many different contracts, up to 18 at some stages, in operation simultaneously, a key task has been to minimise traffic disruption. To provide a suitable overview a resident engineer was appointed with the sole task of monitoring and co-ordinating road closures as well as ensuring suitable diversions are in place.
The construction phase of the scheme is due to be completed late in 2003, with commissioning and operation due in spring 2004. There is a lot of work to be done, but a great deal has already been achieved, and Denis Duggan believes that although it will mean more hard work the deadline will be met.
The public health and environmental benefits of the scheme will be significant and despite the upheaval, the people of Cork seem to be aware of the need for the work and appreciative of its benefits. The city relies heavily on tourism and it is hoped the prospect of the Lee and Lough Mahon achieving EU bathing water quality standards will further increase visitor numbers as well as providing a new resource for local people. Asked whether he felt the public were in favour of the scheme Denis Duggan replied: “I think they are. The estuary should be a selling point for Cork, rather than something you don’t want to talk about, and I think people do recognise that.”