Civil Engineering Reference
In-Depth Information
Oxidation Treatment—General
Effective treatment techniques for control of or-
ganic tastes and odors include oxidation, adsorption, or a combination of these pro-
cesses. Generally, oxidation followed by filtration is the most cost-effective approach,
but particularly tough applications may require a combined oxidation / adsorption pro-
cess to lower tastes and odors to acceptable levels.
The following sections discuss the potential applicability of alternative oxidants for
taste and odor control. Although general conclusions can be drawn about their com-
parative performance, there is no definitive guide as to which oxidant to apply for
each odor type or source. This is due in part to the transient nature of taste and odor
episodes and the potential that there may be multiple origins to the odor problem.
Also, water quality characteristics and competing oxidant demands may significantly
impact the effectiveness or cost of different oxidants. Finally, oxidants have the po-
tential for creating new taste and odor problems. For these reasons, bench or pilot
studies should be conducted to establish design criteria and select the appropriate
oxidant for specific taste and odor problems.
Chlorine
The occasional success of heavy chlorination to control tastes and odors
from algal growth or seasonal reservoir conditions has been reported.
39,41,42
However,
these applications required prechlorination at sufficient doses to produce a free chlorine
residual of 1-5 mg / L. Regulations on chlorinated organics renders this high-dose
prechlorination practice impractical for most source waters.
At lower doses, chlorine has been found most effective with organic sulfides, di-
sulfides and mercaptans.
34
At practical dosage rates and contact times, chlorine was
found ineffective for oxidation of the algae products 2-isopropyl-3-methoxy-pyrazine
[IPMP] and 2-isobutyl-3-methoxypyrazine [IBMP],
43
and could not oxidize geosmin
and MIB even at extreme doses and contact times.
43,44
Chlorination itself is a leading cause of odor complaints in potable water, either by
itself or through its by-products. Studies have shown that the odor threshold for chlo-
rine in water at neutral pH is about 0.2 mg / L.
45
The threshold increases to about 0.5
mg / L at pH 9.0. The odor threshold for certain reaction products of chlorine, such as
nitrogen trichloride, is much lower.
For odors of certain industrial or algal origin, chlorination often increases odor
intensity or character. In this case, superchlorination to a free residual is necessary,
followed by partial dechlorination. An example of this is the reaction of phenolic
compounds with chlorine. At low chlorine doses, chlorophenol compounds are formed
and impart an objectionable medicinal taste to the water. As the chlorine dose in-
creases, the taste-producing intensity of the water increases up to a maximum, after
which greater chlorine doses reduce and finally eliminate the chlorophenolic tastes.
34,42
Chlorine Dioxide
Chlorine dioxide has been used effectively to destroy taste-
producing phenolic compounds, and will eliminate chlorophenol taste caused by prech-
lorination. Chlorine dioxide also oxidizes some other off-taste and odor-causing
compounds such as mercaptans and disubstituted organic sulfides. Studies in Los An-
geles achieved removal efficiencies for IPMP, IBMP, and TCA of greater than 50
percent for a practical range of chlorine dioxide dosages and contact times.
43
Generally,
chlorine dioxide has been found ineffective for oxidation of geosmin and MIB.
44
As described earlier, restrictions on chlorine dioxide by-products in the finished
water limit use of this chemical to applications requiring low dosage rates. For surface
and groundwaters containing significant organic matter, the overall oxidant demand
