The Vartry Water Supply Scheme provides drinking water for over 200,000 consumers in the City of Dublin, Ireland. It was constructed in the 1860s and treats raw water from two reservoirs and deliver it, via 40km of trunk mains, to an open storage reservoir system in Stillorgan – the largest remaining in Europe – in Dún Laoghaire, Dublin. In 2015, the Irish Environmental Protection Agency’s (EPA) placed Stillorgan Reservoir on its Remedial Action List due to the risk of contamination by Cryptosporidium, which presents a public health risk. The then newly formed Irish Water, which operates the reservoir in conjunction with Dublin City Council, set out to solve the problem quickly. It decided to take a two stage approach, starting with immediate installation of Cryptosporidium in water treatment, then launching a longer term capital project to construct a new, covered reservoir on the site.
Cryptosporidium is a parasitic protozoan that causes cryptosporidiosis in humans when its eggs or oocysts are consumed in poor drinking water. This is an illness characterised by diarrhoea, often accompanied by stomach pain, nausea or vomiting, a high fever, dehydration and weight loss. Once infected, healthy people usually recover quickly but those with other medical conditions may be more seriously affected; cryptosporidiosis can be fatal, especially for those receiving chemotherapy or for other immune compromised patients. The pathogenic infection cycle begins with infected animals excreting the organism in the form of oocysts – a sort of egg about 5µm in diameter with a tough outer shell that protects it from the environment –which find their way into drinking water sources. A conventional water treatment plant normally removes the oocysts, but there is a potential for post-treatment contamination entering open storage reservoirs such as Stillorgan.
Cryptosporidium oocysts are resistant to normal chlorine dosing levels and other oxidising biocides and physical barriers, like microfiltration or ultrafiltration, are widely used for as a control measure. A 2004 World Health Organisation report flagged up a 3-log (99.9%) inactivation of Cryptosporidium by ultraviolet irradiation at a UV dose of less than 10mJ/cm2 – a significantly lower dose than is required to inactivate most bacteria – and this was borne out by experience in the US where UV at a validated dose of 12mJ/cm2 has been used for Cryptosporidium in water treatment for more than a decade.
CRYPTOSPORIDIUM IN WATER TREATMENT: SOLUTIONS
Due to the requirement for a quick provision of a suitable control measure in the Stillorgan Reservoir, Irish Water looked at the provision of suitable UV Lamp solution, once the required raw water parameters (Colour, UVT, Hydraulic Flow, etc) and treatment criteria was met. Irish Water commissioned Engineering Consultants Ryan Hanley to design and manage the Stillorgan Reservoir Disinfection Upgrade. The challenge was to design the largest fully automatic UV plant in Ireland within the existing reservoir’s Valve House. There were severe spatial constraints within the Valve House and the project also demanded complex interfaces with an existing live 900mm trunk distribution manifold, which was required to provide uninterrupted supply to a large area of Dublin City during the project works. In parallel with installing the new UV system, the existing chlorination system was to be modified to a marginal chlorination system with the chlorine dosing points being relocated to downstream of the UV reactors, which allows for improved control of chlorine residuals in the distribution system and reduces chlorine usage significantly.
Irish Water appointed Veolia Water Ireland as the principle contractor, and Evoqua, as the selected UV system supplier, working to develop a suitable design to handle a flow of up to 240Ml/d. The new UV plant was to be installed in an existing plant room which had a highly restricted pipe gallery. Evoqua carried out an in depth CFD study of the UV installation and piping network to ensure performance would not be affected by the tight installation space, and developed a design based on utilising three ATG UV™ SX-1873-30 UV reactors operating as duty/assist/standby, in a configuration that minimised the size of UV reactors, actuated flow control valves, pipes, fittings and flow measurement. This meant that the UV system could be installed without adding substantial building and installation costs. It also allowed a more efficient use of power consumption, reducing ongoing operating costs. Each reactor is equipped with eighteen ultra-efficient 7.3kW UV lamps featuring the very latest in medium pressure lamp technology.
The efficacy of UV disinfection is dependent on the dose of radiation required to handle the microbiological load and the ability of the UV equipment to deliver that dose consistently. That depends on the output of the UV lamps, the UV transmissivity (UVT) of the water and the design of the UV reactor. To ensure that UV systems meet these requirements, most European countries have adopted the 2006 Ultraviolet Disinfection Guidance Manual issued by the US EPA Office of Water, which requires independent third party validation testing by biodosimetry. This determines the log inactivation of specific challenge microorganisms passing through a UV reactor delivering a corresponding Reduction Equivalent Dose (RED). The UV control system must maintain this RED over the full range of works flows and UV transmissivities by monitoring UV intensity in each reactor and automatically adjusting the dose. Not all UV reactors are capable of achieving this, but the ultra-compact UV systems at Stillorgan are designed to meet these validation protocols as well as the latest UK, Irish and European potable drinking water regulations, including the recent WIMES 801.B specification guidance for closed vessel drinking water UV systems.
As part of the upgrade, the existing SCADA system was modified to incorporate digital screen mimics of the new UV primary disinfection system, the reconfigured chlorine disinfection and additional flow measurement and controllers. It also included the provision of continuous monitoring UVT and chlorine residual instrumentation with automatic control to modulate input power from 40% to 100% to match the operational power to varying flow rates and seasonal water quality changes.
Irish Water’s Project Manager, William McKnight, said “The successful implementation of the €3.4m UV disinfection project has meant that the Stillorgan reservoir was successfully removed from the Irish EPA’s Remedial Action List and allows for the construction of the new covered reservoir to commence in 2018 with a projected completion date of 2020. Once completed, the final scheme will ensure a safe, reliable water supply to over 200,000 people in Dublin.”