Ultraviolet Radiation (UV)
Other Disinfection Methods
Drinking water should be free of pathogens that cause such illnesses such as typhoid fever, dysentery, cholera, and gastroenteritis. Whether or not a person contracts these diseases from water depends on the type of pathogen, the number of organisms in the water, the strength of the organism (its virulence), the volume of water ingested, and the susceptibility of the individual.
The purification of drinking water that contains pathogens requires a specific treatment called disinfection. Disinfection does not produce sterile water but it does lower the concentrations of pathogens to acceptable levels. Also, disinfected water is quickly contaminated with many types of benign heterotrophic bacteria that are ubiquitous (present everywhere) in the environment. These benign bacteria are regulated and listed in the NPDWS as Heterotrophic Plate Count HPC.
Disinfection reduces pathogens in water to levels designated
safe by public health standards. This prevents the transmission
of diseases. Ideally, an effective disinfection system
should kill or inactivate (render harmless) all pathogens
in the water. It should be automatic, simple to maintain,
safe, and inexpensive. The ideal system should treat
all the water and provide residual (long-term) disinfection.
Chemicals should be safe and easy to store and not make
the water unpalatable. Thus, water supply operators
must disinfect and, if necessary, filter the water to
remove Giardia lamblia, Legionella, coliform
bacteria, viruses, turbidity, and to USEPA-mandated
Private water sources, including wells, are vulnerable
to contamination from septic fields, improper well construction,
and poor quality water sources.
More than 30 million people in the United States rely on private wells for drinking water, and in Arizona there are over 81,000 wells.
|Typhoid fever deaths since the beginning of water chlorination in the US. Source: US Centers for Disease Control and Prevention, Summary of Notifiable Desaease, 1997 .
click to enlarge
Chlorine readily reacts with many contaminants found in water and, in particular, with natural organic matter (NOM), microorganisms, and plant matter. These include natural organic chemicals associated with taste and odor. Many types of reactions of chlorine and water contaminants produce a chlorine demand. The chlorine that remains after is residual. The chlorine breakpoint is the point at which residual chlorine is available for continuous disinfection. An ideal water disinfection system provides residual chlorine at a concentration of 0.3 0.5 mg/L. DPD (diethyl phenylene diamine) is a common water color test used to measure chlorine breakpoint and residual levels. Good test kits must measure free chlorine, not total chlorine, in drinking water.
Types of Chlorine Used in Disinfection
Pubic water systems use chlorine in gaseous forms, which are considered too dangerous and expensive for home use. Private systems use liquid chlorine (sodium hypochlorite) or dry forms of chlorine (calcium hypochlorite). To avoid hardness deposits on equipment, manufacturers recommend using soft, distilled, or demineralized water when making up chlorine solutions.
Equipment for Continuous Chlorination
Continuous chlorination of a private water supply can be done by various methods: chlorine pump, suction device, aspirator, solid feed unit, and batch disinfection. The injection device should operate only when water is being pumped, and the water pump should shut off if the chlorinator fails or if the chlorine supply is depleted. Consult with a professional on equipment selection and tank requirements. For example, in a private well system, the minimum-size holding tank is determined by multiplying the capacity of the pump by a factor of 10. Thus, a 5 gallon-per-minute (gpm) pump requires a 50 gallon holding tank. Other methods to control contact time include the use of pressure tanks and coils.
Ultraviolet Radiation (UV)
This method uses a UV lamp (source) enclosed in a transparent protective hollow sleeve through which water flows. RNA/DNA-damaging UV light is absorbed by bacteria and viruses, making them inactive and unable to reproduce. Class A UV systems are more effective at reducing pathogens than Class B units. Class B UV systems may be used at home to reduce the levels of heterotrophic bacteria present in tap water, although this many not necessarily make tap water safer. However, UV systems may provide an extra level of protection against pathogenic bacteria and protozoa. In summary, home UV Class B treatment systems are well suited to treat clean tap water with only residual levels of bacteria. Industrial grade UV Class A treatment units may also be used to kill or inactivate viruses, yeast, mold spores, and algae.
UV systems are simple and relatively maintenance-free. However, their efficiency depends on several things: the design and energy of the UV chamber and source, the flow rate of the water, the amounts and types bacteria and other microorganisms present, and the clarity of the water.
No chemicals are needed with this method of disinfection. But UV treatment provides no residual disinfection and it is not effective with cloudy or turbid water.
UV System Maintenance and Costs
The UV lamp must be replaced annually (having a 9-month to 1-year lifetime). The cost is approximately $80. A UV sensor is recommended to determine the UV dose needed to kill bacteria. The cost of a home UV disinfection system starts at around $500.
Other Disinfection Methods
Although chlorination is the method of choice for most municipal and private water treatment systems, alternatives do exist:
• ozone is a more powerful disinfectant than chlorine for microbes such as Giardia and Cryptosporidium, but is approximately four times as expensive
• its disadvantage is that it cannot be purchased but must be generated on-site
• the machinery to generate ozone is complicated and difficult to maintain
• the effects of ozonation chemical byproducts are not fully understood
Ozonation uses a four-step process: 1) Pure oxygen
is converted to ozone gas (O3)
using electricity (electric discharge generator). Ozone
gas, the strongest oxidizing chemical available, is
injected into the water where it reacts with and destroys
viruses, bacteria, and cysts and spores. The residual
(unused) ozone must be removed (degassed) from the disinfected
water stream, since it is toxic to humans.
Note that like UV radiation, ozone treatment does not
provide residual disinfection. Thus, long-lasting chlorine
chemicals (such as chloramnes) must be added to ozone
and UV-treated water to prevent microbial contamination.
• two minutes of vigorous boiling assures biological safety
• boiling kills all organisms in water (whereas chlorination reduces them to safe levels)
• boiling is practical only as an emergency measure
• once boiled, cooled water must be protected from recontamination
• pasteurization uses heat to disinfect but not boil water
• flash pasteurization uses high temperature for a short time (160° F, 15 seconds)
• low-temperature pasteurization uses lower temperature for a longer time (140° F, 10 minutes)
The use of household chemicals (such as bleach or iodine) to disinfect water without the appropriate equipment or technical supervision should only be considered under emergency situations. For a list of these chemicals and their safe use, see the EPA website.
(Portions of this text have been adapted from U.S. EPA, "Water on tap: What you need to know," Washington, DC: U.S. Environmental Protection Agency, Office of Water, EPA 816-K-03-007, Oct. 2003, and from Wagenet, L. and A. Lemley, "Chlorination of drinking water," Ithaca, NY: Cornell Cooperative Extension, Fact Sheet 5, Sept. 1988.)
Go to other treatment methods:
Particle and Microfiltration Filters
Activated Carbon Filters
Ion Exchange Water Softening
Other Treatment Methods