5 UV Disinfection am

UV Water Treatment

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UV Water Treatment

Solids / Turbidity reduce UV light
Transmission thus reducing the effectiveness
of the treatment it is therefore common to
install filters upstream from the UV system
to remove fine silt/rust present in the water.

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Ultraviolet systems (UV)

– Disrupts bacteria/pathogen “life cycle”

Advantages

•

Automatic

•

Low contact time

•

Low cost to operate

•

Compact and easy to maintain

•

No taste or odor

Disadvantages

•

Not effective in turbid water (cloudy)

•

Blub housing can become coated, less light penetration

•

Bulbs wear out

•

No way to test effectiveness (i.e., can’t test for residual
chlorine levels)

•

No Residual property

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UV Systems

Not just fit and forget they should be monitored

• New UV systems should measure UV intensity, Bulb

operational hours and should have an alarm to indicate
bulb failure. Bulbs should be changed once intensity
figures have dropped out of spec or after running a set
number of hours.

• The Quartz glass housings will require regular cleaning.

Newer models are fitted with rubber wipe blades.

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How Does UV Disinfect ?

UV light penetrates and permanently alters the DNA of the
microorganisms in a process called

thymine dimerization.

The microorganisms are “inactivated” and rendered unable to reproduce
or infect.

Analogy: a broken zipper.

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Certain wavelength of UV light more effective than others

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Certain wavelength of UV light more effective than others

LP, LPHO lamp Output Spectrum



Approximately 90% of UV output is at 254nm and 5-7% is at 185nm



254nm wavelength is well suited to microbial disinfection

Low Pressure Lamp UV Output

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Medium Pressure Lamp UV Output

Broad spectrum of UV

output (<200 to >600
nm) is characterized as
polychromatic

Though less efficient,

medium pressure
lamps emit
significantly more UV
energy - allowing very
compact treatment
systems capable of
treating large flows

Medium Pressure Lamp UV Output

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UV light will also remove Free Chlorine !

A strong enough UV lamp emitting the right wavelengths will remove free chlorine

Between the wavelengths 180 and 400 nm, UV light produces photochemical reactions that

dissociate free chlorine to form hydrochloric acid.

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Effects of Iron on UV Transmittance

UV are affected by various
compounds that are able to
absorb UV, including iron,
tannins.

Fine particles will also reduce
UV effectiveness through light
scatter.

Iron and hardness are often a
factor in sleeve fouling

UV Dose Influencing Factors

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Sleeve Fouling



The deposition of minerals on the sleeve protecting the UV lamp

LAMP

Quartz Sleeve

Scale Coating



Reduces the intensity of light getting into the water



Iron and hardness are often a factor in sleeve fouling

Pretreatment and/or an appropriate cleaning schedule allow for the use of

UV where iron and hardness are factors

Effect of Hardness and Iron on UV Dose

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UV Dose Influencing Factors

UV Transmittance (UVT)

UVT is a measure of how well the water is able to transmit UV light
(water clarity). It affects the intensity of light reaching pathogens.

Less UV light
getting through
the water

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Ultraviolet systems (UV)

Major

Disadvantage

• UV light does not provide residual protection because

bacterium will persist in biofilms where UV light cannot
penetrate. Thus, UV light is unsuitable as the only
control measure for an entire water system;

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Ultraviolet systems a requirement for Norwegian
sector

• Giardia Lamblia outbreak in Bergen 2004-05
• Caused by presence of Giardia cysts in drinking water supply
• Around 2500 people were treated for Giardiasis during the

outbreak. Contamination though to come from Lake
Svartediket

which supplied water to 60,000 of Bergen’s

250,000 population at that time.

• Chlorine only partially effective against Giardia cysts cold

water reduces chlorine disinfection speed.

• Bergen water supply not protected by UV systems.

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