Environmental Engineering Reference
In-Depth Information
they lack cell walls but have a plasma membrane that is used to take in food
and discharge waste. They can exist as solitary or independent organisms
(e.g., stalked ciliates such as
Vorticella
), or they can colonize (e.g., the seden-
tary
Carchesium
). Protozoa are microscopic and get their name because they
employ the same type of feeding strategy as animals. Most are harmless,
but some are parasitic. Some forms have two life stages: active trophozoites
(capable of feeding) and dormant cysts. As unicellular eucaryotes, protozoa
cannot be easily defined because they are diverse and in most cases only
distantly related to each other (Patterson and Hedley, 1992). Each protozoan
is a complete organism and contains the facilities for performing the body
functions for which vertebrates have many organ systems.
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Protozoa have been found in almost every kind of soil environment
from peat bogs to arid desert sands. They teem in the deep seas as
well as near the surface of waters and can be found even in frigid
Arctic and Antarctic waters.
Certain types of protozoans can cause disease. Of particular interest to the
drinking water practitioner are
Entamoeba histolytica
(ameobic dysentery and
ameobic hepatitis),
Giardia lamblia
(giardiasis),
Cryptosporidium
(cryptospo-
ridiosis), and the emerging
Cyclospora
(cyclosporosis). Sewage contamination
transports eggs, cysts, and oocysts of parasitic protozoa and helminths (tape-
worms, hookworms, etc.) into raw supplies, leaving water treatment and dis-
infection as the means by which to diminish the risk of contaminated water
for the consumer.
To prevent the occurrence of
Giardia
and
Cryptosporidium
in surface water
supplies and to address increasing problems with waterborne diseases, the
USEPA implemented its Surface Water Treatment Rule (SWTR) in 1989. The
rule requires both filtration and disinfection of all surface water supplies as a
means of primarily controlling
Giardia
and enteric viruses. Since implemen-
tation of the SWTR, the USEPA has also recognized that
Cryptosporidium
is
an agent of waterborne disease (Badenock, 1990). In its next series of surface
water regulations (1996), the USEPA included
Cryptosporidium
.
To test the need for and the effectiveness of the Surface Water Treatment
Rule, LeChevallier et al. (1991) conducted a study on the occurrence and dis-
tribution of
Giardia
and
Cryptosporidium
organisms in raw water supplies
to 66 surface water filter plants. These plants were located in 14 states and
a Canadian province. A combined immunofluorescence test indicated that
cysts and oocysts were widely dispersed in the aquatic environment.
Giardia
was detected in more than 80% of the samples.
Cryptosporidium
was found
in 85% of the sample locations. Taking into account several variables,
Giardia
