How does UV work?
Ultraviolet or UV energy is found in the electromagnetic spectrum between visible light and x-rays and can best be described as invisible radiation. In order to kill microorganisms, the UV rays must actually strike the cell. UV energy penetrates the outer cell membrane, passes through the cell body, and disrupts its DNA, preventing reproduction. UV treatment does not alter water chemically; nothing is being added except energy. The sterilized microorganisms are not removed from the water. UV disinfection does not remove dissolved organics, inorganics, or particles in the water.
The degree of inactivation by ultraviolet radiation is directly related to the UV dose applied to the water. The dosage, a product of UV light intensity and exposure time, is measured in microwatt second per square centimeter (µws/cm2). The accompanying table lists dosage requirements to destroy common microorganisms. Most UV units are designed to provide a dosage greater than 30,000 µws/cm2 after one year of continuous operation. Notice that UV does not effectively disinfect some organisms (most molds, protozoa, and cysts of Giardia lamblia and Cryptosporidium) since they require a higher dose.
UV Units for water treatment
Special low-pressure mercury vapor lamps produce ultraviolet radiation at 254 nm, the optimal wavelength for disinfection and ozone destruction. The UV lamp never contacts the water; it is either housed in a quartz glass sleeve inside the water chamber or mounted external to the water which flows through UV transparent Teflon tubes.
Some ultrapure water systems use 185 nm UV units for reducing TOC (total organic carbon).
Important variables for successful disinfection
Although 100% destruction of microorganisms cannot be guaranteed, it is possible to achieve 99.9% reduction in certain applications and with proper maintenance. In order for a UV unit to successfully disinfect water, the following variables must be considered.
Water Quality
Certain contaminants in water can reduce the transmission of UV light through the water, which reduces the UV dose that reaches the bacteria. These UV absorbing contaminants include turbidity, iron, and humic and fulvic acid, common to surface water supplies. Suspended particles are a problem because microorganisms buried within particles are shielded from the UV light and pass through the unit unaffected. UV disinfection is most effective for treating high-clarity purified reverse osmosis or distilled water. (View the table that shows the Recommended Maximum Concentration Levels for water to be treated by UV.)
Flowrate
All UV units have a maximum flowrate capacity and some have a minimum flowrate as well. If the flow is too high, water will pass through without enough UV exposure. If the flow is too low, heat may build up which can damage the UV lamp. The water flow in an animal drinking water system is usually low and intermittent so a UV unit with minimum flow requirements should not be placed on the water line supplying pressure stations in a non-recirculating system. UV units are most often used in constant flow recirculating systems. (View a table that shows the Ultraviolet Dosage Required for 99.9% Destruction of Various Organisms (µWs/cm2 at 254 nanometer.))
Under ideal conditions, a UV unit can provide greater than 99% reduction of all bacteria. Even with this performance, ultraviolet disinfection has the following limitations:
"Point" Disinfection
UV units only kill bacteria at one point in a watering system and do not provide any residual germicidal effect downstream. If just one bacterium passes through unharmed (100% destruction of bacteria cannot be guaranteed), there is nothing to prevent it from attaching to downstream piping surfaces and proliferating.
Cells Not Removed
Bacteria cells are not removed in a UV unit but are converted into pyrogens. The killed microorganisms and any other contaminants in the water are a food source for any bacteria that do survive downstream of the UV unit.
