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Analysis of Thickening Performance
of Final Settling Tanks
RICHARD I. DICK, Professor
Department of Civil Engineering
University of Illinois
Urbana, Illinois
K. W. YOUNG, Senior Engineer
Linde Division
Union Carbide Corporation
Tonawanda, New York
INTRODUCTION
The importance of proper performance of the final settling tank to the overall
effectiveness of the activated sludge process is well known, and operational problems in
existing plants more often are associated with solids separation than with biological
utilization of the waste material. This is because ineffective capture of the biological solids
increases the suspended BOD in the effluent and ineffective consolidation of the captured
solids prior to recycle to the biological phase of the process results in inefficient use of the
aeration tank (and hence, to potential increase in the soluble BOD of the effluent and to
deterioration of settling properties).
It appears that conventional practice recognizes that performance of the biological
phase of the activated sludge process is within the control of the design engineer and the
treatment plant manager. However, similar control over performance of the solids
separation phase normally is not attempted. Instead, conventional design parameters based
on hydraulic loading, retention time, and/ or solids loading are used. Use of such
parameters is based on the apparent tacit assumptions that all biological sludges have like
settling properties and that the designer is powerless to influence the relative degree to
which sedimentation proceeds.
In those instances when consideration has been given to the actual settling
characteristics of the particular activated sludge involved, the SVI of the sludge has been
used as a basis of design. Specifically, the concentration to which activated sludge solids can
be consolidated in the final settling tank for recycle to the biological portion of the process
commonly has been taken as 106/SVI [for example by McKinney (1) Lawrence and
McCarty (2) and Vosloo (3)]. That is, it has been assumed that the concentration of the
recycled sludge sludge will equal the average concentration in the bottom of a ^graduated
cylinder following 30 min of subsidence. Yet, the SVI is highly dependent on the initial
suspended solids concentration and can be significantly influenced by the geometry of the
laboratory settling vessel (4). Furthermore, use of an estimated return sludge concentration
based on the SVI denies the engineer the opportunity of considering alternative final
settling tank designs to accomplish greater or lesser degrees of consolidation. By exercising
the same control over the design and operation of the solids separation phase of the process
as is conventionally exercised on the biological phase, optimal cost effectiveness can be
achieved (5).
Given that final settling tank design can affect activated sludge process performance by
its influence both on effluent clarity and on recycled sludge concentration, it follows that
both clarification and thickening must be taken into account in the design of the tank. In
this paper, primary attention is given to the thickening function. However, by computing
the surface overflow rate (the fundamental parameter relating to clarification performance ) corresponding to the various thickening design and operating conditions, the
relative load on the clarification capabilities of the tank can be evaluated.
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