Oxidation - Public Water Systems - page 534

adjust the pH or even to use softening treatment before the AOP. High levels of NOM
can also increase scavenging of hydroxyl radicals, making application of AOPs less
effective and less economical in these conditions. Again, pretreatment to reduce the
bulk organic level prior to oxidation may be needed.
AOPs employing UV can be adversely affected by the presence of color or turbidity,
which can reduce the UV transmittance. Key design considerations include light in-
tensity and contact time.

APPLICATION OF OXIDANTS IN WATER TREATMENT

The following sections describe applications for oxidants in drinking water treatment.
Most cases involve the oxidation of organic or inorganic materials to achieve a desired
treatment objective. In some applications, the oxidant is used as a biocide. The two
most common uses of oxidants—disinfection and iron / manganese removal—are dis-
cussed in Chapters 19 and 14 and are not addressed here.

Control of Zebra Mussels
As described in Chapter 8, proliferation of zebra mussels in U.S. surface waters has
created serious problems associated with clogged intakes and structures, and water
quality degradation. 21,22 To control zebra mussels, a variety of oxidants have been
tested with varying degrees of success. The most extensive applications have involved
free chlorine. Investigators report success using continuous chlorination in which a
free residual of 0.5-2.0 mg / L is maintained throughout the intake pipe. 23 Typically,
the chlorine dosage requirement will vary seasonally for a variety of reasons, with one
of the major factors being temperature. Application of chlorine is often convenient for
municipal water suppliers who use chlorine or chlorine solutions as disinfectants and
have facilities that can be modified or expanded. Either gaseous chlorine or hypochlo-
rite may be used.
Relocating the chlorine injection point to the intake will increase THM production
by increasing the exposure of the chlorine to organics and lengthening contact time.
Consequently, use of chlorine as a zebra mussel control agent is a workable alternative
only if chlorine by-products can be kept to an acceptable level.
To reduce THM formation, other approaches may be used. A number of systems
along the Great Lakes are supplementing chlorination with potassium permanganate
to hold THM formation in check; including some systems that use KMnO 4 exclusively
for this purpose. 24 Unfortunately, potassium permanganate control is considerably more
expensive than chlorination. Chlorine dioxide is reported to be especially effective in
controlling zebra mussels, 25

and at least one investigator has found ozone to be an

effective control chemical. 26

Control of Biofouling Within the Treatment Plant
Oxidants may be used when it is necessary to control biological growth within the
treatment plant. Basins and filters that are open to sunlight are particularly susceptible
to growth of algae and other microorganisms. Growth may be minimized by applying
chemical oxidant in sufficient quantity to maintain a residual throughout the susceptible
treatment processes. Alternatively, occasional doses at high concentrations may be used