For the past 100 years science has recognized the bactericidal effects of
the ultraviolet area of the electromagnetic spectrum.
The specific wavelengths responsible for this reaction are situated between
240 - 280 nanometers (referred to as nm) with a peak wavelength of 265 nm.
and are known as UV-C,
Fig. ( 1 ) shows: UV-C in the spectrum of electromagnetic radiation
Fig (2) shows: The spectral energy distribution curve for germicidal action
and the spectral power distribution for low and medium pressure UV lamps
EFFECT OF ULTRAVIOLET
When a micro-organism is exposed to LTV-C, the nuclei of the cells, due to photolytic
processes, are so changed that cell division, and therefore reproduction is
prevented.
UV-C PRODUCTION
The Ultra Violet source is basically a fused silica quartz tube, typically l5mm
to 25mm diameter ranging from 100mm-1200mm long. The inert gas with which the
tube is filled, provides the primary discharge and the necessary action to excite
and vaporize the miniscule deposits of mercury within.
The low pressure UV lamp is only capable of producing lines at 185nm and
254 nm. An increase in the current supplied causes the UV lamp to rapidly
heat up so increasing the mercury pressure to produce the typical medium pressure
spectral output shown in Diagram 2.
ULTRAVIOLET DOSE
The UV dose is the product of UV intensity (expressed as energy per unit surface
area) and residence time.
Therefore:- DOSE = I x T
This is commonly expressed as 1mJ/cm2 =
1000 micro Watt second/cm2
Doubling of the dose applied will increase the
destruction by a factor of 10. Therefore doubling the dose required for 90%
destruction will produce 99% destruction of the target organism. Tripling
the dose will produce a 99.9% destruction of the target organism and so on.
Some 90% destruction values are shown in Fig. 3 and the relationship between
UV dose and destruction are shown in Fig. 4
FIG. 3.- DOSE REQUIREMENTS - COMMON MICRO-ORGANISMS
FIG. 4. -E.coli (Waterborne indicator Pathogen) DOSE = 5.4 mJ/cm2
APPLICATIONS
DISINFECTION
• LIQUIDS :- Water, Syrups, Emulsions, Brines.
• SURFACES :- Packaging, Conveyors, Food, Working Surfaces.
• GASES/AIR :- Food preparation, Clean rooms, Air conditioning.
PHOTOCHEMICAL REACTIONS
• OXIDATION :- TOC reduction, Ozone destruction, Chlorine removal.
• CATALYSIS :- Pesticide removal, Effluent treatment, Ground Recovery.
• DEODORISATION :- Sewage and Industrial emissions.
MEDIUM AND HIGH PRESSURE ARC TUBES
Power ratings are from 0.4kW to 7.0kW with a maximum treatment capacity of 600
m3/hour with a single lamp.
The high energy output is equally effective on both hot and cold fluids.
The broad spectrum output performs more efficiently than low pressure lamps
on flows > 13 m3/hour. Conversion of power input to biocidal output is > 15%.
Useful arc tube life between 4000 - 8000 hours depending on operating conditions.
Full spectrum output 185 - 480 nm available for photochemical reactions.
LOW PRESSURE LAMPS
Ideal for low flow situations with power ratings from 15w to 200w.
Single wavelength output at 254 nm.
Conversion to UV-C typically 30% - 35%.
120 - 200 watt lamps unaffected by water temperatures.
IRRADIATION CHAMBERS
The disinfection process involves the exposure of fluids with microbiological contamination to a UV energy
source which is mounted centrally in an irradiation chamber.
Correct chamber design is a significant part of effective disinfection and to this end computer modeling
is used to establish turbulent flow, which ensures good mixing and balanced exposure at high and low flows
and residence time characteristics.
Information necessary to correctly size a UV unit includes, but is not limited to:
1. Peak flow (liters per second or gallons per minute).
2. Fluid sample for free transmission test.
3. Microbiological challenge.
4. Required standard after UV treatment.
Advantages:
- UV does not alter taste, odour, colour or pH
of the water
- UV does not require the addition of chemicals
- UV does not impart toxic by-products into the water
- UV systems are compact and easy to install
- UV systems require very little maintenance
- Running costs are often lower than those of a household lightbulb
