Volume 43 Number 6
UV: Evaluating Ultraviolet Light Reactor Performance -- Utilizing Bioassay
Summary: While being a reliable tool for drinking water disinfection, UV light is difficult to measure for effectiveness. In fact, no method provides precise results. Yet, by using a bioassay, relatively accurate UV doses can be assigned, which result in a better comprehension of the method's dynamics.
Ultraviolet (UV) light is known to be an effective disinfectant of most microorganisms by damaging an organism's DNA thus preventing the organism from multiplying, and making it incapable of causing infection in a host. Due to varying absorbance characteristics of different organisms and abilities of organisms to repair their DNA following exposure to UV, the amount (dose) of UV required to cause irreversible damage varies among species.
UV dose is expressed as millijoules per square centimeter (mJ/cm2), which is equal to milliWatt-seconds per square centimeter (mWsec/cm2) or 1,000 microWatt-seconds per square centimeter (uWsec/cm2). For example, to achieve 4-log (99.99 percent) inactivation of vegetative bacteria, such as Escherichia coli, 5-to-10 mJ/cm2 are necessary. To achieve the same level of inactivation of spore forming bacteria and viruses, 25-to-50 mJ/cm2 are needed. Recent studies have shown that protozoa (Giardia and Cryptosporidium) are quite sensitive to UV, so that 2-to-10 mJ/cm2 would provide at least 4-log inactivation.[1,2]
James Bolton and Larry Henke provided an excellent review of UV disinfection in a 1999 WC&P article. It's useful to reiterate two of their points. First, UV is a physical process. Photons of germicidal UV (those roughly in the 260-to-265 nanometer [nm] range) impact the DNA of target microorganisms. The result, or photoproduct, of this particular interaction is existing bonds within the DNA are broken and new ones (thymine dimers) are formed. As the DNA strands are now altered, they're incapable of proceeding through steps necessary for replication, and thereby the organism is no longer infectious. Second, UV dose is defined as the amount of UV light, or irradiance, reaching an object multiplied by the exposure time, in seconds. In UV reactors, the primary factors that influence UV dose are the output of the UV lamp(s), the spatial relationship of the lamp(s) to the chamber, retention time, and water quality (especially parameters contributing to UV absorbance). Particulate matter and/or turbidity can reduce UV performance so effective pretreatment may be required in some instances.
Need for a UV tool
Once the photons have been absorbed by the water, particles, microorganisms, or the reactor walls, they're gone without a trace. A grab sample of the UV reactor effluent will yield no clues as to the amount of UV light to which the water has been exposed. To address this gap, UV reactor manufacturers have, for all but the most basic units, incorporated in-line UV sensors designed to measure the amount of UV reaching a distinct point(s) on the reactor wall. From this reading(s) and the UV absorbance and flow rate of water through the reactor (which determines average residence time) and knowing the distance from the lamp to the sensor, a delivered dose can be calculated.
To translate the readings of one or a few sensors to an estimated UV dose, it's necessary many assumptions be made. As a sensor measures UV at a single point on the reactor wall, some water is traveling near the lamp and some near the wall. Use of the sensor reading assumes a complete lateral mixing of water as it moves through the unit. Assignment of a UV dose value based on the sensor reading assumes all water is present in the reactor for the same amount of time. Baffles or diffusers may be employed to assure that mixing and consistent residence time occurs.
In an ideal reactor, with plug flow and complete lateral (radial) mixing, a properly functioning sensor can be relied upon to indicate average irradiance; however, in actual practice, the confidence one has with this indication may be compromised by several uncertainties: 1) A potential end-user may require validation a reactor performs as promised, 2) the sensor reading may lose sensitivity, and thereby not respond to fluctuations in lamp output or water quality, and 3) hydraulic dynamics may cause flow patterns outside the optimal, resulting in short-circuiting of some part of the water stream through the reactor. In these or any other situations where verification of reactor performance is necessary, some foolproof method is needed.
Dose response curve
The exposure times are selected to give incrementing UV doses across the range of interest. Log inactivation of the test organism is plotted against UV dose to provide a dose response curve. A non-specific example is given in Figure 1. By comparing the inactivation achieved in an actual reactor against the plot of this collimated beam generated curve, the UV dose that was delivered by the reactor can be determined. In this example, if the reactor achieves 2.2-log inactivation, we find an equivalent of 30 mJ/cm2 was delivered.
Choice of organisms
A 2-log inactivation of a resistant surrogate might suggest greater inactivation of a UV sensitive organism could be achieved; however, if some condition such as poor reactor hydraulics or a failed lamp allowed 1 percent of the organisms to pass without receiving any irradiation, 99 percent or 2-log inactivation would be the limit for this reactor, regardless of how sensitive the target. Four organisms, which meet the above requirements, in the drinking water treatment UV dose range are E. coli, Saccharomyces cerevisiae, Bacillus subtilis spores and MS2 coliphage. Typical dose response curves for the latter three are given in Figure 2.
A comparison of these curves shows that E. coli, which is much more sensitive to UV than the latter two organisms, would be suitable for bioassay tests of a unit designed to provide only 4-to-10 mJ/cm2. To test systems in ranges more often applied to drinking water, the more resistant organisms are generally relied upon. For example, ANSI/NSF Standard 55 evaluates two types of UV drinking water treatment units with S. cerevisiae (point-of-use) and B. subtilis spores (point-of-entry) to verify the required 16 mJ/cm2 and 38 mJ/cm2, respectively, are delivered. B. subtilis is also the surrogate used by both Germany and Austria, where extensive testing of UV systems is carried out to verify the required dose of 40 mJ/cm2 is applied.[4,6,7]
With the exception of the original California guideline (1993), a premise common to the above guidelines, protocols, or regulations regarding UV verification, is that each reactor style must be tested, and across the range of potential service flow rates. It's widely felt there can be little confidence in testing a small-scale pilot version and extrapolating results to larger units. A noted exception would be where a single reactor is validated, and the whole system scaled up, by introducing multiples of the tested unit in parallel.
About the author Tom Hargy is senior scientist at Clancy Environmental Consultants of St. Albans, Vt., which provides research and development services to the drinking water, wastewater and high purity water industries. He is a member of the International Ultraviolet Association, American Water Works Association and Water Quality Association. He is also a member of the WC&P Technical Review Committee. And he holds a bachelor's degree in geology from Macalester College in St. Paul, Minn. Hargy can be reached at (802) 527-2460, (802) 524-3909 (fax) or email: firstname.lastname@example.org
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