Environmental Engineering Reference
In-Depth Information
water treatment plant operations (especially those related to water retention
time and, therefore, to chlorine contact time) vary considerably in different
parts of the United States, neither the USEPA nor the CDC has been able
to identify a chlorine concentration that would be safe yet effective against
Giardia
cysts under all water conditions. For this reason, the use of chlorine
as a preventive measure against waterborne giardiasis generally has been
utilized under outbreak conditions when the amount of chlorine and con-
tact time have been tailored to fit specific water conditions and the existing
operational design of the water utility.
In an outbreak, for example, the local health department and water utility
may issue an advisory to boil water, may increase the chlorine residual at
the consumer's tap from 0.5 mg/L to 1 or 2 mg/L, and, if the physical lay-
out and operation of the water treatment facility permit, increase the chlo-
rine contact time. These are emergency procedures intended to reduce the
risk of transmission until a filtration device can be installed or repaired or
until an alternative source of safe water (a well, for example) can be made
operational.
The long-term solution to the problem of municipal waterborne outbreaks
of giardiasis involves improvements in and more widespread use of filters
in the municipal water treatment process. The sand filters most commonly
used in municipal water treatment today cost millions of dollars to install,
which makes them unattractive for many small communities. The pore sizes
in these filters are not sufficiently small to remove
Giardia
(6 to 9 µm by 8 to
12 µm). For the sand filter to remove
Giardia
cysts from the water effectively,
the water must receive some additional treatment before it reaches the filter.
The flow of water through the filter bed must also be carefully regulated.
An ideal prefilter treatment for muddy water would include sedimentation
(a holding pond where large suspended particles are allowed to settle out by
the action of gravity; see
Chapter 11
) followed by flocculation or coagulation
(the addition of chemicals such as alum or ammonium to cause microscopic
particles to clump together). The large particles resulting from the floccula-
tion-coagulation process, including
Giardia
cysts bound to other micropar-
ticulates, are easily removed by the sand filter. Chlorine is then added to kill
the bacteria and viruses that may escape the filtration process. If the water
comes from a relatively clear source, chorine may be added to the water
before it reaches the filter.
As should be clear to you, successful operation of a complete water
treatment works is a complex process that requires considerable train-
ing. Troubleshooting breakdowns or recognizing the potential problems
in the system before they occur often requires the skills of an engineer.
Unfortunately, most small water utilities with water treatment facilities that
include filtration cannot afford the services of a full-time engineer. Filter
operation or maintenance problems in such systems may not be detected
until a
Giardia
outbreak is recognized in the community. The bottom line is
that, although filtration is the best protection against waterborne giardiasis
