Pilot Scale Evaluation of Clarifier Performance
ROY O. BALL, Waste Consultant
RICHARD F. SMULLEN, JR., Waste Consultant
Engineering Service Division
Engineering Department
E.I. du Pont de Nemours & Company
Wilmington, Delaware 19898
INTRODUCTION
Separation of activated sludge into a concentrated stream of biological solids and
clarified effluent is an essential part of activated sludge biological treatment systems. Rules
of thumb have been developed for "secondary clarifiers," initially based upon theoretical
experiments with primary clarifiers and other discrete particle settling devices.
Typical rules of thumb included recommendations for overflow rate (gpd/ft2-day),
solids flux (lb MLSS/day-ft2) and effluent weir loading (gpd/ft). With the use of these and
other design recommendations, secondary clarifiers were sized and, in general, operated
satisfactorily. However, recurrent upset was a common feature of systems designed in this
manner.
With the passage of the Federal Water Pollution Control Act Amendments of 1972, it
became clear that time honored methods of clarifier design, with their attendant chronic
upsets, would not be satisfactory in many cases. The levels of performance specified in the
Act preclude discharge of significant levels of solids for even brief periods of time.
Furthermore, the Act requires a similar high level of control for the discharge of organics,
such as BOD5, from a treatment facility. Recent studies have shown the relation between
clarifier thickening (or MLSS recycle) and soluble organic removals (1, 2). From these
studies, it is clear that clarifiers must continually and efficiently thicken and clarify if
standards are to be met.
Although advances have been made in the theoretical understanding of the operations
of clarification and thickening, implementation of these theories as equipment design has
not occurred. Also, evaluation of full scale clarifier performance is difficult due to
variations in equipment geometry and design. Therefore, the usual source for settleability
data has been laboratory batch settling studies in one-liter graduated cylinders of 3 to 8-inch
diameter columns. The sludge is either generated in laboratory reactors or is obtained as
batch sludge samples from a full-scale system.
There are, however, shortcomings in these methods. If the objective of the study is to
determine the operational limits of existing equipment and/or to predict the necessity for
additional equipment for expansions, laboratory studies may only provide a conservative
answer, due to the inherent precision of the method. When the precision of present
laboratory methods is examined in relation to cost of equipment, it is clear that an
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