Dublin STW project underway
Work on the Dublin Bay STW in Ireland is now underway. The process will include lamella plate settlement, sequential batch reactors (SBRs) and thermal hydrolysis of sludge.The Dublin Bay STW project is the largest wastewater treatment contract in the history of the Irish state and the first to be awarded on a design, build and operate (DBO) basis. With a total cost of IR£200M, the project will provide Dublin City with a new STW serving a population equivalent (PE) of around 1.7M.
Paterson Candy is involved in the project as part of the ABA Consortium, along with another leading British firm, Anglian Water, and the Irish construction firm Ascon. Paterson Candy is closely involvement in the design, procurement, installation and commissioning of the process, including mechanical and electrical equipment. Anglian Water has won a valuable contract to operate the new treatment works once completed, in addition to the existing works.
The design and build phase of the contract should last 130 weeks, followed by the operational phase which is scheduled to run for up to 20 years.
John Devlin, Paterson Candy's business development manager in Ireland, said: "As part of the ABA consortium we are absolutely delighted to be involved in the design and construction of a wastewater treatment plant with the size and complexity of the Dublin Bay project."
The new works will be located on the site of the existing works at Ringsend, where the River Liffey flows into Dublin Bay. The existing STW currently provides preliminary and primary treatment. A sludge treatment facility has also recently been installed to treat the primary sludges until the new plant comes on line.
In order to meet the new design brief the old plant needs to be provided with additional treatment processes as well as an increase in capacity. The new design will provide the following;
- preliminary treatment in the form of screening fats, oils and grease and grit removal,
- primary treatment in the form of primary sediment-ation, using lamella plate settlement technology,
- secondary treatment using sequential batch reactor (SBR) technology to achieve BOD/COD removal and partial nitrification,
- tertiary UV disinfection.
Sludges generated from the primary and secondary stages will be treated by thickening, hydrolysis, digestion, dewatering and thermal drying. Disposal options under consideration include disposal to agricultural land or forestry.
In the year 2020...
The plant itself has been designed to accommodate flows and loads relating to a 2020 design horizon, with a full flow to treatment of 11.1m3/s and an average biological load of 98,400 kg/d BOD. This equates to a PE of 1.64M. The STW will incorporate storage for 2hrs flow under storm conditions.
A phased construction will be required to ensure the continuing operation of the existing facilities. The existing STW occupies two sites and covers an area of around 15ha. This is a very small amount of space for a plant of this size with multiple-stage treatment.
In order to find the best solution, the ABA consortium carried out an assessment of the treatment options, giving consideration to space limits and ground conditions. In the early stages of the project, account was also taken of the issues raised in the environmental impact study which had already been carried out by Dublin Corporation. ABA declared the most technically and commercially appropriate options to be lamella settlement for primary treatment, followed by sequential batch reactors (SBRs) and thermal hydrolysis of sludge.
The primary stage of treatment will involve two blocks of lamella plate settlers, each block comprising six settlement units. Use of the existing primary tanks will be maximised by locating each block within the structure of one of the existing primary tanks. By doing so, the ABA will not only save money, but also minimise construction difficulties when building foundations in poor ground conditions.
The secondary biological stage of treatment will utilise sequential batch reactor (SBR) technology. Again, the design has been partly determined by space. Carbonaceous BOD removal is the main treatment objective, plus nitrification to ensure ammoniacal nitrogen discharge is less than 18.75 mg/l. SBRs have been proposed in three blocks of eight, with each block arranged into a two-tier configuration.
The biogas produced by digestion will be used to fire four combined heat and power (CHP) engines, which with an output of 1MW, have the potential to provide 60% of the plant's power. Hot exhaust gas from the CHP units will also be used to produce steam for the thermal hydrolysis process, thereby helping to maximise plant efficiency.
|Maximum energy recovery|
One of the most innovative features is the design of the sludge treatment plant. ABA will install Simon Hartley Cambi's thermal hydrolysis technology, in order to maximise the recoverable energy potential of the sludge. This process subjects thickened primary and secondary sludges to extremes of pressure (10 bar) and temperature (160°c). Benefits of thermal hydrolysis include: