basic principles of uv disinfectionbasic principles of uv disinfection dr michael templeton...

© Imperial College London

Basic Principles of UV Disinfection

Dr Michael Templeton Department of Civil and Environmental Engineering, Imperial College London IUVA Vice-President for Europe, Middle East and Africa region [email protected]


Gamma Xrays UV Visible Infrared Microwave Wavelength (nm) 1 10 340 760 106

vacuum UV UVC UVB UVA Wavelength (nm) 200 280 315

germicidal sunburn tan


• In order to inactivate microorganisms, UV energy must be absorbed somehow

• DNA & RNA happens to absorb light in the UVC range emitted by UV lamps

• DNA & RNA are the master instructions for the cell

• UV damages these nucleic acids and prevents cell replication

Microbial Inactivation by UV Light

Presenter

Presentation Notes

Reactivation


Dose = Intensity x Time

Fluence = Fluence rate x Time

• J/m2, mJ/cm2, mW·sec/cm2 are commonly used units

• 10 J/m2 = 1 mW·sec/cm2 = 1 mJ/cm2

• So, 400 J/m2 is the same as 40 mJ/cm2

UV Dose Terminology

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Presentation Notes

How do we express UV dose?


Con

c. o

f Org

anis

ms

UV Dose (mJ/cm2)

106

105

104

103

102

1-log inactivation

2-log inactivation

Log Inactivation


UV dose (mJ/cm2) required for 4-log inactivation

0

20

40

60

80

100

120

140

Cryptosp

oridium parv

um

Giardia

lamblia

cysts

Vibrio ch

olerae

Shigella d

ysen

teriae

E.coli O

157:H

7

Salmonell

a typ

hi

Shigella s

onnei

Salmonell

a enter

itidis

Legionell

a pneu

mophila

Hepati

tis A vi

rus

Poliovir

us Typ

e 1

Coxsac

kievir

us B5

Rotaviru

s SA11

Adenovir

us 40

Typical UV dose = 40 mJ/cm2


Protozoa (Crypto, Giardia)

Viruses

(Adenovirus)

Bacteria (E. coli)

Cl2

Harder to inactivate

Easier to inactivate

Viruses (Adenovirus)

Bacteria (E. coli)

Protozoa

(Crypto, Giardia)

UV


• ‘Light’ and ‘dark’ repair mechanisms exist, but likely not a drinking water concern

• If you apply enough UV, you destroy the ability to repair

• UV doses in the 40+ mJ/cm2 range are thought to be easily high enough to prevent repair

A Note about DNA Repair

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Presentation Notes

Reactivation

Where Do You Get UV?

Humour Credit: Ron Hofmann


water

lamp

quartz sleeve air gap

UV Reactors

mercury drop

electrode reactor vessel

connected to ballast sensor


UV Lamps

0

20

40

60

80

100

150 200 250 300 350 400

Wavelength (nm)

Rel

ativ

e O

utpu

t (%

)

Low pressure

Medium pressure

Note: The y-axis scale is different for LP and MP lamps on this graph. MP lamps emit MUCH MORE energy than LP lamps.

0

1

2

3

200 220 240 260 280 300 320

Wavelength (nm)

Abs

orba

nce

(M-1

cm-1

x 1

0-3)

Uracil(in RNA)

Thymine (in DNA)

UV lamp

Abso

rbed

Ene

rgy


0

20

40

60

80

100

0 2000 4000 6000 8000 10000Time (hours)

Rel

ativ

e O

utpu

tLamps Age and Need Replacing!

Example data


Other Sources of UV…


UV Disinfection Benefits

• Cryptosporidium and other pathogens inactivated at relatively low, economical UV doses

• No formation of regulated disinfection by-products at typically applied UV doses for disinfection

• Small space requirements (no contact tank)

• Competitive costs versus alternatives (e.g. ozone, membrane filtration)


UV Disinfection Limitations

• No taste and odour control (on its own…)

• Does not remove colour (on its own…)

• No iron, manganese oxidation

• No residual disinfecting capabilities

Design and operation of UV reactors must take into account relevant water quality factors and include a dose validation/monitoring strategy


Water Quality Considerations

Relevant water quality parameters:

• UV Transmittance (UVT)

• Fouling

• Turbidity

Note: pH, temperature have no major direct impact on UV performance

Presenter

Presentation Notes

How does water quality affect UV dose delivery?


UV Transmittance (UVT)

• Definition of %UVT: Percent of light emitted (254 nm) that passes through 1 cm of water

Light source 1 cm Detector

100% 95% 95% UVT



• Arguably the most important water quality parameter

• Clean source water: > 90% UVT

• Wastewaters: often 30% to 50%

• Can vary seasonally for surface waters; often more stable for groundwaters


• UVT affected by dissolved and particulate matter; best to apply UV post-filter in a conventional treatment works

• Can always design a powerful enough UV system to handle any UVT

• UV reactor design should consider lowest expected UVT

• Rough rule of thumb: For every 5% decrease in UVT, 50% reduction in UV dose (i.e. Need to build 2X the UV system!)



0102030405060708090

100

0 5 10 15 20 25Distance from lamp (cm)

% T

rans

mis

sion

97% UVT per cm

87% UVT per cm

71% UVT per cm


Presenter

Presentation Notes

Effect of UVT on log inactivation

From Jim Malley, IUVA website

Fouling

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Presentation Notes

Reactivation


Fouling

• Caused by minerals that accumulate on quartz sleeve (i.e. hard waters have greater fouling potential)

• Blocks the light

• Will occur in any water

• Hardness < 100 mg/L as CaCO3 = “slow” fouling

• Iron can be a problem (e.g. 0.5 mg/L iron may require chemical cleaning every few days)

• Good news: Can always be controlled using existing lamp cleaning technology (mechanical/chemical)

• (Clean the sensors too…)


Turbidity • No direct correlation with UV effectiveness in the

drinking water context • Turbidity = scattering. Scattered light can still disinfect • Turbidity = particle enmeshed organisms • To forbid UV for turbid waters implies another

disinfectant works better with turbidity - No good data substantiating this

• Turbid waters may be most in need of multi-barrier disinfection (e.g. some form of filtration before UV)

• Turbidity does affect UVT, which should be accounted for in the design (by measuring UVT)


UV Basics: ‘Take-Home Messages’

• UV is a very effective disinfectant, but not a panacea • UV handles protozoa and bacteria easily at typically

applied doses; some viruses may require higher doses • UV is not an ‘install and forget’ technology • Water quality matters • Pre- and post-treatment may be necessary

basic principles of uv disinfectionbasic principles of uv disinfection dr michael templeton...

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