UV Sterilizer Basics for Ponds and Aquariums

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Ultraviolet light or UV is part of the electromagnetic spectrum; its wavelength is shorter than visible light but longer than x-rays.
Invisible to the human eye, the U.
V spectrum can be subdivided in a number of ways according to energy level and wavelength.
The most common classification is into UVA, UVB and UVC.
UVA has the lowest energy and longest wavelength (400-315nm) followed by UVB (315-280nm) and UVC which has the shortest wavelength (280-100nm) and highest energy level.
You may be familiar with UVA and UVB as those components of the Sun's rays which cause tanning, skin aging, and skin cancer.
The Sun also emits large amounts of UVC which is the most powerful and dangerous type of UV, fortunately for us this is filtered out by the upper atmosphere.
UV was discovered to be a cellular mutagen over 100 years ago when it was shown to be effective against the bacteria which cause tuberculosis.
Since then it has found use in a variety of applications from waste water treatment to manufacturing.
Germicidal UV or UVC is typically used in this situation to disrupt the DNA of unwanted microorganisms leading to death.
In this way a U.
V filter can effectively remove bacteria, viruses and algae from the treated water.
In the public aquarium industry UV sterilizers are often used as a component of the complex filtration systems employed to maintain water quality.
The technology is also used by aquarium and pond keepers typically with the aim of improving clarity and fish health.
Properly specified and installed UV sterilizers are extremely effective at preventing disease in ponds and aquariums; they achieve this by killing many types of suspended bacteria and viruses as well as controlling certain parasites.
Problems with green water caused by suspended algae may also be successfully controlled with UVC which causes the suspended algae to clump together and die.
These clumps can then be removed by mechanical filtration.
UV sterilizers typically utilise a UV emitting lamp held inside a sealed quartz sleeve which in turn is housed within a water proof exposure chamber.
Treatment water is pumped through a exposure chamber and the UVC light shines through the quartz sleeve and into the water as it passes by.
Quartz is used instead of glass as it allows the most UVC to penetrate the water as it flows past.
The effectiveness of any UV filter is dependent on the level of irradiation the water is exposed to as it passes through the unit; this is the total UV dosage and is measured in µWsec/cm2.
The UV dose delivered by a sterilizer is a function of lamp intensity and exposure time, whilst simply reducing the flow through a unit will effectively increase the dosage, this is not effective as it does not take into consideration another important factor which is the turnover rate.
Normally a UV is sized to turnover ponds once every 1 - 2 hours whilst in marine aquariums the turnover rate may be as frequent as every 12 to 20 minutes.
Good manufacturers will provide sizing charts to enable the choice of the correct unit for the application.
Microorganisms have different capacities to withstand UVC irradiation.
The Koi Herpes Virus (KHV) for example requires a comparatively low dose of 4000µWsec/cm2 to be killed whilst the protozoan parasite, Cryptocaryon irritans, ( Marine white spot) requires a treatment rate of 280,000µWsec/cm2 to kill the free swimming tomite stage.
Importantly many microorganisms like Cryptocaryon irritans have life cycles with several stages and each stage may have a different susceptibility to UVC.
As a guide, U.
V systems designed as "clarifiers" aimed at reducing green water in ponds will usually have much lower dose rates than those designed as true sterilizers.
The UV dose rate should be tailored to the application but typically rates are between 30,000 and 90,000µWsec/cm2 Other factors important in determining the effectiveness of a UV sterilizer are:
  • Turbidity: Suspended particulates will effectively absorb and block the radiation, reducing penetration and therefore the amount of UVC reaching the target.
    Installing the U.
    V after a mechanical filter will ensure optimum results.
  • Lamp Age: UVC lamps degrade over time effectively reducing the amount of U.
    V delivered.
    Follow the manufacturer's instructions which will explain how often to replace the lamps.
  • Quartz Sleeve: This protects the lamp from contact with the water and like glass allows the UV light to shine through it.
    Over time this will become dirty and if you are in a hard water area you may get some scaling and mineral deposits which will reduce UV penetration.
    Keep the sleeve clean with periodic maintenance.
UV sterilizers are useful tools in the maintenance of captive aquatic environments.
Failure of any UV sterilizer to function properly is generally down to poor installation or bad quality equipment.
There are a great number of products on the market which offer varying performance, buying based on cost is often a false economy.
Research the manufacturer and product carefully and install the equipment as recommended to ensure success.
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