By Thomas M. Hargy

Summary: With heightened interest in alternative water disinfection methods as a result of increasing regulation of chemical disinfection by-products, ultraviolet (UV) irradiation has received considerable attention this past year. Among results of this attention has been formation of the International Ultraviolet Association and recent efforts in California to update guidelines for UV installations in wastewater reclamation as well as to apply similar guidelines for UV drinking drinking water facilities. The process described below is still under way at the time of this writing, so details are subject to change.

In 1993, California established the “UV Guidelines for Wastewater Reclamation”1 in recognition of the equivalency of properly applied UV technology to accepted chlorine disinfection. The purpose of this document was to give guidance for the use of UV disinfection in California of reclaimed waters which, in the absence of pathogenic bacteria and viruses (defined as less than 2.2 total coliform per 100 milliliter, and 4-log inactivation of poliovirus), might then be reused in various specified applications.

Due to technical advances in the UV industry, recent awareness that the technology can be effective against Cryptosporidium2,3 and Giardia4,5 and concern over chemical disinfection by-products (DBPs), California Department of Health Services (DHS) regulators and other interested parties have seen the need to facilitate and improve applications of UV technology in water reuse and extend the application to drinking water. DHS, in association with the National Water Research Institute (NWRI), and the American Water Works Association Research Foundation (AWWARF), brought UV experts together from around the world, in a series of workshops with three major objectives:

  • Update the 1993 guidelines for use of UV disinfection in wastewater reclamation applications,
  • Establish parallel guidelines for application of UV to drinking water, and
  • Draft a testing protocol to allow confident determination of a UV system’s performance.

The chief obstacle to ready acceptance of UV lies in the lack of any measurable post-disinfection “residual.” These guidelines hope to address this need in different ways for wastewater and drinking water. Common to each would be a verification test procedure linking field performance to controlled laboratory bioassays wherein the UV dose can be precisely quantified.

The original wastewater guidelines, written in 1993, narrowly specified use of low pressure UV systems with flow parallel to horizontal lamps in open channels. Units with any other lamp type or configuration were considered non-conforming, and were required to undergo performance validation prior to acceptance. The new draft guidelines regard any UV reactor/lamp system similarly, as all would require preliminary site specific testing at full scale or an alternative pilot scale with verification that the full-scale unit is hydraulically equivalent and technologically identical to the pilot unit tested. Primary system operation monitoring would consist of flow rate and water UV transmissivity monitoring.

The required UV dose level in wastewater treatment would depend on upstream processes. Media filter systems would require 100 milliJoules per square centimeter (mJ/cm2), which is equivalent to 100 milliwatt seconds per square centimeter (mW-sec/cm2­). Research in wastewater irradiation has found this dose to be effective by a significant safety factor even when microorganisms are sheltered from UV by particulate material found in media filter effluent.6 When more effective filtration is used and particles are less likely to shield microorganisms, the guidelines would call for lower UV doses. Downstream of microfiltration and ultrafiltration, 80 mJ/cm2 would be targeted and, following reverse osmosis where UV transmittance can be expected to be high, 50 mJ/cm2 would be needed.

Achievement of these target doses wouldn’t be directly monitored but would be assured by design of the UV disinfection system, which would define the acceptable flow rate range and minimum UV transmittance of the water to be treated. Having limited the range of these two parameters, the UV dose would be assigned, based on the assumptions that lamps of any valid age were operating at only 50 percent of new output and quartz sleeve transmittance was no more than 80 percent.

Whether proper operational dose was achieved then would be verified by performance of the reactor in a bioassay test or, in the case of a pilot scale unit, with subsequent scale-up to full-scale validation by hydraulic tests. Upon installation following a successful completion of these tests, the effective delivery of at least the minimum dose would be assumed if:

  • Lamp age didn’t exceed specifications,
  • System flow rate was within the validated range, and
  • UV transmittance of the water was above a minimum value.

Alarms would be triggered if these conditions weren’t met, or if other equipment failures occurred. These alarms would either bring standby equipment on-line or initiate contingency actions. Performance would be confirmed by microorganism sample analysis.

Drinking water
The guidelines drafted for drinking water are intended for applications to any public water supply—and so would provide guidance to small and large systems, although the redundancy and monitoring requirements would likely become disproportionately burdensome as system size decreases. The drinking water guidelines follow a similar format to those for wastewater, but with several notable exceptions. One is that the dose to be delivered wouldn’t be specified, as this would be dependent on site specific target organism(s) and level of inactivation desired.

Another difference between drinking water guidelines and those for wastewater is drinking water equipment monitoring would rely on UV irradiance sensor readings rather than UV transmittance for determining the UV dose being delivered, with verification of UV reactor and sensor performance prior to installation. This concept follows that of the German standard7 for UV disinfection systems for drinking water, which requires bioassay testing of full-scale units at a central testing facility.

For drinking water applications, site specific testing of a reactor would be required only for unfiltered or minimally filtered surface water or groundwater under the influence of surface water. For applications in water of high quality (£ 0.3 NTU), any given model would only need to undergo a single bioassay performance study and then could be installed anywhere the water quality conditions were met—without on-site validation.

The guidelines discussed here wouldn’t utilize a central facility but would require, at a minimum, similar independent validation testing of any model reactor and its sensor(s), using a specified protocol (see Figure 1) wherein the inactivation achieved by the reactor is compared to that achieved in controlled bench scale exposures. Once validated, reactors of this identical design could be used at any site where high quality drinking water was to be treated. For sites where 0.3 nephelometric turbidity units (NTU) is exceeded, site specific testing would be necessary. At all locations, a reference sensor probe—which would normally be removed from the flow stream (and thus from the likelihood of becoming fouled)—would be kept on-hand to insert into the reactor at regular intervals as a check against the in-line reactor sensor. This check would verify the in-line sensor or trigger its correction (calibration). Both reference and in-line sensors would require periodic factory calibration.

Once validated and installed, a UV monitoring system would depend primarily on flow rate and irradiance readings. As with wastewater guidelines, alarms would be triggered if these parameters were outside the specified operating range or if other equipment failures occurred. Alarms would either bring standby equipment on-line or initiate contingency actions.

These guidelines, as proposed, are intended to aid regulators, engineers, UV manufacturers, and utility managers and operators in designing, implementing and operating UV systems for effective disinfection of wastewater or drinking water. As guidelines, they themselves wouldn’t be enforced as standards, although any regulatory agency could adopt them as requirements for a given jurisdiction. By precisely defining the conditions under which it’s certain effective UV disinfection is delivered, these guidelines would facilitate installation of UV systems and offer added confidence successful treatment is being provided.


  1. National Water Research Institute, “UV Disinfection Guidelines for Wastewater Reclamation in California and UV Disinfection Research Needs Identification,” Fountain Valley, Calif., 1993.
  2. Bukhari, Z., T.M. Hargy, J.R. Bolton, B. Dussert, J.L. Clancy, “Medium-pressure UV light for oocyst inactivation,” Journal American Water Works Association, 1999, 91(3):86-94.
  3. Hargy, T.M., “UV Equipment Proven against Cryptosporidium,” WC&P, March 2000, p. 36-38.
  4. Finch, G.R., and M. Belosevic, “Inactivation of Cryptosporidium parvum and Giardia muris with medium pressure ultraviolet radiation,” Proceedings, USEPA Workshop on UV Disinfection of Drinking Water, 2000.
  5. G-A Shin, G-A, K.G. Linden, M.D. Sobsey, “Inactivation of Cryptosporidium parvum oocysts and Giardia lamblia cysts by monochromatic UV radiation,” Proceedings, Paris 2000 Conference on Health Related Microbiology, HRM-40, 2000.
  6. Chen, C.L., and J.F. Kuo, “UV Inactivation of Bacteria and Viruses in Tertiary Effluent,” Presentation to the California Department of Health Services UV Disinfection Committee on Research at the County Sanitation Districts of Los Angeles County, 1992.
  7. Deutscher Verein des Gas und Wasserfaches e.V.—DVGW (German Association on Gas and Water), “Technical Standard W 294: UV Systems for Disinfection in Drinking Water Supplies—Requirements and Testing,” 1997.

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 also is a member of the WC&P Technical Review Committee. 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:


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