Department of Civil, Construction and Environmental Engineering
April 15, 2011


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 20 th century

Limited field application: Low cost and maturity of Cl
2 disinfection technology coupled with operation problems associated with early UV systems

9. Fouling of UV lamps


Chlorinated disinfection byproducts (DBPs): THM, HAA etc.

Potential to inactivate protozoan: Cryptosporidium - resistant to Cl
2
Radio IR
Visible
Light
400nm
UV
100nm
X-Rays

UV light: 100 to 400 nm
UV spectrum – 4 regions o Vacuum UV:100–200 nm l
UV-A UV-B UV-C
Vacuum
UV o UV – C : 200 – 280 nm o UV – B : 280 – 315 nm o UV – A : 315 – 400 nm
300nm 200nm
Germicidal Range

 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


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 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

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


The effectiveness of UV disinfection is based on the UV dose to which the microorganisms are exposed

UV dose is analogous to Cl
2
Cl
2 dose = Cl
2 dose conc. x contact time (t) or Cx t
UV dose (D) = I x t or
 t
I .
dt if intensity not constant
0
Where, D = UV dose, mW.s/cm 2 or mJ/cm 2
I = UV intensity, mW/cm 2 t = exposure time, s

UV dose can be varied by varying either the intensity or the contact time

UV Disinfection Kinetics – Similar to Cl
2
Disinfection dN
  kIN dt dN/dt = Rate of change in the concentration of organisms with time k = inactivation rate constant, cm 2 /mW.s
I = average intensity of UV light in bulk solution, mW/cm 2
N = number of microorganisms at time t t = exposure time, s
N
N o
 e
(
 kIt )
I
 t

UV dose
Residual microorganisms protected in particles
N

N
0 e
 kIt 
N p

UV dose required for a 4log inactivation of selected waterborne pathogens
Pathogens
Cryptosporidium parvum oocysts
Giardia lamblia cysts
Vibrio cholerae
Salmonella typhi
Shigella sonnei
Hepatitis A virus
Poliovirus Type 1
Rotavirus SA11 http://www.trojanuvmax.com/institutions/disinfection_article2.html
2.9
8.2
8.2
30
30
36
UV dose mJ/cm 2
4log inactivation (99.99)
<10
<10

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

a. Intensity Attenuation

Dissipation: I

P /( 4

R
2
) b. Calculation Protocol
 Absorption (Bear’s law):

Divide lamp into N sections

Power output of each section
P
 S
N
• Intensity at a given distance from a single point source of energy:
I



S / N
4

R
2

 exp(
 
R )

Add all point-source contributions:


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


- Inactivation governed by the DNA/RNA content
Pathogens
Cryptosporidium parvum oocysts
Giardia lamblia cysts
Vibrio cholerae
Salmonella typhi
Shigella sonnei
Hepatitis A virus
Poliovirus Type 1
Rotavirus SA11 http://www.trojanuvmax.com/institutions/disinfection_article2.html
8.2
30
30
36
UV dose mJ/cm 2
4log inactivation (99.99)
<10
<10
2.9
8.2

Effect of Water constituents on UV Disinfection