Environmental Engineering Reference
In-Depth Information
Note:
The free-swimming ciliated protozoa have hair-like projections,
cilia
, that cover all or part of the cell. The cilia are used for motion and
create currents that carry food to the organism. The free-swimming cili-
ates are sometimes divided into two subcategories:
free
swimmers
and
crawlers
. The free swimmers are usually seen moving through the fluid
portion of the activated sludge, while the crawlers appear to be walking
or grazing on the activated sludge solids. The free-swimming ciliated
protozoa usually predominate when large numbers of dispersed bacteria
are present that can be used as food. Their predominance indicates a
process nearing optimum conditions and effluent quality.
The process continues with floc particles beginning to stabilize,
taking on irregular shapes, and beginning to show filamentous growth.
At this stage, the crawling ciliates dominate. Eventually, mature floc
particles develop and increase in size, and large numbers of crawling
and stalked ciliates are present. When this occurs, the succession pro-
cess has reached its terminal point. The succession of protozoan and
mature floc particle development just described details the occurrence
of phases of development in a step-by-step progression. Protozoan suc-
cession is also based on other factors, including dissolved oxygen and
food availability.
Probably the best way to understand protozoan succession based
on dissolved oxygen and food availability is to view the aeration basin of
a wastewater treatment plant as a “stream within a container.” Using the
saprobity system
to classify the various phases of the activated sludge
process in relation to the self-purification process that takes place in
a stream, we can see a clear relationship between the two processes
based on available dissolved oxygen and food supply. Any change in the
relative numbers of bacteria in the activated sludge process causes a
corresponding change in the population of microorganisms. Decreases
in bacteria increase competition between protozoa and result in seces-
sion of dominant groups of protozoa.
The success or failure of protozoa to capture bacteria depends on
several factors. Those with more advanced locomotion capability are
able to capture more bacteria. Individual protozoan feeding mechanisms
are also important in the competition for bacteria. At the beginning of
the activated sludge process, amoebae and flagellates are the first pro-
tozoan groups to appear in large numbers. They can survive on smaller
quantities of bacteria because their energy requirements are lower than
other protozoan types. Because few bacteria are present, competition
for dissolved substrates is low; however, as the bacteria population
increases, these protozoa are not able to compete for available food.
This is when the next group of protozoa (the free-swimming protozoa)
enters the scene.
The free-swimming protozoa take advantage of the large popula-
tions of bacteria because they are better equipped with food-gathering
mechanisms than the amoebae and flagellates. The free swimmers
are important for their insatiable appetites for bacteria and also in
