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Problem Solver - August 2012

Ensure Your UV System's Effectiveness

UV systems have become popular for controlling chloramines in indoor facilities, as well as inactivating chlorine-resistant pathogens like Cryptosporidium at both indoor and outdoor facilities. Several states and provinces have adopted codes requiring UV systems for supplemental disinfection on spray parks and pools with interactive water features. On a national level, the CDC has addressed RWIs (recreational waterborne illnesses) through the development of the Model Aquatic Health Code Module, Disinfection & Water Quality. As a result, almost all aquatic designers are specifying UV systems on new construction and retrofits. In a recent industry survey, aquatic facilities respondents ranked UV systems as their No. 2 choice in planned purchases. All of this has led to a huge influx of UV systems into the commercial marketplace, leaving designers, owners and operators asking several questions.

Q: How do I know what to use?

A: UV systems for recreational water are used in several different applications, which require different sizing parameters, indoor requirements for chloramine destruction and outdoor requirements for crypto disinfection. If not sized properly for the specific requirement, especially if code requirements are involved, the system will show the lamp is on, but that does not mean it is working effectively. Correct UV system selection is based on flow rate, UV transmittance and dose level required. Each application requires a specific dose of energy. This is measured as mj/cm2. Effective treatment requires a certain minimum dose of energy. This energy is a calculation based on the flow rate through the UV chamber, the length of time the water is treated in the chamber and the lamp intensity supplied.

Q: Why is validation important?

A: A reputable manufacturer designs a system based on CFD modeling to determine how large a chamber they need for a given flow and how much lamp intensity is required for that particular flow at a specific transmittance. This is, however, only modeling. Until the UV system is actually tested by a recognized third party, with various transmittance conditions, the manufacturer cannot be certain that the UV system will perform properly under the actual conditions of use. As the transmittance changes, the ability of the UV lamp to destroy chloramines or inactivate pathogens like Crypto- sporidium at the same given flow changes dramatically.

Q: Why is transmittance significant?

A: UV light is only effective if it can see the organics or inorganics in the water that it is destroying or inactivating. The ability of UV light to see through the water it is treating is measured in a percentage of light being transmitted for a certain distance. In a lab, this is typically measured through a centimeter-wide cell full of the actual water that is going to be treated. The percentage of light that remains at the end of that distance it is being transmitted varies in very simplistic terms on the clarity of the water. Recreational water is much lower in transmittance than drinking water. The difference between transmitting through "recreational water" that typically lets 92 percent to 94 percent of the UV light intensity produced through and transmitting through drinking water, which allows 98 percent of the UV light intensity produced through is almost a 40 percent variance. In other words, actual effectiveness can be 40 percent lower in performance if the UV system flow rate is based on a transmittance of 98 percent and the transmittance is actually 94 percent.

Q: Why is a UV sensor/monitor critical?

A: A UV system cannot be tested and validated without the ability of the test agency to actually read and compile data on the energy produced (dose) in mj/cm2 and the intensity of light in w/m2 being applied at a given flow and transmittance. The end user must be able to verify that the effective dose and correct intensity is being produced at all times that the UV system is in operation to really know if the UV system is providing the proper supplemental disinfection to keep their patrons safe.


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