Determining dosages
Achieving a sufficient dosage is critical when applying UV technology to recreational water systems. Dosage is essentially the amount of UV energy to which a contaminant is exposed; different micro-organisms require different dosage levels for neutralization. Dosage is a function of time and is measured in mJ (milijoules/cm2). Subsequently, the amount of contact time between light and water within a vessel determines the UV dose. Therefore, the rate at which water flows through the vessel (measured in gallons per minute [gpm]) affects the applied dosage to the micro-organism—the contaminant is exposed to more energy the longer it spends in the vessel. Alternatively, if the flow rate exceeds recommended levels, the applied dosage will decrease (UV devices are sized according to flow rate, not gallonage, as with other pool equipment).
Recent notable Crytposporidium outbreaks throughout the U.S. underscore the importance of UV technology, given its effectiveness at inactivating chlorine-resistant parasites. Results from a study carried out under the NSF International-USEPA Environmental Technology Verification Program show a nearly four-log inactivation of Cryptosporidium oocysts at UV dosages as low as 19 mJ/cm2. In addition to the aforementioned study, a second analysis was conducted to verify the degree of Cryptosporidium oocyst inactivation when exposed to low dosages of UV radiation. Data from Clancy et. al. show a five-log inactivation of the oocysts at 33 mJ/cm2, which equate to a 99.999 per cent reduction of infection capacity.
UV is a one-pass treatment application, which means the system needs to be sized to achieve the target dosage for inactivation during a single pass through the unit. Dosage increases as flow rate is reduced, so system design plays a crucial role in unit selection. It is important to note that due to general recirculation patterns within a pool, it takes approximately four turnovers to expose the entire pool volume to the UV light within a 24-hour time period, based on a six-hour turnover rate.
UV technology has been proven to be an effective and systematic approach to recreational water treatment. The ability to inactivate microbiological contaminants, especially those resistant to primary sanitation programs, makes a UV system an attractive option when addressing concerns about overall public health. Additional efficacy against undesirable chlorinated disinfection byproducts only enhances its value.
