39    THE INFLUENCE OF OPERATIONAL CONDITIONS ON THE
VARIABILITY OF ACTIVATED SLUDGE SETTLING
CHARACTERISTICS AT FUNCTIONING TREATMENT
PLANTS
James W. Morris, Assistant Professor
Dept. of Civil and Mechanical Engineering
University of Vermont
Burlington, Vermont 05405
Hugh G. Tozer, Engineer
Woodard and Curran, Inc.
Portland, Maine 04102
Laurie A. Batchelder Adams, Project Engineer
The Johnson Company
Montpelier, Vermont 05602
INTRODUCTION
The settleability of activated sludge solids are a central consideration in treatment system clarifier
design and operation.1"4 Empirical settling tests, such as the sludge volume index (SVI), have long
formed the basis of judgment in operational management;5 however, research has indicated that this
empirically-based approach can lead to system failure.1'6 An alternative management method known
as the mass-flux state-point approach has been developed by numerous researchers. It combines a
system mass balance with the settling characteristics of the sludge to create a rational design and
management tool.
Keinath and co-workers2-6 and McHarg7 have been instrumental in developing the mass-flux state-
point approach. This technique permits an activated sludge management system to be mathematically
modeled. Design alternatives or operational responses may then be rationally evaluated.
The operating state of a generalized activated sludge system shown schematically in Figure 1 may be
graphically portrayed as in Figure 2. For details of this model's development and component descriptions the reader may refer to several sources.2'8>9 Total flux through a clarifier (G, mass/area time)
has two components; flux due to gravity (Gg) and underflow flux (Gu) induced by solids recycle (QR,
vol./time). When the response of separation velocity to slurry concentration is known, the empirical
expression delineated by Vesilind and supported by Smollen10'11 may be used to model settling
velocity:
V = V0e-^ (1)
where c = concentration of slurry, mass/vol
V0 = settling velocity at c = 0, length/time
k = experimentally derived settling constant, vol/mass.
Figure 1.   Conceptual definition schematic of
an activated sludge treatment system.
43rd Purdue Industrial Waste Conference Proceedings, © 1989 Lewis Publishers, Inc., Chelsea, Michigan 48118.
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