A Theoretical Study of Activated Sludge
Transient Response
C. P. L. GRADY, JR., Assistant Professor
School of Civil Engineering
Purdue University
Lafayette, Indiana
In order to improve environmental quality it is anticipated that over ten billion
dollars will be spent by federal, state and local governments to provide secondary
wastewater treatment for municipalities in the United States and to up-date existing
plants so that they can handle the demands of population growth (1). If changes in
the layout and design of wastewater treatment plants can reduce their construction
costs without sacrificing water quality, then those changes should be adopted. This
paper will investigate the feasibility of one possible change: the reduction in size of
aeration basins.
One recent economic analysis of activated sludge treatment plants revealed that
approximately 13 percent of the construction cost is due to the aeration basins (2).
If the usual design retention time of those basins could be reduced by half with no
deterioration in effluent quality, the cost of construction of the basins would be
reduced by approximately 40 percent, resulting in a net savings of about five percent
in the total construction cost of new plants (2). In addition, if the activated sludge
process could perform just as well with shorter retention times, then it should also be
possible to expand the capacity of existing plants without having to increase aeration
basin capacity, thus saving money in plant up-dating as well.
From the standpoint of the biochemical process, the feasibility of a reduction
in aeration basin volume depends upon several factors: among them, steady-state
kinetics, oxygen transfer, and response to shock loads. Research over the last twenty
years has led to the conclusion that micro-organism specific growth rate, and not
hydraulic retention time, is the key factor in determining the steady-state
performance of activated sludge (3). Though some workers use the terms "loading"
or "biological solids retention time" or "sludge age" instead of growth rate, it is
generally agreed that hydraulic retention time is a secondary factor in determining
steady-state effluent quality (4). Therefore, as far as steady-state performance is
concerned, it should now be possible to reduce hydraulic retention times in aeration
basins because growth rate can be controlled through cell recycle in a manner
independent of the retention time.
The second factor which must be assessed in determining the feasibility of
reduced aeration basin volume is oxygen transfer. Since the same quantity of organic
matter would have to be removed daily no matter what the aeration volume, the
same total quantity of oxygen would have to be transferred. However, since the total
volume of liquid in a small basin would be less, the oxygen transfer equipment would
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