Civil Engineering Reference
In-Depth Information
Alternative Disinfectants-Oxidants
Switching to alternative disinfectants-oxidants
may be feasible provided the following criteria are met:
DPBs are not produced at undesirable levels.
•
Microbial inactivation is at least as effective as disinfection with chlorine.
•
A stable disinfecting residual is provided in the distribution system.
•
From an economic standpoint, the ideal alternative disinfectants-oxidants should be no
more expensive than chlorine. Unfortunately, on a cost basis, free chlorine is by far
the most effective disinfectant. Moreover, no single alternative disinfectant-oxidant
(e.g., ozone, chlorine dioxide, chloramines, and UV radiation) can satisfy all of the
above requirements. Hence, to replace free chlorine, a combination of disinfectant-
oxidants is usually needed.
For example, a utility might decide to use ozone, which is much more effective
than chlorine for raw water microbial inactivation and produces no significant halo-
genated organics. However, it is known that ozone creates a variety of other DBPs,
for which little is known relative to health effects, and because ozone leaves no long-
lasting residual, chloramines are commonly used as a secondary disinfectant.
Although such a strategy is adequate for reducing THM and total halogenated
organic by-product levels, the combined use of these disinfectants will produce other
DBPs. Studies have shown that the use of ozone with chloramines can increase levels
of chloropicrin, cyanogen chloride, and total aldehydes. Consequently, utilities using
this strategy to reduce THMs may have to reevaluate treatment as new DBP regulations
are developed.
Removal of NOM
Removal of natural organics, or precursor materials, prior to dis-
infection represents an optimal approach for controlling DBPs. Because precursor ma-
terials are constituents of the dissolved organic carbon (DOC) in raw water, optimizing
treatment to remove DOC before adding disinfectant-oxidant provides the best strategy
for reducing DBPs. Treatment technologies to remove NOM include conventional
treatment, oxidation, adsorption, and membrane processes.
Application of these technologies for organics removal is discussed in this topic's
respective chapters on coagulation / flocculation, activated carbon, and membranes.
MTBE
Methyl tertiary butyl ether (MTBE) poses a historically unprecedented challenge to
the management of the nation's water supplies. It is a chemical produced in massive
quantities and distributed throughout almost all areas of the United States. It has unique
properties that make it a water quality threat, as well as exceptional resistance to
conventional treatment, including aeration and activated carbon adsorption. Of little
concern prior to the late 1990s, MTBE literally burst onto the regulatory scene with
reports of serious water supply contamination in almost all states. Ironically, the wide-
spread use of MTBE, and consequent problems, is the result of environmental initia-
tives designed to address air quality issues.
MTBE is the most common oxygenated fuel additive used in reformulated gasoline.
In 1996 alone, over 18 billion pounds were produced, making it the second most
widely produced organic chemical in the United States. Initially, MTBE was formu-
