Re: purifying water...page 6...IF YOU SKIP THE REST ,READ THIS!... -- Food Storage & Self-Reliance Forum

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Date Posted: 16:52:48 10/17/99 Sun
Author: clover
Subject: Re: purifying water...page 6...IF YOU SKIP THE REST ,READ THIS!...
In reply to: clover 's message, "purifying water" on 18:14:46 10/15/99 Fri

6
Removal from Municipal Water Supplies

During the past 10 years, scientific knowledge about what is required to kill or remove Giardia cysts from a contaminated water supply has increased considerably. For example, it is known that cysts can survive in cold water (4 deg C) for at least 2 month s and that they are killed
instantaneously by boiling water (100 deg C) (23,24). It is not known how long the cysts will remain viable at other water temperatures (e.g., at 0 deg C or in a canteen at 15-20 deg C), nor is it known how long the parasite will survive on various environment surfaces, e.g., under a pine
tree, in the sun, on a diaper-changing table, or in carpets in a day-care center.

The effect of chemical disinfection, such as chlorine, on the viability of Giardia cysts is an even more complex issue. It is clear from the number of waterborne outbreaks of Giardia that have occurred in communities where chlorine was employed as a disinfectant 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 experimental conditions (25,26,27). This does not mean, however, that chlorine does not work at all. It does work under certain favorable conditions. Without getting too technical, one can gain some appreciation of the problem by understanding a few of the variables that influence the efficacy of chlorine 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. Thus, chlorine does not work well in ice-cold
water from mountain streams. 3) Organic content of the water: mud, decayed vegetation, or other suspended organic debris in water chemically combines with chlorine making it unavailable as a disinfectant. 4) Chlorine contact time: the longer 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 chlorine will kill Giardia cysts. Most water treatment facilities try to add enough chlorine to give a free ( unbound) chlorine residual at the
customer tap of 0.5 mg per 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, either the chlorine contact time or chlorine
concentration, or both could be increased. In the case of
Giardia-contaminated water, it might be possible to produce safe drinking
water with a chlorine concentration of 1 mg per liter and a contact time
as short as 10 minutes if all the other variables were optimal (i.e., pH
of 7.0, water temperature of 25 deg C, and a total organic content of the
water close to zero). On the other hand, if all of these variables were
unfavorable (i.e., pH of 7.9, water temperature of 5
deg C, and high organic content), chlorine concentrations in excess of 8
mg per liter with several hours of contact time may not be consistently
effective. Because water conditions and 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 U.S. Environmental Protection Agency nor the CDC has
been able to identify a chlorine concentration that would be safe yet
effective against Giardia cysts under all water conditions. Therefore,
the use of chlorine as a preventive measure against waterborne giardiasis
generally has been used under outbreak conditions when the amount of
chlorine and contact 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 per liter to 1 or 2 mg per liter, and, if
the physical layout and operation of the water treatment facility permit,
increase the chlorine 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, such as a well, can be made operational.

The long-term solution to the problem of municipal waterborne outbreaks of
giardiasis will involve 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.
Moreover, the pore sizes in these filters are not sufficiently small to
remove a Giardia (6 to 9 micrometers x 8 to 12 micrometers). For the sand
filter
to remove Giardia cysts from the water effectively, the water must
receive some additional treatment before it reaches the filter. In
addition, the flow of water through the filter bed must be carefully
regulated.

An ideal prefilter treatment for muddy water would include sedimentation
(a holding pond where the large suspended particles are allowed to settle
out by the action of gravity) 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
flocculation/coagulation process, including Giardia cysts bound to other
microparticulates, 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, chlorine may
be added to the water before it reaches the filter. The point here is that
successful operation of a complete water
treatment facility is a complex process that requires considerable
training. Troubleshooting breakdowns or recognizing potential problems in
the system before they occur often requires the skills of an engineer.
Unfortunately, most small water utilities that have a water treatment
facility that includes 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, in reference to municipal
systems, water filtration is the best that water treatment technology has
to offer against waterborne giardiasis, it is not infallible. For
municipal water filtration facilities to work properly, they must be
properly constructed, operated, and maintained.

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