UV: Pulsed Ultraviolet Irradiation for Drinking Water Disinfection
By Jim Bender
Summary: A variation on UV technology involving pulse intensity at a broadband wavelength shows promise of providing an innovative and non-chemical solution to flux enhancement and virus kill for microfiltration hollow-fiber membrane water treatment.
Emerging technologies such as pulsed blackbody ultraviolet (PBUV) light are easily integrated with hollow-fiber microfil-tration membranes to provide the next generation of advanced municipal water treatment.
Grasping the concept
Before continuing, a brief description of various terms used throughout this article may be necessary. PBUV refers, in the most general sense, to a “broadband” emission of radiation that’s dependent on the temperature of the radiating “surface.” The surface, in this case, is a “plasma” generated in a long quartz tube from a combination of inert gases such as xenon, argon and krypton by a specifically shaped electrical pulse. This pulse—in microseconds—quickly ionizes the gas and compresses it to a thin “shell” near the inside surface of the quartz tube, which drives the temperature of the plasma shell to the range of 12,000 degrees Kelvin (°K) to 20,000°K. These temperatures, and the shell geometry of the plasma, produce UV that spans wavelengths from 185 nanometers (nm) to 400 nm in a profile like that of Figure 1.
The smooth curvature of that radiation response—the “continuum”—encompasses wavelengths near their “maximum power flux” levels, measured in watts per centimeter squared per micron (W/cm2/µm). This is also known as “spectral emittance or spectral irradiance,” which refers to the power emitted from the surface of the quartz tube or “flashlamp.” The term used to describe the combination of high spectral emittance and broadband continuum is “high emissivity continuum.” Emissivity here refers to how well the surface radiator (plasma) emits radiation. If the plasma shell was thick, most of the radiation—especially at the shorter wavelengths—would be absorbed by the plasma itself, resulting in lower spectral irradiance and ultimate UV dosage—low emissivity. However, since in the PBUV lamp, the plasma shell is made thin thereby minimizing the absorption losses and increasing the UV efficiency—high emissivity.
Mercury lamps, by comparison, emit wavelengths from excited mercury, though not in a smooth arc continuum but rather in discrete “atomic lines” that have very narrow bandwidths. Most of the energy is contained in the 254-nm line with lesser amounts in the 185-nm and 365-nm lines. Mercury lamps also produce—especially in the case of medium pressure mercury lamps—a low emissivity continuum. That is, a very small level of “background” UV that spans from 185 nm to 400 nm.
Putting it all together
The integration of PBUV together with hollow-fiber microfiltration provides an effective multiple barrier for particles, bacteria, virus and protozoan cysts. By placing the PBUV unit on the raw water supply upstream of the membrane treatment, not only does pathogen and total organic carbon (TOC) removal take place but membrane flux enhancement (fewer membrane canisters can be used for the same permeate flow), reduced fouling and extended time before required chemical cleaning is realized.
PBUV, when integrated with mi-crofiltration, has also been shown to provide oxidation and removal of iron and manganese1 thereby making possible effective filtration of those constituents.
Microfiltration (MF) is a size-exclusion, pressure-driven membrane process that operates at ambient temperature in either a crossflow or flow-through configuration. It’s usually considered an intermediate between ultrafiltration and multi-media granular filtration with pore sizes ranging from 0.10-to-10 microns. It operates at pressures between 10 pounds per square inch gauge (psig) and 30 psig. MF membranes are available that can provide high quality drinking water—with turbidity of less than 0.05 NTU that exceeds Surface Water Treatment Rule (SWTR) requirements for Giardia and Cryptosporidium log removal.2 By integrating MF membranes with PBUV—a more effective virus kill—as well as secondary barrier control of bacteria can also be accomplished.
Ultraviolet (UV) radiation is used, in some cases, for disinfection of secondary effluents but has only in recent years come into use for disinfection of drinking water in the United States by municipal utilities.1 UV light is generated in many ways. The most widely used today is to excite mercury vapor in a low- or medium-pressure lamp.
Low or medium pressure ultimately indicates the amount of mercury available in the plasma stream inside of the lamp. The implications of higher pressures are:
- More actinic wavelengths become available (atomic lines),
- More input power is required,
- More actinic output power is developed—meaning radiation by which chemical changes are produced—with up to 35 percent contained in a few discrete wavelengths—185 nm, 254 nm, 365 nm—and low level emissivity,
- Higher lamp surface temperatures (800oC in medium pressure lamps), and
- A higher probability of explosion.
The term pulsed UV encompasses a number of technologies. As an example, pulsed white light (PWL) should not be confused with the topic of this paper. PWL, as its name implies, predominantly radiates in the visible region (visible blackbody peak at 450 nm)4 with a much lower emissivity continuum and even UV atomic-line radiation similar to that produced in mercury lamps.
PBUV predominantly radiates in the UV region with a high emissivity blackbody peak of 260 nm (see Figure 1). The high emissivity PBUV radiation profile equates to high peak power and, consequently, a “high photon flux,” which refers here to the number of photons emitted per pulse per lamp surface area. By comparison, with mercury UV, the term photon flux refers to the number of photons emitted per second per lamp surface area (see Figure 2).
With PBUV, wavelengths from 185 nm (the smallest wavelength the quartz will transmit) through the visible range and into the infrared at 3,000 nm are present in a high emissivity continuum. Approximately 38-to-52 percent (depending on how the lamp is used in a particular application) of the output is in the interval of 185-to-400 nm. Figure 1 shows the blackbody spectral output of PBUV. This graph illustrates the strong dependence on the pulse duration and temporal shape of the electrical pulse.
Lamps run hard
By changing certain lamp electrical parameters, a significant change in plasma temperature occurs that results in a shift in the blackbody peak. One type of electrical change results in the lamp being run harder, i.e. hotter plasma, and the blackbody peak shifts deeper into the ultraviolet. However, not all applications require a lamp to run so hard. The tradeoff in operating a lamp hard is a decline in lamplife because of the increased heat and inefficiency because any wavelengths developed by the plasma less than 185 nm are strongly absorbed by the quartz envelope of the flashlamp and the treatment water.
The radiant “excitance”—or power density at the lamp surface—ranges from 40,000 watts per square centimeter (W/cm2) to 170,000 W/cm2 per pulse with peak power ranging from 2 megawatts (MW) to 6 MW per pulse respectively. Figure 2 shows the comparison between mercury lamps and PBUV photon flux.
The root-mean-square power (RMS), or the power that you pay the electric company for, ranges from 300-to-5,000 watts. In pulsed lamps, the peak power and not the RMS power is responsible for the large number of UV photons, with the RMS power serving as a function of the number of pulses per second (pps) that are applied—generally in the range of 0.1-to-30 pps.
Manchester case study
The Manchester Water Works Lower Station (Manchester, N.H.) was the site of the first series of a PBUV membrane flux enhancement and virus kill study. The source water came from Lake Manabessic via a canal system that runs for about two miles through residential and wooded country, undoubtedly picking up fertilizers and making the treatment stream more eutrophic as it becomes heavily contaminated with algae, diatoms and humic substances.
The PBUV unit is placed upstream of the 6-inch hollow-fiber membrane unit. The flow rates chosen for the test were 26gpm average, which challenged the membrane at a flux value of 62 gallons per square foot per day (gfd), which is approximately twice the design flux. Figure 3 illustrates the flow dynamics at the challenge flowrate.
What’s immediately apparent is the increase in flux at the higher flow rate—about 50 percent—due to the action of PBUV on the raw influent treatment stream. The maximum RMS power required for this treatment is 1,700 watts that would allow, in practice, a reduction in the number of membrane units while still maintaining the flux. This has the potential to reduce both capital and operating-and-maintenance costs of hollow-fiber membrane systems for potable water treatment.
Additionally, the microbial kill associated with PBUV is high. In this study, MS2 coliphage was the chosen virus and the raw treatment stream was inoculated with 106 plaque-forming units (PFU) from a 2-liter titer—or standard concentration of solution by titration—of 108 PFU. Duplicate samples were collected at 10-minute intervals for 30 minutes on the influent and effluent of the PBUV reactor.
Because of discharge problems with potential surviving viruses, a much smaller flow rate had to be employed. A reactor influence test was first conducted at the challenge flow rate of 3-gpm to see if the reactor alone (without the PBUV unit active) has any influence on the virus kill. As suspected, it didn’t. Following this, the PBUV was activated at 1 pps (590 watts RMS) with peak power at 2.5-MW total spectrum (~1.2 MW UV from 185-to-400 nm) and the virus sampling commenced. A 6-log removal was demonstrated and is summarized in Table 1 (the 0.00 effluent values are actual counts and not detection limits—the bioassay was performed by the University of New Hampshire).
Preliminary testing of pulsed blackbody ultraviolet promises potable water treatment efficacy in municipal water treatment systems by enhancing the flux of hollow-fiber MF membranes and providing effective microbial kill. This is done without use of chemical additions to the water.
- Pall Corporation/Pulsar Environmental verification/challenge testing, “Comparisons of PBUV to chemical agents for the oxidation of iron and manganese in groundwater,” Pall Corporation, Port Washington, N.Y., October 1997.
- Kothari, N. (water systems manager, Manitowoc Public Utility), and E. St. Peter (assistant general manager, Kenosha Water Utility), “Utility Perspective on Regulatory Approval for Microfiltration Treatment Facilities in Wisconsin,” Proceedings of the AWWA Annual Conference 2000, American Water Works Association, Denver, June 2000.
- Soroushian, F., and W.D. Bellamy (CH2MHill), “Assessment of Ultraviolet Technology for Drinking Water Disinfection: Facts of Light,” Proceedings of the AWWA Annual Conference 2000, American Water Works Association, Denver, June 2000.
- Friedman-Huffman, Dr. Debi (research associate at the University of South Florida), “Pulsed White Light Takes Aim at Conventional UV,” WC&P, July 1998.
About the author
Jim Bender is president of Pulsar Environmental Technologies of Roseville, Calif., and inventor of pulsed blackbody ultraviolet (PBUV) generation systems. He is the inventor and holder of two PBUV patents used for decoating (paint removal) and has just recently received notice of allowance for two more such patents for water treatment. He has 19 years of experience in the laser and high-intensity light business, and formed Pulsar in October 1997 with New Star Lasers also of Roseville, Calif. Bender can be reached at (916) 677-1900 or email: firstname.lastname@example.org