Ultraviolet (UV) Disinfection in Water Treatment

1. Ultraviolet (UV) Disinfectionin Water Treatment Samir Kumar Khanal, Ph.D., P.E. Department of Civil, Construction and Environmental Engineering Iowa State University March 5, 2004

2. History of UV Disinfection • Ancient Hindu source written at least 4000 years ago - raw water be boiled, exposed to sunlight, filtered, and then cooled in an earthen vessel. • Germicidal properties of sunlight: 1887 • Artificial UV light (Mercury lamp) developed: 1901 • First application in drinking water: Marseilles, France in 1910 • Substantial research on UV in the first half of 20th century • Limited field application: Low cost and maturity of Cl2 disinfection technology coupled with operation problems associated with early UV systems

3. Advantage and Disadvantage of UV Disinfection 9. Fouling of UV lamps

4. 400nm 100nm Visible Light Radio IR UV X-Rays l Vacuum UV UV-A UV-B UV-C 300nm 200nm Germicidal Range Increasing Popularity of UV Disinfection • Chlorinated disinfection byproducts (DBPs): THM, HAA etc. • Potential to inactivate protozoan: Cryptosporidium - resistant to Cl2 UV Radiation • UV light: 100 to 400 nm UV spectrum – 4 regions • Vacuum UV:100–200 nm • UV – C : 200 – 280 nm • UV – B : 280 – 315 nm • UV – A : 315 – 400 nm

5. Germicidal Range of UV Light • Vacuum UV- most effective – attenuates rapidly in short distance – not practical • UV-A : less effective – long exposure time – also not practical • UV disinfection – germicidal action mainly from UV- C and partly from UV - B

6. ULTRAVIOLET RADIATION • Physical Process • Damages Nucleic Acids in Organisms • Stops Reproduction of Organisms by Breaking Apart the DNA Bonds • Wavelengths Between 100-400 nm

7. Mechanisms of UV Disinfection • Disinfection by UV radiation- physical process- electromagnetic waves are transferred from a UV source to an organisms cellular materials (especially genetic materials) • UV light does not necessarily kill the microbial cell • UV light inactivates microorganisms by damaging nucleic acids (DNA or RNA) thereby interfering with replication of the microorganisms and therefore incapable of infecting a host • Different microorganisms have different degree of susceptibility to UV radiation depending on DNA content • Viruses are the most resistant • Microbial repair: regain of infectivity

8. UV Lamps • UV light can be produced by the following lamps: • Low-pressure (LP) mercury vapor lamps • Low-pressure high-output (LPHO) mercury vapor lamps • Medium-pressure (MP) mercury vapor lamps • Electrode-less mercury vapor lamps • Metal halide lamps • Xenon lamps (pulsed UV) • Eximer lamps • UV lasers Full-scale drinking water applications : LP, LPHO, or MP lamps

10. Mercury vapor Lamp Comparison

11. UV Lamp and UV Absorbance of DNA

12. LOW PRESSURE 20-25 Seconds 30% power efficiency 0.3 kW $2500 per lamp 85% at 253.7 nm MEDIUM PRESSURE 2-5 Seconds 20% power efficiency 3.0 kW $25,000 per lamp Equals 7-10 low pressure lamps Wide range wavelength LOW AND MEDIUM PRESSURE MERCURY LAMPS

13. ULTRAVIOLET WAVELENGTHS

14. UV Dose • The effectiveness of UV disinfection is based on the UV dose to which the microorganisms are exposed • UV dose is analogous to Cl2 dose Cl2 dose = Cl2 conc. x contact time (t) or Cx t UV dose (D) = I x t or if intensity not constant Where, D = UV dose, mW.s/cm2 or mJ/cm2 I = UV intensity, mW/cm2 t = exposure time, s • UV dose can be varied by varying either the intensity or the contact time

15. UV Disinfection Kinetics – Similar to Cl2 Disinfection dN/dt = Rate of change in the concentration of organisms with time k = inactivation rate constant, cm2/mW.s I = average intensity of UV light in bulk solution, mW/cm2 N = number of microorganisms at time t t = exposure time, s Residual microorganisms protected in particles

16. UV dose required for a 4log inactivation of selected waterborne pathogens http://www.trojanuvmax.com/institutions/disinfection_article2.html

17. Components of UV Disinfection System • Components of UV system 1. UV lamps 2. Quartz sleeves: to house and protect lamp 3. supporting structures for lamps and sleeves 4. Ballasts to supply regulated power to UV lamps 5. Power supply 6. Sleeve wiper – to clean the deposit from sleeves UV Reactors • Open-Channel System • Closed-Channel System

18. Open-Channel Disinfection System • Lamp placement: horizontal and parallel to flow (a) : vertical and perpendicular to flow (b) • Flows equally divided into number of channels • Each channel - two or more banks of UV lamps in series • Each bank - number of modules (racks of UV lamps) • Each module: number of UV lamps (2, 4, 8, 12 or 16)

19. Closed-Channel Disinfection System • Mostly flow perpendicular to UV lamp • Mechanical wiping: clean quartz sleeves Drinking Water installation, Busselton, Australia

20. Lamp Array

21. Point Source Summation • a. Intensity Attenuation • Dissipation: b. Calculation Protocol • Absorption (Bear’s law): • Divide lamp into N sections • Power output of each section • Intensity at a given distance from a single point source of energy: • Add all point-source contributions:

22. Factors Affecting UV Disinfection • Reactor Hydraulics:reduced activation due to poor reactor hydraulics resulting short-circuiting • density current – incoming water moving top/bottom of UV lamp • inappropriate entry and exit conditions : uneven velocity profiles • dead zones within reactor Short circuiting/dead zone reduces the contact time • Remedial measures for open-channel system • Submerged perforated diffuser • Corner fillets in rectangular • channel with horizontal lamps • Flow deflectors with vertical • lamps • Ideally plug-flow reactor

23. Remedial measures for closed-channel system • perforated plate diffuser • Plumb correctly • Presence of Particles: - reduce the intensity of UV dose • acts as shield to protect the particle-bound pathogens

24. Characteristics of Microorganisms - Inactivation governed by the DNA/RNA content http://www.trojanuvmax.com/institutions/disinfection_article2.html

25. Effect of Water constituents on UV Disinfection