Environmental Engineering Reference
In-Depth Information
The so-called microstrainer types of filters are true filters. Manufacturer
data accompanying these filters indicate that some have a sufficiently small
pore size to physically restrict the passage of some microorganisms through
the filter. The types of filter media employed in microstraining filters include
acrylic, ceramic, and proprietary materials. Theoretically, a filter having an
absolute pore size of less than 6 µm might be able to prevent
Giardia
cysts
8 to 10 µm in diameter from passing through. When used as a water sam-
pling device during community outbreaks, portable filters in the 1- to 3-µm
range more effectively removed
Giardia
cysts from raw water than filters
with larger pore sizes. For effective removal of bacterial or viral organisms
that cause disease in humans, microstraining filters with pore sizes of less
than 1 µm are advisable; however, the smaller the pores, the more quickly
the filters will tend to clog. To obtain maximum filter life, and as a matter of
reasonable precaution, the cleanest available water source should always be
used. Keep in mind, however, that even sparkling, clear mountain streams
can be heavily contaminated.
Another thing to consider when choosing a filter includes whether the fil-
ter element can be cleaned or replaced without posing a significant health
hazard to the user, because infectious organisms can be concentrated on the
filter element or media. Properly engineered portable filters should mini-
mize the possibility of contaminating the clean water side of the filter with
contaminated water during replacement or cleaning of the filter element.
Because filters used in the field are often rinsed in a stream or river that
may be contaminated, this is an especially important consideration for rec-
reational outdoor use.
Cryptosporidium
In 1907, when Ernest E. Tyzzer recognized, described, and published an
account of a parasite he frequently found in the gastric glands of laboratory
mice, he and his new discovery were overlooked—just another scientist going
quietly about his normal, tedious, out-of-the-limelight research, buried in
obscurity. Initially, his studies focused on describing the asexual and sexual
stages and spores (oocysts), each with a specialized attachment organelle, and
he noted that the spores were excreted in the feces (Tyzzer, 1907).
Tyzzer identified the parasite as a sporozoan, but of uncertain taxonomic
status, and he named it
Cryptosporidium muris
. Later, in 1910, after more
detailed study, he proposed
Cryptosporidium
as a new genus and
C. muris
as the type species. Amazingly, except for developmental stages, Tyzzer's
original description of the life cycle (see
Figure 6.5
) was later confirmed by
electron microscopy. Later, in 1912, Tyzzer described another new species,
Cryptosporidium parvum
.
For almost 50 years, Tyzzer's discovery of the genus
Cryptosporidium
remained (like himself) relatively obscure, because it appeared to be of no
medical or economic importance. Slight rumblings of the importance of the
