Thickening of Compressible Sludges
JON A. COLE, Associate Professor
Wala Wala College
College Place, Washington 99324
LAWRENCE B. POLKOWSKI, Professor
JOHN A. HOOPES, Associate Professor
Civil and Environmental Engineering
University of Wisconsin
Madison, Wisconsin 53706
INTRODUCTION
To determine the cost of wastewater treatment which is attributable to solids handling
would require a definitive description of waste characteristics and the treatment process
employed. General capital and operation costs place the proportion of waste treatment
funds expended in the handling of solids in the range of one-third to one-half of total costs
(1.2. 3.4, 5).
Units used in production and processing of solids are many, including a variety of
modifications of basic forms. Solids concentration increase is a major objective in some
operations, such as flotation, gravity thickening, centrifugation, and drying beds. While
legal requirements dictate that the primary objective of final settling tanks is clarification,
considerable design effort and operational attention is given toward concentration of the
settled material, expecially when return sludge consistency affects other unit operations.
Disposal efficiency for excess solids is also dependent upon underflow concentration, often
leading to use of a separate thickening unit. Sludge concentration is likewise a necessary
aspect of second stage and single stage digestion.
Concentration of solids involves drawing particles together in closer proximity as well
as release of water from the suspension. Gravity thickening, which contains both of these
aspects as well as being related to the aforementioned unit operations, was chosen as a most
likely component for the study of compressible sludges. In as much as design techniques
employed for gravity thickeners are largely a result of theory developed for incompressible
slurries, it is not surprising that these units are commonly over designed (6, 7, 8) or at the
least poorly understood with respect to wastewater applications.
THICKENER DESIGN FOR NONCOMPRESSIBLE SLUDGES
Rational procedures for the design of continuous sludge thickening facilities were not
developed with highly theoretical considerations but rather employed the batch laboratory
procedure of observing the time dependent subsidence of the interface which forms between
a concentrating material and the liquid being released. This procedure, which is still widely
preferred more than fifty years after its inception, was used by Coe and Clevenger (9) to
calculate the required cross-sectional area for a continuous thickener. Although their
design formula was used for investigation of concentrations ranging from the feed to the
underflow, the authors understood that other factors affected the settling rate giving
different shapes to batch interface subsidence curves under different conditions. It was
suggested that the conditions found in practice should be duplicated or that multiple tests be
conducted over the range of conditions expected.
Kynch (10) made a major contribution to thickening theory using continuity
relationships to describe batch interface subsidence velocities. Assuming that the settling
velocity v, is a function only of local particle concentration, c, he showed that the layers of
uniform concentration, which are propagated upward through the slurry, travel at constant
velocities, U, (traces in Figure I a).
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