All About UV'sULTRAVIOLET LIGHT FILTRATION IN BACKYARD PONDS
Ultraviolet Light Filters, commonly referred to as "UV Sterilizers" or "Clarifiers" are used to render waterborne microorganisms incapable of reproduction. They are used in many types of applications in industry and homes from disinfecting cosmetics, to household drinking water. The primary use in a backyard pond is to rid the water of suspended algae cells that cause green, murky conditions, thereby creating/maintaining very clear water. They may also be used to kill other unwanted organisms that may have a negative impact on fish health. They will not affect the good aerobic, nitrifying bacteria colonies that are part of every healthy pond ecosystem, because these bacteria colonize on surfaces and will not pass through the light. It should also be noted that a UV Filter has no effect on filamentous or "stringy" algae since it grows attached to surfaces and/or due to its physical nature will not pass through the light chamber. The use of barley products is recommended for control of this type of algae.
UV Light Filters are available as a separate component, or as part of a bio/mechanical filter, or skimmer. The "stand-alone" UVs provide more effective sterilization/ clarification with less worry. UV lamps inside of filters/ skimmers are certainly effective, though they operate in a less clean environment.
Does every pond need a UV Filter? No. Proper stocking/ maintaining of aquatic plants/ scavengers can achieve clear water nearly year round. Even ponds with moderate fish populations can achieve clear water without a UV Filter. Ponds with high fish populations, or large "fish loads" (total mass/weight), and/or fewer plants will certainly benefit from UV Filtration. Ponds designed specifically for Koi definitely need one, along with a good biological filter. The bottom line: A properly sized, installed, and maintained UV Filter on any backyard pond assures clear water with virtually no attention except annual bulb replacement.
How to choose the proper UV Filter.
There are three elements critical to a UV Filter's effectiveness.
1) Assuring that the entire pond water volume passes through the filter at least once every 2-3 hours. This means a 1,000 gallon pond requires a minimal flow rate of 400 to 500 GPH (gallons per hour) entering the filter. Complete turnover every three hours is okay, every two hours is ideal. Anything less than this allows microorganisms/ algae to reproduce before passing through the filter and getting "zapped". Remember to consider vertical lift impact on the pump.
2) Not exceeding the maximum flow rate specified for the filter. Every UV Filter specifies maximum flow rates for effectiveness. The flow rate is critical because the water must remain in contact with the lamp long enough for proper irradiation of the organism/cell. Normally the higher the UV lamp wattage the higher the allowable flow rate. A good analogy to help understand this is to imagine holding a lit candle and passing your open hand just over it quickly. It does not burn. Now slowly pass your hand over and obviously the burn increases.
3) Assuring that all of the water in the pond is circulated through the light chamber. This means the pump, intake port, or point that water leaves the pond to enter the filter should be as far in distance as possible from the water discharge or return point into the pond, such as the waterfall. There should be no "bays" or "coves" in the pond that avoid water movement. Ponds with surface skimmers and waterfalls require some way, such as an additional pump or air stone, to move the bottom layer of pond water that otherwise may remain unmoved. If a portion of the water remains unfiltered the unaffected microorganisms will continue to affect the entire pond.
If these criteria are met a UV Filter will keep pond water crystal clear and healthy.
FYI: All UV Filters for ponds may be used as a "sterilizer" by slowing down the flow rate, while higher flow rates make them "clarifers" as long as the criteria above is followed.