Water Journal : Water Journal July 2012
refereed paper UV disinfection water JULY 2012 63 Abstract Over the last 10 years, use of ultraviolet (UV) disinfection in municipal drinking water treatment has grown rapidly. Most installations at drinking water facilities are focused on Cryptosporidium inactivation in response to the US Environmental Protection Agency (USEPA) Long-Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR). Other systems target Giardia, viruses or bacteria. Despite widespread use of UV disinfection, practical information can be difficult to locate. Regulatory reporting requirements may not be well understood or overlooked until a UV system has been commissioned. Although USEPA's UV Disinfection Guidance Manual (UVDGM) provides excellent general information, state, provincial or other site-specific entities can override UVDGM requirements. Site-specific constraints may limit UV disinfection options. In most cases, detailed conversations with regulators are required to identify specific requirements. Participating Utilities This article summarises general operation and maintenance (O&M) requirements for several North American municipal drinking water UV disinfection installations. The utilities providing information are: • Seattle Public Utilities, Cedar Treatment Facility (Seattle, Washington, US); • City of North Bay, Water Treatment • Facility (North Bay, Ontario, Canada); • City of St Johns, Windsor Lake Treatment Facility (St Johns, Newfoundland and Labrador, Canada); • North Shore Water Commission, Water Filtration Plant (Glendale, Wisconsin, US); • City of Cedar Rapids, J Avenue and Northwest Water Treatment Plants (Cedar Rapids, Iowa, US); • Ketchikan Public Utilities, Water Treatment Plant (Ketchikan, Alaska, US). These facilities have operated UV systems ranging from one year to over 10 years. Total flow capacity ranges from 50 million litres per day (MLD) to 600 MLD. Most of the utilities target Cryptosporidium and Giardia inactivation, while one utility targets virus inactivation. All facilities use medium-pressure lamp technologies and pre-validated UV reactors. Most of them have automatic sleeve-cleaning systems. Minimum lamp power ranges from 30% to 60% of maximum. Design Elements Before the UV systems became operational, key elements were incorporated during design to ensure proper levels of disinfection were achieved and maintenance requirements were not cumbersome for operations staff. Some of the key design elements included: Installing adequate straight pipe diameters upstream of the UV reactor to ensure good hydraulic conditions in the UV reactor; • Utilising online UV transmittance (UVT) analysers and flow meters to optimise the UV dose and energy delivered for varying water quality conditions; • Providing adequate space around the UV reactors and lift equipment for easier maintenance; • Incorporating provisions for future expansion or more stringent disinfection targets; • Supplying sufficient ancillary devices such as air relief valves, drain lines and sample taps; • Locating operator interface units (OIU) close to the UV reactors to facilitate daily maintenance and monthly reporting activities; • Perform sufficient factory, functional and performance testing to validate performance and warranties. Monitoring of the UV Reactor The UVDGM includes recommended monitoring and recording frequencies for required parameters such as UV intensity, lamp status, flow rate, UVT, validated dose and off-specification events. Typically, these parameters are automatically tracked by the UV process control system. T Elliott, P Swaim A North American perspective OPERATING AND MAINTAINING UV DISINFECTION SYSTEMS AT DRINKING WATER TREATMENT FACILITIES A UV facility incorporating some of the design elements described above.
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