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 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 encounter 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 therefore 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 Oodinium sp. and 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 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.
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.
Each TMC Commercial UV Steriliser uses a high frequency control gear, which should, ideally, be operated on its own power supply circuit. If this is not possible then it is important that the UV Steriliser is not installed on a circuit that may be susceptible to induced loads that lead to high energy surges (spikes), as these high energy surges will damage the UV control gear. Examples of equipment that could potentially cause high energy surges or spikes are: most large electric motors including some water pumps and light fittings with igniters, such as floodlights. High frequency control gears should not be operated on the same power circuit as traditional magnetic ballasts.
UV Filtration Technical Information
Choosing the Correct UV Steriliser
- Select application level required, bearing in mind the type of pathogen to be controlled or killed (see table below, Exposures needed to kill Micro-organisms). Level 2 or 3 is adequate for most applications.
- Decide on the flow rate that you require in m3/hr for your application.
- Divide this number by the flow rate per lamp given in the table below, according to your selected application level.
- The result tells you the number of 55W UV lamps that are required to treat your given flow rate i.e. the size of the Commercial UV unit, or TitanUV. (Note: If you have an odd number or fraction of a number, you should round up to the nearest whole even number. If the number is greater than 12 then you should use 2 or more UV units to give the total required number of lamps.)
- Use the product data table on the Commercial UV info sheet or TitanUV info sheet to obtain the specification of the correct unit for your application.
Please note that all outputs are given for the end of the maximum lamp life of 8000 hours. However, it is strongly recommended that lamps are replaced every 4000 hours (approx. 6 months) to ensure optimum efficiency of the steriliser.
To maximise germicidal action, it is important to consider the contact time and distance between the UVc source and the microorganisms to be treated. The contact distance is set by the design of the UV steriliser. The actual time it takes for water to pass through the chamber - ‘the contact time’- is a function of the recommended flow rate per lamp. The overall volume of water in the livestock tank or system, and the frequency that the total volume comes into contact with the UVc source, is also significant and must be considered for the correct operation of the filter on a given system. We have carried out extensive trials over many years to ascertain the best relationship between the ‘contact time’ and the total volume of the system, and the turnover requirements of the system. UV output, recommended flow rates and maximum system volume are given for each model.
To achieve desirable levels of sterilisation the total volume of a system needs to be turned over regularly through the steriliser. The following recommendations should be adhered to and used in conjunction with the table on p.41. Shellfish depuration tanks Minimum 3 x per hour, recommended 5x per hour (Sea Fish Industry Authority Recommendation) Marine/freshwater ornamental Minimum 4x per hour, recommended 7x per hour Public display Minimum 2 x per hour, recommended 3-5x per hour.
* UV dosage required to destroy 99.9% of micro-organisms