By Rick Andrew

Disinfection of drinking water is accomplished through several different means. One of the most common and most effective is ultraviolet (UV) disinfection using a low-pressure mercury source. UV disinfection can be used on public water supplies, pools and spas, and also POU/POE drinking water treatment. With enough energy, or dosage, the UV radiation at the 254 nm wavelength has the ability to disrupt DNA in pathogenic microorganisms so they cannot reproduce, which prevents them from causing disease in people drinking the water. Because pathogenic microorganisms cause acute health effects (i.e., can make people very sick very quickly), one of the important considerations in UV system design is fail safe features. It is important for users to be alerted to situations during which their system may not be disinfecting the water effectively.

Two variables in the UV equation

UV disinfection relies on UV energy, measured as dosage, for effectiveness. The greater the energy or dosage, the more effective the disinfection. UV dosage is a function of two variables, UV intensity and contact time with the radiation source:

UV dosage = UV intensity * Contact Time

This is analogous to people getting sunburned – the more intense the sun, and the longer the time spent in the sun, the more severe the sunburn. Essentially, this is the same thing happening to microorganisms in a UV system – they get ‘sunburned’ to the point where their DNA is disrupted and they cannot reproduce. It is possible to stay in intense sun for only a short period of time without becoming burned, but if the sun is less intense, longer periods of exposure without burn are possible. Likewise, a UV system with high intensity will require less contact time to achieve adequate disinfection, whereas a system with low intensity will require longer contact time.

UV sensors warn of low intensity

Several factors can cause a system to suffer reduced UV intensity. These factors include turbidity in the water, scaling of the quartz sleeve, failing lamps and other factors. A sensor calibrated to the system will be able to detect reduced UV intensity and warn the user that the system may not be performing adequately.

Flow control as a fail safe for contact time

UV intensity is only half of the dosage equation, however. The other factor, contact time, must be accounted for when considering fail safe for UV systems. The way NSF/ANSI 55 Ultraviolet Microbiological Water Treatment Systems addresses fail-safe for contact time is to require a flow control in the system. This flow control prevents the system from flowing too fast, thereby providing a guaranteed minimum contact time.

Disinfection testing with fail-safe considerations

Fail-safe design features are taken into account when testing UV systems for disinfection under NSF/ANSI 55. Class A UV systems are first evaluated for maximum achievable flow rate. This involves laboratory testing at a variety of inlet pressures, making sure that the test apparatus in no way restricts flow through the system. The flow rates are measured and recorded at 15 psig, 20 psig, 30 psig, 40 psig, 50 psig, 60 psig, 70 psig, 80 psig, 90 psig, 100 psig, and the system’s maximum working pressure. The highest flowrate achieved is then used for disinfection testing.

UV intensity fail safe is taken into account by conducting disinfection testing with reduced intensity all the way to the alarm point, or to 70 percent transmittance, whichever is lower. Parahydroxybenzoic acid (PHBA) is added to water flowing through the system little by little until the alarm is activated. If the UV transmittance is less than or equal to 70 percent, the water as adjusted can be used. Otherwise, additional PHBA is added until the transmittance is reduced to 70 percent.

Ultimately, the two variables for determining dosage are incorporated into system design fail safe and disinfection testing fail safe by incorporating flow control and UV sensors in the system design, and then testing at the highest achievable flowrate at the alarm set point or 70-percent UV transmittance, whichever is lower. By testing at the highest achievable flowrate under alarm conditions, consumers can be assured that the system will function properly unless the alarm is indicating a system problem, no matter what.

Fail safe – an important consideration for acute health effects

Fail safe is such an important consideration with UV systems because of the potential immediate and possibly severe impact of a system failure. Pathogenic microorganisms, should they be present in drinking water, can very quickly affect consumers in a very serious way. Those who are immuno-compromised, or frail in some way (such as elderly or very young), can die from consuming drinking water contaminated with certain pathogens such as Cryptosporidium. These outbreaks have happened and people have died (Milwaukee, WI, 1993, and Walkerton, Ontario, 2000).

Because of the critical nature of fail safe in drinking water treatment systems protecting consumers from acute health contaminants, testing under fail-safe conditions is built into NSF/ANSI 55. By considering the two aspects of fail safe, UV intensity and contact time, and developing the test method around both of them, the standard assures that conforming systems will perform effectively for consumers or alarm and warn consumers that they are not functioning properly.

About the author

Rick Andrew is the Operations Manager of NSF’s Drinking Water Treatment Units Program for certification of POU/POE systems and components, and he has previously served as the Technical Manager for the program. Andrew has a Bachelor’s Degree in chemistry and an MBA from the University of Michigan. He can be reached at (800) NSF-MARK or email: [email protected] .


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