Environmental Engineering Reference
In-Depth Information
TAble 6.6
Chlorine (Bleach)
Liquid chlorine bleach used for laundry usually has 4 to 6%
available chlorine. Read the label to find the percentage of chlorine
in the solution and follow the treatment schedule below. Mix
thoroughly by stirring or shaking water in container for 30 minutes.
Let stand for several hours or overnight. A slight chlorine odor
should be detectable in the water.
Number of Drops (0.05 mL) per Quart/Liter
Available Chlorine
Clear Water
Cold or Cloudy Water
1%
10
20
4-6%
2
4
7-10%
1
2
Unknown
10
20
giardiasis for international travelers, backpackers, campers, sportsmen, or
persons who live or work in areas where water is known to be contaminated.
Unfortunately, very few published reports in the scientific literature detail
both the methods used and the results of tests employed to evaluate the effi-
cacy of these filters against
Giardia
. Until more published experimental data
become available, consumers should look for a few common-sense indica-
tions when selecting a portable or household filter.
The first thing to consider is the filer media. Filters relying solely on ordi-
nary or silver-impregnated carbon or charcoal should be avoided, because
they are not intended to prevent, destroy, or repel microorganisms. Their
principal use is to remove undesirable chemicals, odors, and very large
particles such as rust or dirt. Some filters rely on chemicals such as iodide-
impregnated resins to kill
Giardia
. Although properly designed and man-
ufactured iodide-impregnated resin filters have been shown to kill many
species of bacteria and viruses present in human feces, their efficacy against
Giardia
cysts is less well established. The principle under which these fil-
ters operate is similar to that achieved by adding the chemical disinfectant
iodine to water, except that the microorganisms in the water pass over the
iodide-impregnated disinfectant as the water flows through the filter.
The disinfectant activity of iodide is not as readily affected as chlorine
by water pH or organic content, but iodide disinfectant activity is markedly
reduced by cold water temperatures. Experiments on
Giardia
indicate that
many of the cysts in cold water (4°C) remain viable after passage through
filters containing triiodide or pentaiodide disinfectant (Marchin et al., 1983).
Longer contact time (compared to those required to kill bacteria) is required
when using chemical filters to process cold water for
Giardia
protection.
Currently available chemical filters are also not recommended for muddy or
very turbid water. Note that filters relying solely on chemical action usually
give no indication to the user when disinfectant activity has been depleted.
