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Disinfection, taste & odor control.

Sparkling, clear water is esthetically appealing and a desirable goal of the water treatment process. However, no matter how clean it looks, if the water has an odor to it or leaves an unpalatable taste on the tongue, the consumer's reaction will be outrage. If pathogens remain in the water the reaction could be even worse. Thus of all the unit processes disinfection is unquestionably the most important. The disinfection process includes filtering, adding a primary disinfectant, and maintaining a residual disinfectant throughout the distribution system to guard against subsequent contamination or microorganism regrowth.

Adequate disinfection is achieved by maintaining a minimum concentration of the disinfectant material times the contact time value that is specific for each disinfectant and set of conditions. Oxidative disinfectants are used exclusively in potable water. In order of current use they are free chlorine, chloramine, chlorine dioxide, and ozone. Each has its own unique characteristics that make it more or less suitable for particular applications. Which method or combination is best depends on disinfection objectives, available contact time, type and concentration of reactive contaminants, disinfection by-products, and economics.

Free chlorine has been used for potable water disinfection since the early 1900s. Today it is still the most widely used because of low cost and proven effectiveness. It is an excellent bactericide, viricide, and cysticide. Chlorine's effectiveness is reduced at higher pH levels and in the presence of ammonia and organic nitrogen compounds. Free chlorine is a strong reactive oxidant that may not be sufficiently stable to provide a protective residual in some distribution systems. As do all disinfectants, free chlorine will produce by-products, the most notable being trihalomethanes or THMs.

Some water treatment plants have used chloramine as the primary disinfectant as far back as the 1920s. Hundreds use chloramine as a residual disinfectant in conjunction with a stronger primary disinfectant. Chloramine are formed by combining chlorine and ammonia. Based on taste and odor characteristics, monochloramine is the preferred form. Chloramine are a substantially weaker disinfectant and oxidant than its free chlorine counterpart. Chloramine may take 100 times or longer to effect the same bacteriological kill. It is however, relatively stable and may be the preferred residual disinfectant in distribution systems. Of all the disinfectants chloramine produce the least amount of by-products.

Chlorine dioxide has traditionally been used as an oxidant. In the last five years it has been increasingly applied as the primary disinfectant. Chlorine dioxide inactivates Giardia, a disinfectant resistant pathogen, five times faster than free chlorine. Its effectiveness is not significantly affected by pH and it does not form chlorinated by-products. It is, however, more expensive than chlorine and does produce inorganic by-products - chlorite and chlorate. Chlorine dioxide is reactive and is not usually used to maintain a residual in the distribution system. EPA currently recommends that the maximum combined concentration of chlorine dioxide, chlorate, and chlorite be limited to 1 mg/L.

Currently there are several dozen water treatment plants in the U.S. using or planning to use ozone. Ozone is being considered in more treatment systems to meet new disinfection requirements and minimize the formation of regulated by-products. Ozone is the strongest of the disinfectants$covered here. The drawbacks are cost and limited information on by-products. Ozone can reduce or eliminate the need for other processes and chemicals, which will in part offset the cost.

The chemicals used for disinfection are also often used to control taste and odor (T&O). T&O can be imparted to water by natural processes, contamination by man-made wastes, and/or during other water treatment processes. Treatment includes: oxidation by chlorine, chlorine dioxide, ozone, or potassium permanganate; adsorption by granular or powdered activated carbon or by substituting a treatment process (chemical) that does not form a T&O compound. Most T&O is treated by chlorine oxidation and powdered activated carbon adsorption. Free chlorine can form chlorophenol and other chlorinated organics with intense T&O characteristics. Chloramine and potassium permanganate will not form these chlorinated T&O compounds and chlorine dioxide can be used to remove them.
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Title Annotation:Water Supply and Treatment
Publication:Public Works
Date:Apr 15, 1995
Words:674
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