Environmental Engineering Reference
In-Depth Information
that they are killed instantaneously by boiling water (100°C) (Bingham, 1979;
Frost et al., 1984). We do not know how long the cysts will remain viable at
other water temperatures (e.g., at 0°C or in a canteen at 15 to 20°C), nor do we
know how long the parasite will survive on various environment surfaces,
such as under a pine tree, in the sun, on a diaper-changing table, or in car-
pets in a daycare center.
The effect of chemical disinfection (chlorine, for example) on the viability
of
Giardia
cysts is an even more complex issue. The number of waterborne
outbreaks of
Giardia
that have occurred in communities where chlorine was
employed as a disinfectant demonstrates that the amount of chlorine used
routinely for municipal water treatment is not effective against
Giardia
cysts.
These observations have been confirmed in the laboratory under experimen-
tal conditions (Jarroll et al., 1980a,b). This does not mean that chlorine does
not work at all. It does work under certain favorable conditions. Without get-
ting too technical, gaining some appreciation of the problem can be achieved
by understanding a few of the variables that influence the efficacy of chlo-
rine as a disinfectant:
1.
Water pH
—At pH values above 7.5, the disinfectant capability of
chlorine is greatly reduced.
2.
Water temperature
—The warmer the water, the higher the efficacy.
Chlorine does not work in ice-cold water from mountain streams.
3.
Organic content of the water
—Mud, decayed vegetation, or other sus-
pended organic debris in water chemically combines with chlorine,
making it unavailable as a disinfectant.
4.
Chlorine contact time
—The longer that
Giardia
cysts are exposed to
chlorine, the more likely it is that the chemical will kill them.
5.
Chlorine concentration
—The higher the chlorine concentration, the
more likely it is that chlorine will kill
Giardia
cysts. Most water treat-
ment facilities try to add enough chlorine to give a free (unbound)
chlorine residual at the customer tap of 0.5 mg/liter of water.
The five variables above are so closely interrelated that an unfavorable
occurrence in one can often be compensated for by improving another; for
example, if chlorine efficacy is expected to be low because water is obtained
from an icy stream, the chlorine contact time or the chlorine concentration,
or both, could be increased. In the case of
Giardia
-contaminated water, pro-
ducing safe drinking water with a chlorine concentration of 1 mg per liter
and a contact time as short as 10 minutes might be possible if all the other
variables were optimal (pH of 7.0, water temperature of 25°C, and a total
organic content of the water close to 0). On the other hand, if all of these vari-
ables were unfavorable (pH of 7.9, water temperature of 5°C, and high organic
content), chlorine concentrations in excess of 8 mg/liter with several hours of
contact time may not be consistently effective. Because water conditions and
