Folklore In Design of Final Settling Tanks
RICHARD I. DICK, Professor
Department of Civil Engineering
University of Delaware
Newark, Delaware 19711
INTRODUCTION
Conventional waste treatment practices developed as an art out of necessity. The need
for pollution abatement techniques predated knowledge of basic concepts controlling
performance of the unit operations and processes which could be used for wastewater
treatment. Under this conditions, pioneers in the field of waste treatment did the best they
could. They developed sophisticated waste treatment techniques by trial and error and by
continually introducing appropriate refinements.
To permit designs and operational procedures to be predicated in the successes of the
past, early workers in wastewater management tried to identify parameters which might
reasonably characterize process performance. Parameters were selected which intuitively or
empirically could be expected to influence the behavior of treatment processes. An
additional requirement was that the parameters should be readily measurable. By making
the magnitude of these parameters the same as used in successfully performing treatment
processes, it was intended that reasonable process design could be accomplished in the
absence of basic understanding of factors controlling process performance.
Examples of such parameters developed on an empirical or intuitive basis are
widespread in the wastewater treatment field. For example, the organic loading intensity
per unit volume of an aeration tank in the activated sludge process is a fundamentally
unsound, but historically useful parameter. Similarly, the hydraulic loading per unit area of
trickling filters and the volatile suspended solids loading per unit volume of anaerobic
digester capacity are related only circuitously to process performance. Many other
examples of such parameters are included in common waste treatment plant design criteria.
These empirical parameters have served for many decades as bases for design and as
guidelines for operational control of waste treatment facilities. They stand as tributes to
pioneers who got a job done. However, because the parameters don't reflect fundamental
relationships between process loading and process performance, their use can lead to a lack
of cost effectiveness in design and to a lack of precise operational control.
Much of the research in water pollution control in past decades has been directed
toward acquiring fundamental understanding of processes developed previously out of
need. Such improved understanding has enabled new designs to be more cost effective, has
allowed increases in the capacity of existing treatment facilities at nominal cost, and has
permitted improved control of the performance of waste treatment facilities.
At this point, when basic understanding of process performance becomes available, the
"rules-of-thumb" developed over the years as valuable aids to design and operation become
sort of "technical old wives tales." A significant body of such "tales" or "folklore" has
accumulated in the annals of water pollution control technology.
Regrettably, there is a tendency to become comfortable with the familiar parameters of
the past. There is a reluctance on the part of professors, design engineers, equipment
manufacturers, regulatory agencies, and operating personnel to abandon comfortable and
well-tried parameters for less familiar, less well-tried (but potentially more useful) rational
approaches to analysis of performance of waste treatment facilities.
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