This page contains important information about using Ultraviolet Sterilization to reduce algae and pathogens in the water in your aquarium and pond.

UV Sterilization
is a safe and natural method to reduce waterborne pathogens and algae the causes “Green Water”. UV is as natural as sunlight. Unlike chemical treatments that can potentially harm the fish, beneficial bacteria, and plants in your aquarium or pond, UV does not leave any residuals in the water and therefore cannot harm fish and plants.

A UV sterilizer works in harmony with other essential filters by eliminating nuisance green water caused by waterborne algae that can easily create problems and ultimately reduce filter efficiency. 
 
Protecting Your Aquarium Fish.
For almost three decades UV sterilization has proven itself as an alternative to harsh copper sulfate chemical treatments to control waterborne pathogens and algae.
 
The Truth about UV!
Regardless of whether you choose to label UV as a water clarifier or a water sterilizer, the same design, performance, and operating principals apply. Successful UV operation means destroying the targeted microorganisms. Here are five main factors that will help determine the ability of a UV sterilizer (or clarifier) to achieve this desired effect.
 
1. The Lamp Type
There are two types of UV-lamps available: low-pressure or medium/high-pressure. As explained below, low-pressure lamps are much better in aquatic sterilizers.
 
2. The Length of the Lamp ...
being used; also known as ARC length.
 
3. The Physical Design ...
of the UV's water exposure chamber. The distance the UV has to travel from the surface of the lamp to the inner wall of the UV's water containment vessel is the primary design criteria that will determine the unit's "UV dose rate" at any given water flow rate.
 
4. The Condition of the Water ...
to be treated. The term used to express the ability of a body of water to be effectively treated by a UV light source is known as percent transmittance. This is a value expressed in percent, which is used to indicate the quality of the water to be treated. The higher the percent transmittance the higher the effectiveness of the UV sterilizer in treating the water.
 
5. Water Flow Rate ...
through the UV's contact chamber. The amount of water that is passed through the UV filter will ultimately determine the unit's actual UV dose rate, which is expressed in microwatts per second per square centimeter or (u-watts-sec/cm2).
 
6. Other Important Factors
Lamp positioning and multiple lamp system design.

Let's start over at the beginning ...
UV is an abbreviation for Ultraviolet light, which is a spectrum of light just below the range visible to the human eye. That is to say UV light is below the blue spectrum of visible light. So UV light cannot be seen by the human eye, and for that reason is often called UV energy.
 
UV light is divided into four groups as measured by wavelength: (1) Vacuum UV with wavelengths from 100 to 200 nanometers, (2) UV-C at 200 to 280 nanometers, (3) UV-B at 280 to 315 nanometers, and (4) UV-A at 315 to 400 nanometers.

The UV-C spectrum (200 to 280 nanometers) is the most lethal wavelength for microorganisms, because it disrupts the bonds in the between the atoms in the chemicals in microorganisms. This range of wavelengths, with 264 nanometers being the peak germicidal wavelength, is known as the Germicidal Spectrum.
 
When you understand UV light and how it destroys microorganisms, you can properly select the appropriate UV light source and design your UV's water containment vessel around the lamp, or lamps in the case of a multi-lamp unit.

The UV Lamp is the Source of UV-C Light.
There are two primary types of UV light, low-pressure lamps and medium/high-pressure lamps. Low-pressure lamps produce virtually all of their UV output at a wavelength of 254 nanometers, which is very close to the peak germicidal wavelength of 264 nanometers.
 
These lamps generally convert up to 40% of their input watts into usable UV-C watts, much higher than other classes of lamps. For example, a 150-watt low-pressure lamp will produce approximately 58-watts of UV-C power.
 
Low-pressure lamps typically run on low-input power currents of 200 to 1,500 milliamps and operate at temperatures between 100 and 200 degrees Fahrenheit. They have a useful life of 8,000 to 12,000 hours, depending on the operating current of the lamp. Low-pressure lamps are the best lamps for aquatic UV sterilizers.
 
Medium/high-pressure lamps produce a very wide range of wavelengths, from 100 nanometers to greater than 700 nanometers, well into the visible light spectrum. These lamps are very poor producers of usable germicidal wavelengths; they generally convert a maximum of 7% of their input watts into usable UV-C watts. For example, a 175-watt medium-pressure lamp will have approximately 12-watts of UV-C power.
 
The remaining 163-watts are converted into heat and visible light. Medium/high-pressure lamps typically run on high-input power currents of 2,000 to 10,000 milliamps and operate at temperatures between 932 and 1,112 degrees Fahrenheit. They have a useful life of only 1,000 to 2,000 hours, depending on the lamp's operating current.
 
The length of the lamp being used.
Low-pressure UV lamps come in many different styles and lengths. As a general rule, the longer the lamp, the greater amount of UV the water will receive because it will be exposed to the UV source for a longer period of time.

UV Design
The physical design of the UV's water exposure chamber. This element is completely overlooked by some manufacturers, but is key to successful operation.
 
Key Design Criteria:

UV Transmittance
The condition of the water to be treated. This also largely overlooked factor is one of the most critical in determining the ability of a UV sterilizer to treat a given volume of water. Regardless of the type of UV light source used, any body of water with impurities will adsorb UV energy.
 
The impurities of interest are algae, waterborne microorganisms and bacteria, and organic waste. Green water, as water plagued by algae and microorganisms is known, will absorb the UV energy emitted by our UV light source in proportion to its density (or how green the water is).
 
The greater the amount of impurities in the water, the greater the reduction in percent transmittance. Percent transmittance is the ability of a body of water to be effectively treated by a UV light  source. This value indicates the quality of the water to be treated.
 
The higher the percent transmittance, the easier the UV sterilizer will be able to treat the water at a given flow rate. A reduction in percent transmittance means the UV sterilizer will be less effective in dealing with the algae problem. If the sterilizer's water flow rates have not been calculated with a reduced percent transmittance rate, the unit will have considerable trouble dealing with an algae bloom.
 
Water Flow Rate through the UV
The basic criteria for a sound UV sterilizer design revolves around a careful selection of lamp type, lamp length, lamp position, and body diameter. These factors, together with the intended water flow rate, percent transmittance of the water to be treated, and UV dose rate needed to destroy the targeted microorganism or pathogen, should be the basis of your decision when purchasing a unit for your pond or aquarium.
 
Multiple UV Lamp Use
When considering the use of multiple UV lamps there are two proper ways to approach the design. Firstly, one can use a single body exposure chamber with multiple lamps evenly spaced to provide a consistent UV field. This design is very effective provided the UV lamps are properly spaced.
 
Secondly, one can use a double body exposure chamber which effectively reduces the total flow rate through each exposure chamber. The flow is reduced to 50% of the total flow through each chamber.
 
A Common Mistake
is that you can continue to add UV's in series and receive higher kill rates. This is not the case. When adding UV's in a series, you are passing the same flow rate through the same intensity lamp, repeatedly.

UV Exposure is not Cumulative! 
In order to effectively implement a multi body UV unit one must operate the lamps in parallel. For example, if I am operating two 40-watt lamps in parallel I effectively have an 80 watt UV. If I am operating two 40-watt lamps in series then I effectively have two 40-watt lamps each providing the same intensity which is not cumulative.

All UV's are not created equal!
When researching which type of UV to purchase, remember the criteria laid out in this article, read the manufacturer's literature, ask questions, and most of all ask yourself, does this information provided make sense to me? If not, consider another UV manufacturer.