Water Treatment - Disinfection

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Water Treatment

First Water Products




		Disinfection refers to the use of a physical process (e.g., heat or radiation) or chemical treatment to destroy vegetative microbial pathogens. Heat is an effective means of disinfection, and is still used in a clinical context for most sterilization needs. Raising water to boiling, or even to low 60ºC if contact continues for at least 15 minutes, will deactivate most waterborne microbes. However, by consuming scarce fuels and contributing to air pollution, boiling present certain disadvantages, even when the time needed to heat and cool drinking water is not an issue. UV radiation has also shown to be an effective means of disinfecting water. In the field, however, UV systems can be difficult to manage where high turbidity and other solids can protect pathogens from direct radiation. Chemical treatments include halogens, such as chlorine and iodine, and ozone (O3). Chlorine, dispensed as a solid (calcium hypochlorite), liquid (sodium hypochlorite) or gas (elemental chlorine, Cl2) has been the disinfectant of choice in point of entry water treatment systems, particularly in the United States. It is generally effective, fairly low in cost and provides a residue in water distribution systems that help ensure that the water remains disease free. Because of its low cost, chlorination is also the most common means of disinfecting water for victims of emergencies. However, chlorine resistance, particularly by encysted protists (such as Giardia lamblia and Cryptosproridium spp.), certain bacteria (including Vibrio cholerae that can hide inside algae) and certain viruses, has increasingly required chlorination to be supplemented by filtration and other secondary methods. Concern about chlorine by-products such as trihalomethanes (THM’s) has also led some treatment facilities to use ozone, a more common disinfectant in Europe. While ozone is more effective than chlorine against most cysts and viruses, it is impractical in field applications. Iodine is also an effective water disinfectant, though it must normally be accompanied by microfiltration to eliminate cysts. Poly-iodides such as I3 and I5 are even more powerful than conventional I2. When used in an ion exchange system, or combined with an adsorption medium, iodine residuals can be significantly limited so as not to affect taste or present a health hazard. Metal ions, such as silver (Ag+) are used as bacteriostats, but should be combined with microfiltration or germicides for disinfection. It should be noted that most chemical disinfectants are particularly susceptible to differences in turbidity, pH and temperature of the source water. Contact time and/or concentration must normally be increased where turbidity is high and pH and temperature low. Moreover, while disinfectants deactivate microbes in their vegetative state, they may not be effective against cysts and spores. For this reason, disinfectants are usually combined with at least a 1 micron filter. Finally, disinfectants do not address toxins that may be produced by bacteria. Where toxins are a concern, they should be managed both by treating the bacterial source and by using adsorption to remove the chemical toxins.