Section 17. PHYSICAL/BIOLOGICAL SYSTEMS
COMPARISON OF ACTIVATED SLUDGE RESPONSE
TO QUANTITATIVE, HYDRAULIC AND COMBINED SHOCK
FOR THE SAME INCREASES IN MASS LOADING
T. S. Manickan, Assistant Professor
A. F. Gaudy, Jr., Professor
Department of Civil Engineering
University of Delaware
Newark, Delaware 19711
Since the introduction of biological treatment, and in particular the activated sludge process, interest in the stability of its efficiency of performance under changing environmental
conditions, i.e., shock loadings, has been nearly as much interest to engineers involved in
design and operation as has prediction of performance under average "design" conditions.
Similarly, since the introduction over 30 years ago of general kinetic theory for continuous
culture of microorganisms under steady-state conditions, there has been an equal interest in
kinetic description of response to various kinds and modes of introducing changes in environmental conditions (shock loads). Yet, while much has become known about the response
to shock loadings, today only rough guidelines are available regarding biological mechanisms
of response. Also, while rather sophisticated mathematics have been applied to various
model formulations of the growth process, little in the way of useful predictive kinetic
formulations for the transient state resulting from a shock load has evolved. Such statements
do not constitute a negative critique of the efforts of a large number of researchers. Rather,
they attest to the complexity of the subject, particularly as it relates to slow-growing, high-
recycle biomass systems of heterogeneous populations.
Response to shock loadings has been an investigative interest of the authors for many
years. One general conclusion, reached after review of our own work and that of others, is
that after all the effort that has been expended, we can say only that our field has made a
reasonably good beginning toward understanding and kinetic description of biological
response to changing environmental conditions, and that there is need for expanding participation and a more sustained effort by all researchers interested in this field if scientific
insight is to be gained and converted into sound engineering practice. Unfortunately, there
would appear to be a paucity of time, and practical ways in which to simplify the methods
of handling and controlling responses to shock loadings need to be sought. For example, it
would be expedient to be able to consider the response to a change in feed concentration
as having the same effect as a hydraulic shock loading, so long as both shocks resulted in the
same mass rate of increase in organic loading. It is well known that reponse to hydraulic and
quantitative shocks can produce different effects on effluent turbidity. Depending on operational conditions, clarifier size and geometry, and possibly other factors, one usually finds
that hydraulic shocks will lead to greater turbidity in the clarifier effluent than do quantitative shocks. However, making such a generalization may be an oversimplification, since work
of some investigators tends to indicate that increased turbidity is a result of an increase in
daily organic loading rather than of hydraulic shock [ 1 ]. With respect to soluble organic
material, it may not seem unreasonable to expect that for equal increases in mass rate of
organic feed, quantitative and hydraulic shocks or a combination thereof may lead to equal
leakage of substrate, i.e., organic carbon, in the effluent. The equivalence of the effects of
equal increases in organic loading rate via either quantitative or hydraulic shock is implicit
in the work by Adams and Eckenfelder [1] and McLellan and Busch [21. This equivalence
was examined by Grady [3] analytically (analog computer) using Garrett's operational
model [4] with mass balance equations enveloping both the reactor and the clarifier, and no
difference was found computationally in the response to like increases in mass rate or organic
feeding, whether the increase was applied as a quantitative or a hydraulic shock.
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