Population Dynamics of Protozoa in Wastewater,
and Impact of Environmental Changes
MAN M. VARMA, Professor and Director
HAROLD E. FINLEY, Professor of Zoology
GLEN H. BENNETT, Graduate Student
Bio-Environmental Engineering and Sciences
Howard University
Washington, D.C.
INTRODUCTION
Diverse environmental factors, like temperature, availability of food, presence of
water, pH, and predators, influence the presence and growth of protozoa (1,2,3). Different
species of protozoa can live under varying conditions.
Protozoa play an important role in the efficient functioning of sewage treatment
plants. Without the work of certain protozoa, especially some of the ciliated protozoa, the
final effluent of the plant would not be fit to discharge. Much is already known about the
contributions of protozoa to the effectiveness of sewage purification (4,5,6,7,8). Vorticella
and Opercularia are responsible in a great degree for the success of the treatment process
(9). Much still remains to be learned about the ecology of influents and effluents of sewage
treatment plants, especially in regard to protozoa and the framework within which they
help in the treatment of wastewater. Moreover, because each treatment plant seems to have
characteristic features which set it apart from other plants, information from many
different plants is desirable to formulate a generalized principle of the importance of
protozoa in predicting the efficiency and degree of treatment of wastewater.
This study had a multipurpose approach. The objective of the first phase of the
research was to recognize the dominant protozoa in the various segments of the treatment
processes at the Water Pollution Control Plant of the District of Columbia. The second
phase concerned behavior of the dominant protozoa in the secondary effluent, discovered
and recognized in the preceding phase, when the variables governing the growth factors
were not controlled. Behavior of protozoa in relation to pH was especially watched.
Information resulting from this phase of the research may be useful at the plant because p H
is variable in water.
In the last part of the study, referred to as Phase 3, the behavior of the dominant
protozoa was investigated when some of the environmental factors such as pH and
temperature were controlled.
WORK OF OTHER INVESTIGATORS
Curds and Cockburn (10) examined percolating filters and activated sludge for the
presence of protozoa, and also studied the properties of activated sludge in relation to
numbers and kinds of existing protozoa. Their findings showed that the presence of
protozoa in at least these two treatment processes is significant.
Pillai and Subrahmanyan (11) thought that in aerobic purification of sewage,
protozoan activity is of primary importance and bacterial activity of secondary importance.
Heukelekian and Gurbaxani (12) thought that protozoa certainly aid in the work of
clarification but they are secondary in importance to bacteria.
Wagner (13) added inhibitors to a small model sewage purification tank, which
destroyed the protozoa but allowed the bacteria to grow. The latter increased in numbers,
but purification of the sewage was adversely affected. Upon renewal of the protozoan fauna
the bacteria were reduced in numbers and the purification was restored to its original high
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