When Dublin's wastewater treatment plant was built in the 1970s, it answered the Irish capital's population and environmental needs. But as environmental awareness and Dublin's population has grown, so has the need for greater and more environmentally sound wastewater treatment. To answer that need, a new plant is under construction that will fully replace the old system.
The new plant, knows as Ringsend, will greatly improve the level of wastewater treatment in Dublin and better the water quality in Dublin Bay. It’s the biggest project of its kind in Europe, costing £200 million and is one of the largest environmental improvement schemes ever undertaken in Ireland. When completed, the Ringsend plant will provide secondary and tertiary treatment. (Only primary treatment is provided at Ringsend at present).
The 30-year-old system is being replaced by new screens, screenings handling and primary (lamella plate clarifiers) and secondary treatment known as sequencing batch reactor (SBR). The new works will have a capacity of 11.5 cubic meters of water per second, which will pass through from three main pumping stations in Dublin as well as from a gravity feed in Dodder Valley. But the 0.5-square-kilometre site on which the new plant is being constructed is only half of what would have been desirable for a project of this scale.
'The biggest challenge on this project has been the site,' says Richard Dun, project director from the American engineering firm Black & Veatch, one-third of the building consortium ABA. 'The available workspace is limited and we have to maintain plant operations while replacing and adding to the existing facilities.' The other two companies forming the consortium are the Irish civil construction company Ascon and British Anglian Water. aba is commissioned by the municipal service company Dublin Corporation to design, build and also operate the plant for 20 years after its completion.
The plant is situated on a peninsula at the mouth of Liffey, the river that divides Dublin's city center. 'All the surrounding land is owned by the Electricity Supply Board and by Dublin Port, who both are extremely short of land,' explains Battie White, project engineer at Dublin Corporation.
The solution to having too little space was provided by technology from ITT Industries' Sanitaire unit (a subsidiary of ITT Flygt) a company focused on creating innovative wastewater treatment technologies for municipal and industrial wastewater treatment facilities. Sanitaire provided 96 stainless steel decanters for 24 sequencing batch reactor tanks (SBR tanks) for the plant. To save space, the tanks, each measuring 52 by 39 meters were stacked on top of each other, instead of in a row. But because of the location, there are height restrictions of 28.8 meters. Restrictions apply also for the storm overflow settlement tanks that will be built on the site of the original turn-of-the-century works. The storm tanks will have a capacity to hold 56,000 cubic metres.
There will be four decanters in each SBR tank, all moving with one motor. The 40 ft (12.2 m) stainless steel decanters are coupled into pairs and connected by a line shaft to act as one. The decanters work by decanting the treated water from the top down, withdrawing only the uppermost clear water from the basin and prevents disruption of the settling solids. The decanter equipment requires minimum maintenance, and enables plant personnel to observe effluent quality at all times, it also provides emergency overflow control and a uniform rate of discharge.
The SBR Process
The sequential batch reactor (SBR) process can be used at both municipal and industrial wastewater treatment plants including biological nutrient removal. The process is a fully automatic, simple-to-operate, time-based system that responds to flow and load variations and is easily expanded. The processes of biological oxidation, nitrification, denitrification, phosphorus removal and liquids/solids separation are achieved continuously in a single basin.
The Dublin plant also is putting 108,000 Sanitaire diffusers to work. The diffusers are nine-inch (230 mm) Silver Series II membrane. 'Sanitaire is the only supplier of SBR technology that manufactures its own decanters and diffusers,' says Andrew Ware, director ABJ, Sanitaire UK. 'It was this, coupled with Sanitaire's design capabilities and excellent track record on larger projects, that gave the team the competitive edge and secured the order.'
Providing Systems for Waste Treatment
The order of 108,000 diffusers and 96 stainless steel decanters for the SBR (sequencing batch reactor) tanks in Dublin is the largest single order undertaken by Sanitaire Corporation, a subsidiary of ITT Flygt.
'This type of order highlights the extra dimension that Sanitaire Corporation has brought to ITT Flygt, as the extension of our core product lines allows us to tender for more projects in the wastewater treatment industry. It also gives the company an advantage for customers as we can now offer complete submersible systems of pumps, mixers and aeration products,' says Anders Hallberg, ITT Flygt President.
'The order for the Dublin Bay Ringsend Project follows on from earlier successes won by Sanitaire in Europe. The worlds second largest SBR - Sequential Batch Reactor, is currently being built in Cardiff, Wales, using Sanitaire diffusers and ABJ decanters,' comments Jerry Woodhouse, head of European operations for Sanitaire Corporation.
He continues, 'our SBR projects are not just of interest from a technology standpoint. A number of our European SBR projects have involved Sanitaire as a lead technology designer, delivering design services from the initial concept stage onwards. In addition, many of our schemes are executed on a partnering basis, contributing to long business relationships that benefit both our customers and Sanitaire.'
A Steady Record of Improving Waste Treatment
Until last year, all sludge from the population equivalent of 1.2 million in Dublin was dumped into the Irish Sea. To meet EU directives, a sludge treatment plant was built in September 1999 and is now drying the sludge and processing it to be used as fertilizer. This plant will be incorporated in the new Ringsend plant in 2002. A second sludge treatment plant is also planned, designed for 36,000 tons of sewage sludge annually to be dewatered, hydrolysed, digested and dried. Biogas from the digester will be used for running the works.
By the middle of 2002 the workforce of more than 300 construction workers will be gone and the new plant will be run by a staff of five. The capacity will be enough to handle a population of 1.9 million. The pollution removal will be 95 percent and the plant will, according to Dublin Corporation, be meeting the EU standards regarding both suspended solids and biochemical oxygen demand.