57    SOLIDS SETTLING VARIABILITY IN ACTIVATED SLUDGE
SECONDARY CLARIFIERS: EFFECT ON OPERATION
AND CAPACITY
James W. Morris, Assistant Professor
Laurie A. Batchelder Adams, Research Assistant
Hugh G. Tozer, Research Assistant
Department of Civil and Mechanical Engineering
University of Vermont
Burlington, Vermont 05405
INTRODUCTION
The most widely used wastewater treatment technologies employed in the United States are the
various activated sludge modifications. Over 8,000 facilities were operating as of 1986 with many
more being designed.'.2 The efficient separation of secondary solids is crucial to the operation of
activated sludge technologies. Optimum control of these processes requires that both biological
activity in the aeration unit and the settleability of the resultant biological suspension produced be
considered. Solids recycle control using the sludge volume index (SVI), settled sludge volumes, or
simple mass balance approaches coupled with monitoring the secondary clarifier sludge blanket level
are approximate at best. These techniques do not allow for solids return flow optimization, and can
potentially lead to effluent deterioration. Such operational modes involve responding to changes
which have already occurred or the "dog chasing its tail" syndrome.
The use of the gravity solids-flux state-point concept has received widespread attention as a rational
technique for operational control and analysis of activated sludge systems. Use of this approach
requires knowledge of the mixed liquor's settling characteristics in the secondary clarifier. However,
the settling properties of biological sludges vary with time. The magnitude and frequency of solids
flux variation, and its influence on secondary clarifier capacity in operating systems is unknown. This
study describes solids settling variability, its actual and potential operational influence, at three
operating municipal wastewater treatment facilities.
GRAVITY SOLIDS FLUX STATE-POINT APPROACH
The Gravity Solids-Flux State-Point Approach has been forwarded in the last decade by Keinath
and co-workers3'4'5 based on early work by Coe and Clevenger,* Kynch,7 and Talmadge and Fitch.8
Their groundbreaking work was developed for flocculent compressable biological slurries by Dick
and co-workers,9'10'11'12 Vesiland13'14'15'16 and others.1718 This eloquent technique incorporates a
solids mass balance approach together with the settleability of secondary solids. The State Point
Theory allows the activated sludge solids management system to be modeled mathematically, and
response to operational and design changes to be predicted, when the solid settleability characteristics
are known.4'12 Figure 1 represents the basic components of an Activated Sludge System. Using this
simple configuration definitions may be given for the Gravity Solids-Flux State-Point Approach.
Consider the operating condition plot shown in Figure 2 without the goose-neck shaped gravity flux
curve. Such a plot (two intersecting straight lines, ignore dashed lines for now) defines the possible
mass balance relationships for solids transport through the secondary clarifier. The total solids flux
through a sedimentation unit, G, equals the flow of mixed liquor solids into the basin divided by its
cross-sectional area or:
G = C0(Q + Qr)/A (1)
where    G = total flux, mass/area-time
C0 = slurry concentration, mass/volume
Q = influent flowrate, volume/time
Qr = recycle flowrate, volume/time
A = clarifier cross-sectional area, area
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