By Rick Andrew
Ultraviolet (UV) technology has been applied successfully to drinking water treatment for many years. Low-pressure mercury lamps have a very strong UV emission at 254 nanometers. Such radiation at that 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.
An essential requirement of UV drinking water treatment is to ensure that an adequate UV dose is being delivered. This requirement is addressed via various test protocols, generically known as bioassay testing. Different applications have different bioassay test protocols.
Municipal water treatment, POU/POE water treatment and swimming pool water treatment each have their own bioassay test protocols. The accepted protocol for bioassay testing of POU/POE UV drinking water treatment systems is described in NSF/ANSI Standard 55, Ultraviolet microbiological water treatment systems.
NSF/ANSI Standard 55 requires that Class A UV systems must deliver a high enough UV dose at 254 nm (40 mJ/cm2) to inactivate the pathogenic microorganisms that could be responsible for causing disease through contamination of our drinking water. This dose is verified through the bioassay test procedure detailed in the Standard.
UV dose is not only a function of the energy output of the lamp, but also a function of the contact time between the water being treated and the UV source:
UV dose = UV source irradiance intensity x contact time
Given this equation, you can see that it is possible for a relatively weak UV source to deliver a sufficient UV dose as long as the contact time is sufficient. Because of this relationship between the UV source irradiance intensity and contact time, Standard 55 testing (like other bioassay type tests) is structured to determine the actual UV dose delivered by the UV system as opposed to measuring the irradiance intensity of the UV source.
POU/POE UV drinking water treatment system
A typical UV system provides a flow path surrounding a mercury lamp, structured to provide close proximity of the water flow along the length of the mercury lamp (see Figure 1). Because turbidity or particulate matter in the water can occlude the UV source and absorb the UV energy, many system manufacturers recommend the use of pretreatment to reduce turbidity and particulate matter prior to the system.
UV may not be effective against all forms of protozoan cysts. Studies have demonstrated the effectiveness of UV against Cryptosproridium and Giardia, but there have not been studies to demonstrate effectiveness of UV against Toxoplasma or Entamoeba. For these reasons, Standard 55 requires the use of a cyst-rated prefilter for UV devices making general cyst reduction claims or being used to disinfect untreated surface water sources.
All Class A UV systems must also have an alarm triggered by a UV sensor to warn the user when sufficient dose is not being delivered by the system. Insufficient dose can result from a UV source with diminishing power, turbid water, or fouling of the device. Note that sensors operating in the visible light range or other types of alarms such as those triggered by a lack of power to the lamp are not sufficient for conformance to NSF/ANSI 55 Class A requirements.
Given the structure of a typical UV system, one might wonder how the UV dose can actually be measured. It is obviously a function of many factors: the irradiance intensity of the UV source, proximity of the water being treated to the source, contact time of the water with the source and whether any water is occluded or hidden from the source as it passes through the system.
The solution to this complex measurement problem is to use the dose-response method of establishing UV dosage. This is done for all bioassay type dose measurement protocols. And as we all hear in the media lately, “the devil is in the details.”
The basic concept of a dose-response bioassay is fairly simple: subject a known microorganism to the UV treatment system and see how effectively it is inactivated by the system. Then compare the level of inactivation achieved by the system back to the known response of the organism when subjected to a measured UV dosage.
Every time a ‘batch’ of microorganisms is cultured, true strength or response to a given dosage of UV power is not really known. Microorganisms are a bit like any other living thing – when well nourished and given a proper growth environment, they are very healthy and strong. If their nourishment is poorer, or their environment is not ideal, they tend to be weaker and more susceptible to inactivation by UV irradiance.
This variability in organism culture hardiness necessitates the development of a dose-response curve for each batch of microorganisms cultured. Once the batch of microorganisms has been cultured, samples of them are subjected to a known UV source under very controlled conditions, with increasing dosage.
This dosage is established by contact time under the known UV source. Standard 55 requires response to be measured at dosages of 0, 6, 12, 18, 24, 30, 36, 42, 48, 54 and 60 mJ/cm2 for Class A testing. In practice, this is achieved by exposing plates of the microorganisms to a highly calibrated and controlled UV source for increasing lengths of time – for example, 0, 6, 12, 18, 24, 30, 36, 42, 48, 54 and 60 seconds.
The specific microorganism used in Standard 55 for establishing the UV dose for a Class A system is MS-2 Coliphage (ATCC #15597B). MS-2 Coliphage shows very linear, measurable response over the dosage range appropriate to Class A requirements. It is also widely available, can be cultured relatively easily and has a good analytical method that allows it to be enumerated without significant interferences from other organisms.
A typical dose-response curve for MS-2 (Figure 2) shows that at the 40 mJ/cm2 dosage, about 1.6 logs of MS-2 are inactivated. NSF has seen inactivation of various cultures or batches of MS-2 range from about 1.5 logs to about 1.9 logs over the years.
This is a relatively narrow range, but nonetheless the variability in cultures is evident. If dose-response curves were not established for each batch, the true strength of the organisms would not really be known. To simply require a 1.9 log reduction of MS-2 in order to pass the test might understate the performance of many systems.
This is essentially how the UV dose of the test system is determined according to NSF/ANSI 55 and other bioassays. Although some protocols use different microorganisms depending on the target dosage and other factors, the test operation and sampling regime requires a detailed focus.
Test operation and sampling
Once the strength of the culture of organisms has been established through dose-response, the actual UV device can be tested. Component filters or other media that may interfere with the test are removed from the UV devices to be tested.
Two devices are tested and they are plumbed on the test stand in parallel. These devices are conditioned according to the manufacturer’s instructions prior to testing. Test water with the following characteristics is prepared:
- 17.5 – 22.5 ºC
- <1.0 NTU turbidity
- 200 – 500 mg/L total dissolved solids (TDS)
- >96 percent UV transmittance (before addition of parahydroxybenzoic acid, or PHBA)
The test is performed at or above the maximum flow rate obtained through the devices with their integral flow control devices installed. The test water is further adjusted to reduce the UV transmittance with parahydroxybenzoic acid (PHBA). PHBA is added to reach 70 percent UV transmittance.
If the alarm is operating, the test is ready to begin. If the alarm is not operating, additional PHBA is added until the alarm activates. This level of UV transmittance is maintained throughout the test.
By testing at 70 percent UV transmittance or the device alarm point, the device is demonstrated to provide the required UV dosage necessary for disinfection. This is accurate even when the UV transmittance is significantly reduced to the point of alarm activation. (See Figure 3 for testing example.)
The devices are operated over a period of seven days, with samples repeatedly collected on five of the days under two different operating conditions:
- Initial device start-up after an overnight stagnation period.
- When devices are operating at steady state conditions.
This sampling regime helps to assess performance under a variety of usage patterns. For example, if a device is subject to overheating when there is no flow, which will decrease the lamp output, the data resulting from the initial samples at device start-up after an overnight stagnation period will capture this issue.
If a device uses ‘instant on’ technology and the lamp is not fully powered up when initial flow occurs, the data from the initial samples will capture this issue as well. Other bioassay protocols not designed for POU/POE where there will be stagnation periods may not involve sampling under these operating conditions.
The overall dosage is determined by comparing a geometric mean of organism counts in all of the influent samples to that of a geometric mean of organism counts in all of the effluent samples of each of two test devices. The overall log reduction calculated for each device must be equal to or greater than the log reduction of the dose-response curve at 40 mJ/cm2 (1.6 log reduction in the example of Figure 2).
Examining UV devices may raise the question of how their effectiveness is measured. Some relatively small UV devices designed for POU applications may also raise questions as to how a system that small could possibly be effective.
Since it is all about the dose (and how that dose is measured), small UV devices have been proven to deliver high dosages if their flow rates are low enough. Once the dosage is tested according to NSF/ANSI Standard 55, this verifies that certified UV systems do perform as advertised when operated and maintained according to the manufacturer’s instructions.
About the author
Rick Andrew has been with NSF International for over 10 years, working with certification of residential drinking water products. He is currently the Operations Manager of the Drinking Water Treatment Units Program 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 1-800-NSF-MARK or email: [email protected].
Figure 3. UV System on Test in the NSF Laboratory