As we enter the height of new pond set ups and maintenance on older ponds, the subject of algae control via the use of a UV sterilizer/clarifier always comes up. The simple truth is that the proper application of a UV sterilizer can eliminate free floating, “green water” type algae. Unfortunately, it will not eliminate stringy algae or pond scum algae, as it can only eliminate algae that can be pumped through the UV unit. The UV bulb emits light primarily at a wavelength of 254nm, which penetrates the cell and disrupts the DNA, preventing growth and multiplication.
There are several factors that will help determine the proper size UV needed:
- Size of organism to be killed (algae, bacteria, etc)
- Total output of the UV sterilizer (Watts)
- Ability of UV light to penetrate the water
- Contact time between the organism (algae) and the UV bulb
In most applications, we are interested in killing free floating algae, and that requires a “kill dose” of approximately 20,000 micro-watt seconds per square centimeter. (Not a rating that just rolls off the tongue, we usually just refer to the base number.) The kill dose is dependent on the watt rating of the bulb and flow rate of water through the unit, also called the dwell time. Most manufacturers will give recommendations for the flow rate versus wattage of the bulb required to kill different organisms. For most pond applications, you want to be able to use a water pump with a flow rate equal to or greater than 1/2 of the total gallons of water in the pond at the desired kill dose. Mathematics beyond the scope of this discussion have shown that if we take the total number of gallons treated and divide that by the Gallons Per Hour (GPH) of the pump and then multiply that number times 9.2, it will give us the number of hours to guarantee that 99.99% of the water has passed through the UV at least once, or one turn. To control algae, we want 2-3 turns per day. To control bacteria could take up to 6 turns per day. It is for this reason that whenever you use a UV unit, you want to run it continuously for at least 24 hours or longer. Once the pond is clear, you can turn it off and wait until the algae starts to build up and then run the UV for a day or two to clear it out again. The UV bulb will need replacement after 6-9 months of operation, or when it has dropped to about 60% of its original output.
Most modern UV units include a quartz sleeve or tube to separate the UV bulb from contact with the water. While this helps eliminate electrical issues, it also allows the UV bulb to burn at a higher temperature than the water, making it more efficient. You may need to periodically clean the quartz sleeve with rubbing alcohol, just try to not leave any fingerprints on the sleeve or the bulb to avoid “hot” spots. Ideally, you will want to place the UV sterilizer after some kind of mechanical filtration to help remove any turbidity in the water to increase the penetration depth of the UV irradiation. Most units have a 2-3” diameter chamber to bring the water in close contact with the quartz sleeve and may have a “twist” design to increase the length of the flow chamber without increasing the overall length of the unit itself. While it is difficult to use too large of a sterilizer, some care needs to be taken when treating the water with medications or bacterial preparations to eliminate sludge. Turn off the UV for a few days to allow these beneficial bacteria to colonize the surface areas of the pond. With a little bit of planning, the UV sterilizer can be a most effective control of green water in the pond.