Civil Engineering Reference
In-Depth Information
ozone-chloramine
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chlorine dioxide-chlorine
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chlorine dioxide-chloramine
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All these disinfection strategies can produce a wide variety of DBPs, including the
ozone-chloramine strategy. In fact, any strong oxidant, halogen or not, will react with
NOM to produce oxidation by-products. Ozone by itself has been observed to increase
the concentration of nonhalogenated organics, such as aldehydes and carboxylic acids.
When used in conjunction with chlorine, ozone will increase the formation of DBPs,
such as chloropicrin and the haloketones.
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One of the principal treatment concerns
relative to ozonation is that a portion of the refractory NOM compounds are oxidized
to smaller, more biologically degradable compounds, hence increasing the assimilable
organic carbon content. In some circumstances, this may require additional treatment
to achieve a biologically stable water. Even the use of preoxidants for taste and odor
control (i.e., potassium permanganate and hydrogen peroxide) can impact the DBP
distribution formed by subsequent disinfection strategies.
Table 3-8 presents a shortlist of disinfectant and DBP-related compounds that are
currently receiving focused attention from the USEPA relative to health effects,
analytical methods, occurrence, and treatment methods. It is anticipated that Stage II
of the D / DBP Rule (scheduled for promulgation in 2005) will address many of these
compounds.
DBP Control Strategies
Developing a DBP control strategy requires careful planning. The current USEPA
approach strongly favors those strategies involving removal of DBP precursors prior
to disinfectant addition. This may involve optimizing existing processes or adding new
processes to remove NOM.
In response to the original THM MCL (1978), many utilities instituted DBP control
measures based on alternative disinfectants, such as ozone and chloramines. Although
this approach is useful for control of THMs and, to a lesser extent, HAAs, it may not
be adequate for control of other DBPs, such as chloropicrin and the haloketones.
Consequently, utilities that have already installed these approaches for THM control
will need to review the DBP issue once again. A limited number of utilities have
installed processes, such as aeration, that remove THM after their formation. These
utilities will also need to reexamine their existing approach for control of the other
DPBs. The recently promulgated D / DBP rule will be a major driving force behind
the design and operation of drinking water treatment facilities into the foreseeable
future.
Three general strategies (or a combination thereof) are available for reducing DBPs
in finished drinking water supplies:
Remove the DBPs after they are formed.
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Use a disinfectant-oxidant other than chlorine that does not produce undesirable
DBPs.
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Remove the natural organics (precursors) before disinfection-oxidation.
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