How to Select an Ultraviolet Sterilizer
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There are many books available that describe scientific methods for selecting an ultraviolet sterilizer. The most thorough explanation we've seen may be found in Aquatics Systems Engineering by Pablo Escobal, president of Aquatronics^® and Filtronics^® and one of the true aquarium gurus. 

Most UV sterilizer manufacturers provide charts on their web sites for the proper selection based on the design of their sterilizers. We provide examples of these charts in the detail pages of our ultraviolet sterilizer price lists on our storefront. These charts are fairly accurate, but we believe it's important to understand the various factors that influence how well an ultraviolet sterilizer works in aquaria. This will hopefully help you to select the proper size sterilizer for your application.

The goal of proper UV sterilization is to obtain the correct radiation dosage (zap dosage, usually expressed in Microwatt-Sec/cm²) which is the time and intensity required to destroy harmful or unwanted microorganisms from the volume of water in the aquarium. Books talk about many factors necessary to accomplish this, but those we'll worry about for the purposes of our discussion, are "lamp intensity," "dwell time,"  "turbidity," and the total volume "turnover" of the aquarium.

Dwell time is basically the amount of time the volume of water in a sterilizer will remain in the sterilizer when moving at a given flow rate in order to receive enough radiation so that targeted microorganisms are killed. Different microorganisms die at different exposures to radiation, according to their size and physical structure. The longer the time the water in the sterilizer is exposed to UV radiation, the larger the microorganisms that are killed. Speaking very generally, bacteria require the least amount of dwell time and protozoa require the most, with algae and fungal spores somewhere in the middle. The factors that come into play in obtaining the proper dwell time are:

1. Radius and length (volume = r²) of the sterilizer body.
2. Flow rate through the sterilizer.

To find the correct volume (V) of the sterilizer, first calculate the space in cubic inches occupied by the lamp (or the quartz sleeve, if the sterilizer is so equipped), then subtract that volume (LV) from the total volume of the sterilizer (SV) in cubic inches. There are 231 cubic inches in a gallon so:

                 SV (r² x L) - LV (r² x L)
Volume  =     ---------------------------------------------
                    231

The dwell time may then be easily calculated by simply dividing the Volume in Gallons by the Flow Rate in Gallons per Hour.

Intensity of the radiation being produced inside the sterilization chamber is dependent on the wattage of the bulb used, the quality of the bulb, whether the sterilizer is equipped with a quartz sleeve and the clarity (turbidity) of the water passing through the ultraviolet sterilizer. Please note that we recommend and sell ultraviolet sterilizers (see our selection of ultraviolet sterilizers) equipped with quartz sleeves. UV Sterilizers with quartz sleeves allow the lamps to be easily changed and lower the risk the bulb will be damaged by improper installation, but the sleeve should be cleaned monthly to maintain good light penetration into the sterilizer chamber. Most manufacturers supply aquarium size ratings for their lamps like the one at the bottom of this page. 

Clarity of the water passing through a UV sterilizer is important because microscopic particles in the water will absorb UV light and that will lower the intensity of radiation available to kill microorganisms. The cloudier the water, the less light will penetrate. We strongly recommend that the water be well-filtered before it passes through the ultraviolet sterilizer. With our client's aquariums, we always use a separate canister filter to move water through an ultraviolet sterilizer. This way, we can not only ensure that the water being sterilized is clean, but we can also regulate the flow through the UV to increase or decrease dwell time without disturbing the flow through the main filter systems.

Finally, it is important to make sure the turnover of aquarium water is sufficient to provide maximum sterilization in the aquarium. It is impossible to sterilize 100% of the water in an aquarium as sterilized water is constantly being mixed with old water in the aquarium. The best you can hope for is about 90% sterilization, which should be more than sufficient. This may be accomplished by turning the aquarium over about two times per 24 hour period, assuming the sterilizer is sized correctly for the aquarium. Remember that more frequent turnover reduces dwell time. A knowledge of calculus is required in order to calculate the total sterilization turnover for an aquarium allowing for all variables, which is why most major ultraviolet sterilizer manufacturers supply flow rate charts for you with the higher mathematics already done for you.

The following table is from the Emperor Aquatics web site
(opens new browser window) and shows their recommendations for maximum aquarium size and flow rate for their Smart UV^® sterilizers in order to provide the proper zap dosage for two broad classes of microorganisms. Please note that the original table on their site also shows the zap dosage for their sterilizers after the lamp has degraded to 60% of it's lamp-life. We left this information out to conserve space.

Table Copyright © 1999 Emperor Aquatics, Inc.

*These are recommended flow rates when the bulb is new. As the bulb ages, these flow rates decrease over time.

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