General Function of UV Sterilizers
UV sterilizers utilize a germicidal lamp emitting UV light inside of or over the top of a container with water. The water flows around/under the UV light and any bacteria or algae in the water will be killed by the exposure to the UV light. We use a measurement called a “kill dose” to determine if the UV will be strong enough to kill the organism we want killed. The kill dose relies on the wattage of the UV bulb, the flow rate of the water past the bulb, age of the bulb and the path the water takes. Obviously, the lower the water flow rate, the higher the kill dose for a given wattage UV bulb. But, we need to get the water of the aquarium/pond to pass through the UV unit fast enough to get enough “turn-over” of the water per day to be effective. Most manufacturers will list a suggested flow rate that quite frankly is usually too high for most aquarium applications. To be effective, most UV bulbs need to be replaced after 12 months of operation.
The primary use of UV sterilizers with ponds is to control the growth of the free floating single celled algae to eliminate “green water”. The proper sized UV will allow you to have a flow rate equal to or greater than 50% of the volume of the pond. For example, a 2000 gallon pond would need a UV capable of a flow rate of 1000 GPH (most likely an 18W UV). In some cases, this means you cannot use the main circulation water pump to channel water through the UV, you would need a smaller, dedicated water pump. Some of the pressurized canister styles have an UV built into their design to simply the process. There are also Gravity UV filter boxes that have a compartment where the UV bulb shines down onto the water flowing over a divider wall into the mechanical side of the filter. This is a less effective way to use the UV, so usually higher wattages are required.
While you can use an UV to control free floating algae, this is rarely a problem in a home aquarium. Most of the time, the UV is used to control the spread of infectious bacteria. The kill dose required is usually a little higher than that needed for the algae, and the number of turns per day needs to be higher since most bacteria have a shorter replication time than algae. Thus, an 18W UV would be rated for an aquarium up to 250 gallons, not 2000 gallons as seen in the pond example. With the development of the mini-reef aquarium, hobbyists also want to use an UV to eliminate parasites (Ick). The “problem” with this is the fact that a parasite requires a kill dose 4 to 5 times higher than bacteria. If you were to use an 18W UV bulb, the flow rate would have to be less than 70 GPH and would therefore only be effective for an aquarium up to 75-100 gallons! Overkill with an UV sterilizer is usually not a bad thing, but you would not want to run one when adding bacterial starter solutions or during the first 10-14 days that the aquarium is set up. Now, if an UV could control algae growing on the rocks and glass, it would be the perfect aquarium filter!