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Factors affecting UV

Reduction Levels


        Defined as 50 – 99% reduction (2-log)


        Defined as 99.99% reduction (4-log)


        Defined as 99.9999% reduction (6-log)

         DOSAGE is the product of Intensity & Time



         = microW/cm2 X time

         = microWsec/cm2

         Note: 1000 microWsec/cm2 = 1 mJ/cm2

Factors Affecting UV

    UV can only be effective if it is absorbed by the target:

     UV absorption in water is almost entirely caused by dissolved substances. Certain organic substances such as humic or fumic acids (tannins), and certain inorganic substances such as iron and manganese absorb UV.

     Suspended solids shield microorganism and need filtration.

     Hardness causes the formation of scale on the quartz sleeve surface and therefore prevents transmissibility.

     Iron causes the staining of the quartz sleeve thereby impeding UV transmission

     Lamp operating temperature levels is an important factor. The optimum being 40oC. The increase or decrease of temperature will affect UV energy levels. Quartz sleeves will minimize temperature fluctuations.

     Good flow rate design does take into account energy level fluctuations.

Water Quality Effects


        This varies according to the dissolved substances in water(mainly due to iron, humic acids and tannins)


        This has an effect on scattering of light, absorption and light penetration


        Due to chemical and biological deposits (mainly inorganic scale: iron, magnesium, calcium, aluminum, manganese, sodium, and anionic carbonate, phosphate, sulphate and organic biofilms)

pretreatment required

    Iron                < 0.3 ppm

    Manganese          < 0.05 ppm

    Hydrogen Sulphide      < 0.05 ppm

    Hardness              < 7 gpg

    Suspended Solids        < 10 ppm

    Excessive colour, turbidity, iron or organics, require additional treatment

NSF Standard 55


    systems capable of producing an ultraviolet dose at the alarm setpoint equivalent to a UV dose of 40 mJ/cm2 (40,000 microWsec/cm2 ) at 254 nm

     challenge organism is MS-2 Coliphage

     must have a 254nm UV sensor to monitor UV transmission (set at 70%)

     must have a flow control device

     must have either a visual alarm, audible alarm or shut-off device (solenoid)

     must have a NSF 53 turbidity filter for a general cyst claim (crypto & giardia are allowed)

     must pass applicable structural integrity tests (eliminates Teflon)