Ultraviolet (UV) Disinfection in Water Treatment - PowerPoint PPT Presentation

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

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

Advantage and Disadvantage of UV Disinfection

9. Fouling of UV lamps

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

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

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

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

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

Mercury vapor Lamp Comparison

UV Lamp and UV Absorbance of DNA

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

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

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

UV dose required for a 4log inactivation

of selected waterborne pathogens

http://www.trojanuvmax.com/institutions/disinfection_article2.html

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

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)

Closed-Channel Disinfection System

• Mostly flow perpendicular to

UV lamp

• Mechanical wiping: clean

quartz sleeves

Drinking Water installation, Busselton, Australia

Lamp Array

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:

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

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

Characteristics of Microorganisms

- Inactivation governed by the DNA/RNA content

http://www.trojanuvmax.com/institutions/disinfection_article2.html

Effect of Water constituents on UV Disinfection