Whilst residual chlorination is relied upon to provide protection of drinking water in distribution systems, it is increasingly being supplemented by ultraviolet (UV) irradiation as a primary disinfectant for drinking water treatment. As a chemical-free process it avoids many of the issues associated with hazardous oxidising disinfectants and does not generate disinfection by-products. It is also effective against microbiological pollutants such as protozoa, that are resistant to chlorine. Cryptosporidium and Giardia are common protozoa that can cause diarrheal disease. The EU Directive on water quality requires that water treatment works must include Cryptosporidium removal. Ultraviolet (UV) irradiation is now standard practice as an effective Cryptosporidium barrier which is much more cost effective than membrane filtration, the previously most widely used process.
In 2015, Engineering Consultant Ryan Hanley was commissioned by Irish Water to design and manage the Stillorgan Reservoir Disinfection Upgrade. The reservoir had been included on the Environmental Protection Agency (EPA)s Remedial Action List (RAL) due to the absence of a Cryptosporidium barrier. The Stillorgan Reservoir, located in Dún Laoghaire - Rathdown, supplies drinking water from the Vartry and Ballymore Eustace Water Treatment Plants to over 200,000 consumers in the City of Dublin. The reservoirs store treated drinking water and are currently open and at risk of environmental contamination. Additional treatment to provide a barrier for Cryptosporidium was required in the short-term prior to construction of covered storage within the reservoir site to enable the reservoir to be removed from the EPA’s RAL. The challenge was to design a fully automatic UV system with a 900mm NB pipework manifold within the Reservoir Outlet Valve House. There were severe spatial constraints within the existing Stillorgan Valve House and the project also involved complex interfaces with an existing live trunk distribution manifold, which was required to provide uninterrupted supply to a large area of Dublin City during the project works. In parallel with the installation of the new UV system, the existing chlorination system was being modified to provide marginal chlorination to provide a residual in the distribution system, with the chlorine dosing points being relocated to downstream of the UV reactors.
Irish Water appointed Veolia Ireland as principal contractor, and Evoqua worked closely with them to develop a design based on three ATG™ UV reactors operating as duty/assist/standby to handle a flow of up to 240Ml/d. This configuration minimized the size of UV reactors, bends, tapers, tees, actuated flow control valves, air valves, scour valves, and flow measurement which meant that the UV system could be installed without adding substantial building and installation costs. It also allowed a more efficient use of power reducing operating costs.
The project also included an upgrade of the existing SCADA system, incorporating digital screen mimics of UV primary disinfection, the reconfigured chlorine disinfection, the electrical upgrade and the upgrade of the existing Valve House flow mimic to include manifold, additional flow measurement and flow controllers and the provision of continuous monitoring UVT and chlorine residual instrumentation to facilitate plant operation.
The successful implementation of the project, which involved the construction of the largest UV disinfection plant in Ireland, meant that the Stillorgan supply was removed from the EPA RAL thereby securing the safety of the water supply to over 200,000 people. Chlorine dosing at the works has been reduced to simple marginal chlorination for residual control.