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Ultraviolet Sterilizers: Use in Pond Filtration Systems


A dual bulb in line unit

Aquatic Gardens

Ponds, Streams, Waterfalls & Fountains:
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Volume 2. Maintenance, Stocking, Examples

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by Robert (Bob) Fenner

Ultraviolet (UV) radiation is sometimes employed in water purification systems for tropical freshwater, marine, cool water and pond systems. UV sterilization can be very effective in reducing free-floating algae, bacteria and other microscopic planktonic organisms.

UV and near U. radiation, 295 to 400 nm (nanometers) has also been documented to aid in oxidation of organics, phosphate and nitrogenous compounds through the production of ozone (O3).

Though the disease-reducing benefits above are considerable in producing and maintaining a favorable environment, UV sterilization should not be relied on as the principal part of a filtration system. UV purification is a very useful addition to an otherwise appropriate filtration system.

This article describes the pros and cons of UV use and appropriate application in re-circulated pond systems.


UV light is indiscriminate in the destruction of free-floating micro-organisms. It kills "good guys" as well as bad. These beneficial microbes are absolutely necessary in almost all captive environments. For this reason: 1) Initial, break-in periods of new aquatic set-ups are run without the UV sterilizer being turned on. 2) It is suggested by some writers that UV's be left while using some therapeutic treatments but used with others.

Organisms maintained in a "well-filtered", strongly UV sterilized system seem to develop a type of acquired immune deficiency syndrome. Like the "boy-in-a-bubble", organisms kept in an almost sterile environment seem to lose their ability to ward off infectious diseases. Now, let me explain the qualifiers placed upon the terms above. A) There are no captive systems that result in 100% effective kill of all micro-organisms. B) This loss of apparent immunity occurs over long periods of time in a highly variable, non-selective manner. C) This "syndrome" has, to my knowledge, never been scientifically documented; therefore my use of the word seems from my personal and second-hand experiences.


UV purification is widely used in many industries; medical, recreational swimming, alcoholic beverages and drinking water among others.

UV light is a natural part of electro-magnetic-radiation (EMR, Electromagnetic radiation) produced by the Sun. UV lamps generate more of these frequencies of light at a nominal cost.

There are sizes, fittings and models of UV's for virtually any aquatic application; aquaria, pools, ponds and re-circulating multiple tank systems.

The water going through the UV sterilizer should first be run through biological filtration and mechanically filtered to remove particulates. Allowing air bubbles or any solid matter to pass through the contact chamber is contra-indicated. For this reason, using an air-lift system as a means of moving water through the UV is a bad idea. It is suggested that if a heat-exchanger (for freezing weather) and/or separate chemical filtration is utilized that these come before the UV. In other words, the UV system should be the last part of the filtration system to have water passed through before returning to the live-holding systems.

The reasons for this are several and mostly obvious. The most important are:

1) To remove as much "other stuff" from the water so that the UV radiation will operate with highest efficiency.

2) Beneficial microbes will be preserved and their activity promoted.

Flow Rates:

Dwell time, the amount of time a given quantity of water is exposed to a given concentration of radiation may be calculated at about twenty gallons per hour flow per watt UV. This value or more watts per unit flow is adequate for providing a good kill rate per pass and will substantially reduce planktonic micro-organisms and organics concentrations.

The beneficial effects of UV are enhanced by utilizing the filter system order described above and by arranging for as complete a circulation pattern as possible in your live-holding system.

More vigorous flows are not necessarily to be avoided, but reducing dwell time will result in loss of efficiency per pass. This might lead to increased spread of infectious disease.

Passing all the water in the system though the filter mechanisms once or more an hour is ideal; though unrealistic for large volumes of water.


Most UV lamps are engineered to operate at around 72 degrees Fahrenheit. Cold/cooler water systems or systems with debris or pressure that might damage the lamps must be fitted with sleeves of quartz of Teflon. These jackets require periodic inspection and  cleaning; about once a month.


Plumbing By-Pass:

To facilitate cleaning and lamp replacement, the following layout is offered:

Valves A and B can be closed after opening C, allowing continuous operation of the system during UV shutdown. Multiple UV systems can be cut in parallel to aid in cleaning one at a time.



Regular, routine up-keep is necessary to assure peak performance. Sleeves should be removed from their contact chambers and cleaned once a month or so. Generally, this can be accomplished by simply wiping with a clean, dry cloth or towel. If necessary, slime may be removed by wiping with rubbing alcohol.

Lamp Replacement:

For most systems the UV is left on continuous operation, at least for the same time the circulation pump is running. Lamp life for these units is about ten months of continuous duty. Mark your calendar, your "pond log book", and the sterilizer housing/body with a pen for the install and replacement dates.

Remember, the bulb's may still be glowing blue after their effective radiation-killing effectiveness is long gone. Don't spend the money on electricity for a placebo blue-light.

To Build or Buy a UV?

Depending on the size and complexity of the system in question, an adequate UV might cost as little as a hundred to several hundred dollars to buy and approximately half as much to build, with the ratio of cost to savings improving with the size of the system. This guesstimate, of course, figures no charge for your tools and your time to search for, collect and assemble the necessary components.

To Build: A simple do-it-yourself model is described below; assembled with easy to find parts and solvent.


2+ fluorescent fixtures; connectors, ballast, switch, cord, wire.

2+ germicidal (UV) lamp; from medical, pet fish business.

Some plastic (PVC, ABS) pipe of adequate diameter to allow insertion of lamp, possibly sleeve and permit water flow.

Some fittings; tees, reducers, barbs if necessary.

2+ compression fittings for water-tight sealing of lamp and/or sleeve.

Quartz or Teflon tubing for sleeves (You want these for coldwater systems and maintenance sakes.).

A mounting bracket/arrangement.

A pumping source for pushing/pulling water through the UV.

Buying a UV:

There are a large number of hobbyist and industrial grade units made in the U.S. and abroad. Their quality and features vary widely. Features to look for:

1) Remote able ballast's); to position it in a heat and water-damage free place.

2) Indicator light; to check for "on" operation.

3) Automatic-On feature; to turn the U.V. back on in the event of a temporary power loss.

4) Couplings that are easily fitted to your system. Get flexible types of true-union quick (threaded) disconnects.

5) Sleeving at little or no additional cost.

6) Guarantees/warrantees.

7) All Non-Corrosive, water-contact surfaces.

8) Ask Other Users regarding their success/failure, in particular with leakage and ease of use with given brand names and models.


Ultraviolet sterilizers are not a cure-all, but may figure into an integrated pond water quality management scheme. In order to receive maximum benefits, they must be placed in proper sequence in the filtering process and require regular cleaning maintenance.

Bibliography/Further Reading:

Drew, C. 1970. How to build an ultra-violet germicidal filter. Modern Aquarium, December 1970.

Fujita, Grant 1987. Nishikigoi Book. Electrical purification, p.107

Kennedy, Ron & Rosemay, 1986. Electronic U.V. Filter. In The Associated Koi Club's of America's, KOI BOOK, p. 85 & 86

Webb, R.B. & Brown, M.S., 1982. Genetic damage in Escherichia coliK12 AB2480 by broad spectrum near-ultraviolet radiation. Science, Vol 215, 19 Feb. 1982, AAAS

Aquatic Gardens

Ponds, Streams, Waterfalls & Fountains:
Volume 1. Design & Construction
Volume 2. Maintenance, Stocking, Examples

V. 1 Print and eBook on Amazon
V. 2 Print and eBook on Amazon

by Robert (Bob) Fenner
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