True UV Performance

by Steve Zimmer

Unfortunately, the water gardening community has no governing organization, such as the EPA or NSF to oversee claims manufacturers make about their equipment. Until a standard is adopted by our industry, unsubstantiated information regarding UV use will continue to confuse or mislead uninformed retailers and consumers. It is this article’s intent to dispel the misinformation and give the consumer a clear basic guideline to make an informed UV purchase.

1. The type of lamp: low-pressure or medium-pressure.
2. The length of the lamp: also known as ARC Length.
3. The physical design of the UV’s water exposure chamber.
4. The condition of the water to be treated.
5. Water flow rate through the UV’s exposure chamber.

chart to right). UV light is divided into 4 distinct spectral areas, Vacuum UV (100 to 200 nano meters), UV-C (200 to 280 nano meters), UV-B (280 to 315 nano meters), and UV-A (315 to 400 nano meters). The UV-C spectrum (200 to 280 nano meters) is the most lethal wavelength for microorganisms. This range contains the peak germicidal wavelength (264 nano meters) and is known as the Germicidal Wavelength.

When you understand UV and how it works to kill microorganisms, you can properly select the appropriate UV light source and design the UV’s water containment vessel around the lamp(s) selected.

Low-pressure lamps are typically run at low-input power currents of 200 to 1,500 milliamps and operate at temperatures between 100 and 200 degrees F. They have a useful life of 8,000 to 12,000 hours, depending on operating current of the lamp.

Medium/high-pressure lamps produces a very wide range of wavelengths, from 100 nm to greater than 700 nm, well into the visible light spectrum. These types of lamps are very poor producers

of usable germicidal wavelengths; they generally only convert up to 7% of their input watts into usable UV-C watts. (For example, a 175-watt medium-pressure lamp will have approximately 12-watts of UV-C power, while the remaining 163-watts are converted into heat and visible light.)

Medium and high-pressure lamps are typically run at high-input power currents of 2,000 to 10,000 milliamps, and operate at temperatures between 932 and 1,112 degrees F. This class of lamps generally has a useful life of only 1,000 to 2,000 hours, depending on the lamp’s operating current.

As we can see from these comparisons, low-pressure lamps are without a doubt, best suited for use in water Sterilizer applications.

Design of the Water Exposure Chamber. This is an area of design that is completely overlooked by some manufacturers, but is key to successful operation because the distance UV light energy has to travel from the surface of the lamp to the inner wall of the UV’s water containment vessel determines how much UV the water will receive, known as the “UV dose rate,” at any given water flow rate. The amount of water passing through the UV filter ultimately determines the unit’s actual UV dose rate, which is expressed in microwatts per second per square centimeter or (u-watts-sec/cm2). When selecting a UV Sterilizer for your application look for these important features:

• Make sure that the UV lamp is positioned between the water inlet and outlet ports of the unit’s water containment vessel. Any portion of the UV lamp(s) not located between the water ports is rendered useless. Furthermore, when calculating the UV’s performance data only the ARC length located between the water ports can be applied to the calculation, thus reducing its capabilities.
• Select a unit that has the largest diameter water containment vessel in the wattage that you are considering. A unit that has a larger diameter will always have a greater contact time. (For example, a 25-watt model that has a 3” diameter housing will be able to flow more water than a 2” housing model.)
• Make sure that the unit you are considering uses a Quarts Sleeve. The quartz sleeve is needed to isolate the UV lamp from the water to avoid a short circuit path for the lamp’s electrical power and to allow the lamp to operate at its optimum temperature by acting as an insulator.
• Do manufacturer’s list water flow rates at the end of a lamp’s life or the beginning? Most UV manufacturers give a water flow rate, but do not indicate whether it applies to a new lamp or one that is at the end of its useful life. Try to find a manufacturer that includes the water flow rate in the unit’s end of lamp life rating. The end of lamp life rating takes into account the lamp losing UV-C output due to age and so is a more realistic ability of how the unit will perform.
• Do the manufacturer’s water flow rates take into account the reduced effectiveness that the UV light will have when treating green water? This information should be listed as some type of percent transmissibility rate or absorption coefficient (decimal value). Units that account for this will have lower water flow rates.

Condition of Water to be Treated.
This also largely overlooked factor is one of the most critical factors when trying to determine the ability of a UV Sterilizer to treat a given volume of water. Regardless of the type of UV light source used, any body of water with impurities will adsorb UV energy. To those of us in the Pond Industry, the impurities of interest to us are algae and waterborne microorganisms. Green water, as water plagued by algae and microorganisms is known, will absorb the UV energy emitted by our UV light source in proportion to its density (or how green the water is).

The greater the amount of algae in the water, the greater the reduction in percent transmittance. Percent transmittance is the ability of a body of water to be effectively treated by a UV light source. This value indicates the quality of the water to be treated. The higher the percent transmittance, the easier the UV sterilizer will be able to treat the water at a given flow rate. A reduction in percent transmittance means the UV sterilizer will be less effective in dealing with the algae problem. If the sterilizer’s water flow rates have not been calculated with a reduced percent transmittance rate, the unit will have considerable trouble dealing with an algae bloom.

Putting it All Together.
The basic criteria for a sound design of UV Sterilizers, revolves around a careful selection of lamp type, lamp length, lamp position, and body diameter. These factors, together with the intended water flow rate, percent transmittance of the water to be treated, UV dose rate to kill the targeted microorganism, should be the basis for your decision when purchasing a unit for your customers. When researching which type of UV Sterilizer to purchase, remember the criteria laid out in this article, read the manufacturer’s literature, ask questions, and most of all ask yourself, does this information make sense? If not, consider another UV manufacturer.