Page 48 - Commercial Catalogue 2011

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43 Commercial Catalogue
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UV Filtration
Ultraviolet is a term used to describe the wavelength between
the blue violet range of the visible spectrum and the shorter
wavelength or X-rays.The ultraviolet spectrum is divided into
three separate bands:-
UVa From 315nm to 400nm
UVb From 280nm to 315nm
UVc From 200nm to 280nm
UVc is often referred to as the germicidal wavelength, because of its ability to destroy micro-organisms.The
lamps in all of the UV sterilisers listed on p.51 emit a wide range of ultraviolet with its most significant
part at a wavelength of 253.7nm, which has been found to be the most efficient for germicidal action.
When water is passed through a UV steriliser, micro-organisms contained in the water come into
contact with UVc radiation emitted from the germicidal lamp.The UVc radiation penetrates the cell
membrane and thereby either destroys or debilitates the micro-organism. Different micro-organisms
vary in their susceptibility to UVc radiation.The most sensitive and hence, the easiest treated, are
some bacteria and most phototrophic organisms such as those often found in ponds and aquaria.
Bacteria, such as
Escherichia coli
which can contaminate drinking water supplies, are easily controlled by
relatively low levels of UVc and this is why low level UVc radiation has been employed by the drinking
water industry for many years.The most resistant micro-organisms to UVc are protozoans which may,
when compared to simple bacteria, require several hundred times the radiation dose to destroy them.
Generally, the larger the organism, the larger the UV dose required to damage it.There are many
hazardous and pathogenic aquatic organisms that can be controlled given the correct UVc radiation dose.
Correct application will dramatically reduce the likely spread of most primary infections such as
Cryptocaryon sp.,
at the same time almost eliminating the risk of secondary bacterial infection which
is so often the real cause of fish loss.This is achieved by controlling potential populations of heterotrophic
bacteria which are often associated with an unhealthy aquatic environment.
Factors Affecting Germicidal Action
It is important that the UV lamp reaches its optimum operating temperature of 40°C otherwise the
lamp will not generate the maximum amount of UVc radiation possible and the efficiency of the unit
will be compromised. For this reason all UV water sterilisers must be fitted with a protective fused
quartz glass sleeve, which keeps the UV lamp from direct water contact, which would otherwise act
as a coolant. Units should also feature sealed end caps which help to maintain the lamp and the air
around it at its constant optimum running temperature.The protective sleeve employed must be
manufactured from fused quartz glass, which allows the highest possible amount of UV light to
penetrate into the water. Other materials should not be used as they can have a significant impact on
the effectiveness of the steriliser by significantly reducing the amount of UVc that reaches the water.
Water Clarity
Dirty or turbid water with suspended particles will absorb the UVc radiation and dramatically reduce
the effectiveness of the unit. It is important to ensure that the UV steriliser is placed at the end of
the filter circuit, before the water returns to the holding tanks to ensure that the water passing
through the steriliser is as clear as possible.
Dissolved organic compounds also reduce the distance of penetration of the UVc radiation.Typically
in any aquatic environment with fish or other animals present, there is a gradual build up of phenolic
compounds and other organic dyes.This build up of organic dyes is often referred to as ‘yellow water
syndrome’ and can be dramatically reduced by filtering through a good quality activated carbon
(e.g HR Carbon) or by using ozone gas.