Civil Engineering Reference
In-Depth Information
dose of chlorite.
3
From animal studies, a Suggested No Adverse Response Level
(SNARL) of 0.38 mg / L has been derived with an uncertainty factor of 100.
2
Chlorate.
Oral studies with chlorate demonstrate the effects on hematological param-
eters and formation of methemoglobin, but at much higher doses than chlorite (between
157 and 256 mg / kg-day
1
). No carcinogenicity or reproductive effect tests have been
conducted. Data were inadequate to develop a limitation for chlorate as part of the
Stage 1 D / DBP rule.
3
Using concentrated solutions of chlorate in humans, a lethal
dose of 71 mg / kg.
2
Iodine
Formation and Occurrence
Historically, iodine is better known for its use as a
disinfectant for abrasions and other skin wounds than for disinfecting water systems.
It has been used in piped water systems only in emergency situations, and then only
in small, independent utilities.
19
An iodine residual of about 1.0 mg / L is required for
effective disinfection.
Iodine reacts in water to form hypoiodous acid (HOI). In water at pH above 4.0,
HOI undergoes disassociation to form the hypoiodite ion (OI
). At pH values above
8.0, HOI is unstable and will not form OI
, but decomposes to form iodate (
IO
3
iodide (I
). Iodine is not likely to form with ammonia, and does not combine with
many organic compounds.
) and
Health Effects
Iodine is an essential trace element, required for the synthesis of the
thyroid hormone. The estimated adult requirement is 80 to 150
g / day. Most intake
is from food and table salt.
18
Concern has been expressed over the possible harmful
effects of iodinated water on the thyroid function; however, field analyses of human
users of water treated with iodine have failed to detect an adverse effect. Iodine is an
irritant, with acute toxicity caused by irritation of the GI tract. A dose of 2 to3gmay
be fatal. No chronic data are available on iodine.
3
The NAS has calculated a Suggested
No Adverse Response Level of 1.19 mg / L, considering a 20 percent intake from water
with a 7-day exposure calculated at 16.5 mg / L.
2
Ozone
Formation and Occurrence
Ozone is one of the most potent oxidants used in water
treatment. It is widely used in Europe (more than 1,000 systems) and is gaining wide-
spread use in the United States (more than 100 systems) for both disinfection and
chemical oxidation applications. A pale blue gas with an acrid odor, ozone is relatively
unstable in air and must be produced on-site.
In water, ozone dissolves at acid pH values (below 7), and is 11.5 times more
soluble than oxygen. It does not react with water and is present as the molecule O
3
.
As the pH is elevated, spontaneous decomposition of ozone occurs through a variety
of steps to produce very reactive free radicals. Because of its instability, an ozone
residual cannot be maintained for extended periods and essentially no ozone is present
when the finished water reaches consumers.
Health Effects
Because of its dissipation, there are no health concerns related to
ozone itself. Instead, health effect issues are associated with ozonated by-products,
such as bromate and formaldehyde.
