The Use of Final Clarifier Models in
Understanding and Anticipating Performance
Under Operational Extremes
S. L. JENNINGS, Associate
Bucher and Willis
Salina, Kansas
C. P. L. GRADY, JR., Associate Professor
School of Civil Engineering
Purdue University
West Lafayette, Indiana
INTRODUCTION
When an engineer embarks upon the design of an activated sludge treatment plant he
must determine four key elements: the volume of the aeration basin, the concentration of
mixed liquor suspended solids (MLSS) in that basin, the surface area of the final settler, and
the rate of the return sludge flow. Usually the decisions that are made concerning these
elements are influenced strongly by guidelines or accepted standards of practice (I). For
example, the volume of the aeration basin may be chosen to give a mean hydraulic residence
time of around six hours. The concentration of mixed liquor suspended solids may next be
chosen by considering a process loading factor which reflects the amenability of the waste to
biological treatment. Then the area of the final settler might be determined through the use
of a surface overflow rate while the return sludge flow rate may be estimated by
considering the anticipated sludge volume index.
Although the design procedure outlined can lead to reasonable decisions it suffers from
several potential weaknesses. First, the primary consideration in aeration basin design is the
product of mean hydraulic residence time and MLSS concentration implying that
considerable freedom is available to the designer in the choice of either of those elements (2).
Second, activated sludge settlers can be limited by thickening capacity rather than by
clarification ability, but overflow rates are based solely upon the latter (3). Third, the sludge
volume index does not necessarily bear any relationship to the return sludge concentration
and therefore could be misleading if used to size the return sludge system (4). Rather, the
rate of return sludge influences markedly the performance of the final settlerand should be
considered in the actual sizing of that operation (3). Last, the procedure ignores the
interactions between the aeration chamber and the settler by assuming that the two can be
designed independently.
The last potential weakness is perhaps the most important because in seeking to
account for aerator - settler interactions an engineer must consider all of the other points
outlined. In addition he will be able to see how cost trade-offs are made and ultimately
arrive at a design that achieves its objective at least cost—the desired goal of all design.
Because a realization of operational interactions is dependent upon a knowledge of the
performance of the individual operations, only recently has it been possible for the design
engineer to attempt to minimize the cost of the aerator - settler combination. Descriptions
of these performance interrelationships and examples of the effects that they have upon
process costs have been presented by several authors in the form of graphs, but with
relatively little discussion of the techniques employed to develop them (5,6,7). The intent of
this paper is to consider those interrelationships in more detail, showing how they govern
the cost of the initial facility and how they must be anticipated if proper process operation is
to be achieved. This will be accomplished through a review of current research and the
synthesis of that research into a general computational procedure. Finally a method will be
presented whereby the design engineer can prepare operational charts which will aid plant
operators in anticipating settler performance under operational extremes.
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