Geoscience Reference
In-Depth Information
Caution:
Not all filters perform equally well. See the “Portable Water Filters” section fol-
lowing.
Protozoa
Protozoa, such as
Giardia
and
Cryptosporidium
, are single-celled animals. Unlike bacteria,
they are able to move themselves around, and don't just sit there waiting for something external
to push them around. They are microscopic but relatively large (3 to 10 microns), which makes
them considerably easier to filter out of your drinking water. However, they have the capacity
to transform themselves into a cyst, which is a form that is very tough to kill chemically with
traditional iodine and chlorine water treatments. When these little animals get into a harsh en-
vironment, such as one that is too cold or has no food (like clean water), they change into cysts.
A cyst cannot move itself around or feed itself, but it can passively survive in harsh environ-
mental conditions that would kill most bacteria. The cysts remain dormant waiting for a posit-
ive growth environment, such as the intestinal tract of an animal or polluted water. In a positive
growth environment, protozoa change back to the active form and begin to eat and multiply.
Protozoan infections usually take considerably longer to show symptoms—from a week to sev-
eral months. Boiling and ultraviolet sterilization will kill protozoa and their cysts. Protozoan in-
fections can be extremely difficult to treat once they have become entrenched in the body.
Viruses
Viruses are different organisms altogether. They are much smaller than bacteria, on the order of
0.004 to 0.06 microns (Wilkerson 1992, 72) and are much harder to filter out. Viral contamina-
tion of drinking water is not as problematic as bacterial or protozoan contamination, but water-
borne outbreaks are not uncommon. Viruses are so small that they may be visible only through
the use of an electron microscope. Viruses multiply by invading the cells of a host organism
and “stealing” some of the genetic material of the host cell to reproduce the virus. This process
usually destroys the host cells and reproduces the virus. Some common harmful waterborne
viruses are hepatitis A, polio, and Norwalk virus. Over the past few years, the Norwalk virus
has been in the news quite a bit for showing up in the water systems of a number of cruise ships
and seriously sickening several thousand passengers (Sardone 2007). Proper iodine and chlor-
ine chemical treatments, as well as boiling, ozone, and UV sterilization, will kill viruses. Wa-
terborne viruses are usually spread by human feces. Unlike bacteria, viruses generally do not
cross from animals to humans, so the chance of getting viral infections in pristine remote loca-
tions is rather small, unless there has been a viral outbreak in the local human population.
