The Basics of Water Disinfection
Water Treatment 101: Why UV Is Gaining in Popularity for Microbe Control in Well Water
by Pure Water Annie
By its nature, water provides a an inviting growing place for bacteria. Bacteria, viruses, protozoa are small creatures, but when ingested over time and in sufficient quantities they can lead to serious health problems. E coli, a member of the coliform family of bacteria, has received enough public attention to be feared and respected, although it is only one of many microorganisms that can be dangerous.
Actually, bacteria are fairly easy to control as compared with cysts like giardia and cryptosporidium, which are bigger, tougher and very difficult to kill with conventional water treatment disinfection chemicals like chlorine. Cysts have a protective outer shell that protects them from municipal water treatment chemicals.
Essentially, there are three strategies that can be used to control potential disease-causing microbes in water.
1. Chemical treatment with chlorine, chloramine, hydrogen peroxide and other less frequently used disinfectants. Chemicals are not effective against cysts. Chemicals remain the treatment of choice for municipal water supplies, however, because chemicals like chlorine provide a residual effect that stays in the water all the way to the home.
2. Straining through very tight filters. Bacteria, for example, can be removed by very tight sub-micron filters, and cysts, which are much larger, are easily blocked out even by a one or two micron filter. Straining is a popular strategy for emergency filters and small point-of-use drinking water filters. It can be applied for cysts in larger applications, though flow restriction usually makes it impractical for bacteria control where significant flow rates are required.
3. Ultraviolet treatment which eliminates both bacteria and cysts. Ultraviolet treatment (UV) involves passing the infected water by an ultraviolet lamp that has enough intensity to alter the DNA of water-borne pests. UV is becoming the favorite technique for well owners because it is easy to install, easy to maintain, and relatively inexpensive. It is also popular because it provides a complete treatment and adds nothing objectionable to the water.
More About UV
Disinfection chemicals like chlorine are measured in “parts per million” of the disinfectant. Straining devices are measured by the micron size of the filter. UV is a little more complicated. The standard measure of UV dosage is mJ/cm2, millijoules per square centimeter. This number is a measurement of the intensity of the lamp with consideration of how fast the water flows past the lamp. Although NSF standard is 40mJ/cm2, in the water treatment industry it is generally assumed that 30mJ/cm2 is more than enough to treat residential well water. In fact, a 16mJ/cm2 unit is twice as hot as it needs to be. 6-10mJ/cm2 is sufficient for most pathogens. 6mJ/cm2 will do away with 99.99 percent of E. coli.