Ultraviolet (UV) Sterilizers - Tech Talk 96
In order to protect aquatic livestock held in aquariums or holding tanks, biologically treated water should be adequately disinfected before returning it from the filter system. Mechanical and biological filtration does not provide inactivation of pathogenic bacteria, protozoa and viruses.
Ultraviolet radiation (germicidal) energy is unmatched in its efficiency, simplicity and dependability when applied as a microorganism disinfectant! It is most effective for its germicidal value in a clear water application at a wavelength of 265 nanometers. With proper exposure, ultraviolet radiation energy (ultraviolet light) penetrates a microorganism's cell wall. It then destroys the nuclear material, causing abrupt modification and quickly bringing about its destruction.
At AES we hear from a lot of people who have ultraviolet (UV) sterilizers and are still experiencing disease problems. This can almost always be traced back to inadequate UV irradiation. Because it is so difficult to measure the intensity of UV energy hitting the water, many buyers have improperly sized their UV sterilizers by simply following the maximum gpm flow rate published for 15,000 µWs/cm²* (who said aquaculture was easy?).
This is the problem! You cannot compare UV sterilizers by the watt ratings alone. That would be like comparing cars by their engine size alone. The watt rating is just the starting point for comparisons.
The full amount of UV energy required to kill a microorganism must hit the organism after the energy leaves the lamp, after it leaves the quartz sleeve, after the lamp has aged and after it has passed by any turbidity and color that block the light.
Low-pressure mercury type UV lamps are best suited to germicidal action because the primary radiation generated by these lamps consists almost exclusively of a spectral wavelength of 254 nanometers, which is close to the maximum peak germicidal effectiveness wavelength of 265 nanometers. This gives the low-pressure mercury type lamps an exceptional 40 percent UV energy efficiency rate between input watts and UV output watts.
Medium- and high-pressure mercury type lamps are best suited for treatments involving chemical by-products associated with industrial waste water or for the drying of printing inks, paints and adhesives, not germicidal action.
The bulk of their power is in the 320 to 440 nanometer range, well outside the germicidal range.
Ultraviolet light can be very effective at eliminating viruses, bacteria, algae and fungi. The required UV exposure rate to irradiate common bacteria is 15,000 µWs/cm² , while the required UV exposure for waterborne algae is 22,000 µWs/cm² . Since it is the intensity of light that is doing the killing, we must know how much light energy to use and how much is reaching the target. Just as some sunglasses and sunscreens reduce UV intensity, so do discolored water, turbidity, dirty quartz sleeves and even some dissolved salts, such as sodium thiosulfate. Lamp temperatures may even reduce output when operated in cold water (110°F gives maximum UV output).
To ensure sterile water using UV light, first start with clear water, and have a lamp and flow rate that are sized to deliver the correct amount of irradiation for the target organism (see exposures list). If a UV light is flow rated for 15,000 µWs/cm² and you want 30,000, either double the number of lamps or reduce the flow by half, and so on for higher dosages.
Be aware that some lamps age rapidly, and the manufacturer probably states the watts produced when the lamp is new. This wattage can be reduced by as much as 40 percent in as few as six months! We suggest oversizing the UV sterilizer by at least 40 percent to be sure of getting the killing power required when the lamp has aged. We also suggest changing lamps at six-month intervals. For a more in-depth look at UV sterilization and system design, consult the book Aquaculture Engineering (WQB3).
*µWs/cm² = exposure to ultraviolet light of 253.7 nm wavelength in microwattseconds per square centimeter.