ULTRAVIOLET (UV) RADIATION DISINFECTION

ULTRAVIOLET (UV) RADIATION DISINFECTION

UV Radiation

UV DisinfectionPhysical process (inducing

photobiochemical changes within microorganisms)

Two conditions must be met:◦ Radiation of sufficent energy to alter

chemical bonds must be available◦ Such radiation must be absorbed by the

target molecule or microorganism

Eʎ = (h*Cv*AN)/ʎ

Eʎ = Radiant energy associated with given wl(kcal/einstein)

h = Planck’s constant, 1.583 * 10-37 kcal.sCv = Speed of electromagnetic radiation in

a vacuum, 3 * 1017 nm/sʎ = Wl of electromagnetic radiation, nmAN = Avogadro’s number, 6.023*1023

photons/einstein

Eʎ = 112.8 kcal/einstein ʎ = 253.7 nm

Bond Bond Dissociation Energy(kcal/mole)

O – H 110 – 111C – H 96 – 99N – H 93C – N 69 – 75C – C 83 – 85

Physical process (inducing photobiochemical changes within microorganisms)

Two conditions must be met:◦ Radiation of sufficent energy to alter

chemical bonds must be available◦ Such radiation must be absorbed by the

target molecule or microorganism

DNA and RNA are the two most common forms of nucleid acid, that consists of single or double stranded polymers comprising building blocks called nucleotides.

Purines: Adenine, GuaninePyrimidines: Thymine, Cytosine (DNA), Uracil, Cytosine (RNA)Strong absorbers of UV light.

Source of UV RadiationLow-Pressure Low-intensity UV Lamps :Produce essentially monochromatic UV light at

253.7 nm. UV light is produced by mercury at low vapor pressure.

Low-Pressure High-Intensity UV Lamps :Mercury-indium amalgam is used. Allows 2-4

times greater UV-C output. 25% greater lamp life.

Medium-Pressure High-Intensity UV Lamps :

Mercury vapor emission is carried out at higher lamp pressures and temperatures. Produce polychromatic UV light.

UV Reactor Configuration Open – Channel Disinfection Systems

Lamp placement can be◦ Horizontal◦ Vertical

Closed – Channel Disinfection Systems◦ In most design configurations, the direction of flow is

perpendicular to the placement of the lamps

UV Intensity and UV DoseUV Intensity is a measure of

radiative power per unit of exposed area. The total UV intensity at a point in space is the sum of the intensity of UV light from all directions.

UV Dose is the integral of UV intensity during exposure period.

UV Dose D = I x t

D = UV dose, mWs / cm2

I = UV intensity, mW / cm2

t = Exposure time, s

UV Disinfection Kinetics The measured concentration of microorganisms

before and after exposure provides the response, or log reduction of microorganisms from exposure to UV light.

Log Reduction = log (N0 / N), where,◦ N0 is the concentration of infectious microorganisms

before exposure to UV light◦ N is the concentration of infectious microorganisms

after exposure to UV light.

• UV dose – response relationships can be expressed as either the proportion of microorganisms inactivated (log reduction) or the proportion of microorganisms remaining (log survival) as a function of UV dose.

Factors Affecting UV DisinfectionFlow RateUVTSuspended

SolidsWater Quality

◦ Iron◦ Hardness

D = I x t

UVTUVT is the percantage of light

passing through material over a specified distance.

Suspended SolidsShadowingMicrobes within particlesPotential for microbes to pass

through system without seeing UV light

Water QualityIron and HardnessDeposition of minerals on the sleeve

Water QualityIron is a strong absorber of UV light

Advantages of UV DisinfectionEffective disinfectantMore effective than chlorine in inactivating

most viruses, spores, cystsNo chemical addition requiredNo formation of disinfection byproductsWater retains its natural flavour and smellMicroorganism inactivation achieved within

secondsMax operational safetyMinimal operating costs