Transcript UV-disinfection - Iowa State University

Ultraviolet (UV) Disinfection
in Water Treatment
Hans van Leeuwen.
Department of Civil, Construction and Environmental Engineering
Iowa State University
April 15, 2011
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
Increasing Popularity of UV Disinfection
 Chlorinated disinfection byproducts (DBPs): THM, HAA etc.
 Potential to inactivate protozoan: Cryptosporidium - resistant to Cl2
UV Radiation
400nm
Radio
IR
 UV light: 100 to 400 nm
Visible
Light
100nm
UV
X-Rays
UV spectrum – 4 regions
l
o Vacuum UV:100–200 nm
UV-A
UV-B
UV-C
Vacuum
UV
o UV – C : 200 – 280 nm
o UV – B : 280 – 315 nm
300nm
o UV – A : 315 – 400 nm
200nm
Germicidal Range
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
ULTRAVIOLET RADIATION



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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
LOW AND MEDIUM PRESSURE
MERCURY LAMPS
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
ULTRAVIOLET WAVELENGTHS
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
t
UV dose (D) = I x t or
 I . dtif intensity not constant
0
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
  kIN
dt
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
N
 e (  kIt )
No
I  t  UV dose
Residual microorganisms protected in particles
N  N0 ekIt  N p
UV dose required for a 4log inactivation
of selected waterborne pathogens
Pathogens
UV dose mJ/cm2
4log inactivation (99.99)
Cryptosporidium parvum oocysts
<10
Giardia lamblia cysts
<10
Vibrio cholerae
2.9
Salmonella typhi
8.2
Shigella sonnei
8.2
Hepatitis A virus
30
Poliovirus Type 1
30
Rotavirus SA11
36
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:
I  P /(4R )
b. Calculation Protocol
 Absorption (Bear’s law):
 Divide lamp into N sections
S
 Power output of each section P  N
• Intensity at a given distance from
a single point source of energy:
S/N 
I 
exp(R)
2 
 4R 
 Add all point-source contributions:
2
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
Pathogens
UV dose mJ/cm2
4log inactivation (99.99)
Cryptosporidium parvum oocysts
<10
Giardia lamblia cysts
<10
Vibrio cholerae
2.9
Salmonella typhi
8.2
Shigella sonnei
8.2
Hepatitis A virus
30
Poliovirus Type 1
30
Rotavirus SA11
36
http://www.trojanuvmax.com/institutions/disinfection_article2.html
Effect of Water constituents on UV Disinfection