Equalization of Liquid Wastes
IVAN METZGER, Associate Professor
Civil Engineering Department
Newark College of Engineering
Newark, New Jersey
INTRODUCTION
The incoming waste to industrial and municipal treatment plants is often characterized by wide variations in composition or by the sudden and intermittent appearance of certain compounds in high concentration. Wide variations in the concentration of those compounds subject to treatment may reduce the effectiveness of
automatic control (1) and present difficulties in the selection of chemical feeders
and related equipment (2).
The sudden appearance of high concentrations of toxic materials, or shock
loads, can have a harmful effect on aerobic treatment processes (3,4), sludge digestion (5) and primary treatment (6). While biological treatment processes have
a marked ability to adjust to toxic wastes when the waste concentration is raised
gradually over a period of time, certain shock loads may be fatal to organisms.
The harmful effects of a shock load of toxic material may be much greater than it
would be if the same amount of toxic material were discharged over a period of
time. In general, it is desirable that the concentration of pollutional materials in
raw wastes be nearly constant at all times.
The problem resulting from shock loads can be alleviated by attempting to reduce the extremes in concentration and produce a more uniform, or equalized,
raw waste. In some cases sewers can provide a degree of equalization (7); however, this is not the case in relatively short and simple runs (8). The more usual
approach to the problem has been to provide some type of detention ahead of
treatment to even the fluctuations in concentration. The process is known as
equalization, with the terms "averaging" and compositing used in the same
sense. In batch treatment the wastes and shock loads are accumulated in a tank,
the contents mixed, and chemicals added for treatment. More than one tank is
usually provided so that while one is in the treatment stage the other may be used
for equalizing. This type of treatment is satisfactory for small volumes of concentrated wastes; but, continuous treatment is more desirable for large volumes.
In continuous treatment a properly designed tank should equalize the raw wastes
received over a given time interval and permit the plant to operate on a more uniform waste. While in batch operation equalization is straight forward, continuous-flow treatment appears to present more difficulty.
Continuous-flow treatment plants have been provided with either lagoons,
settling basins, mixing tanks or aeration units in an attempt to provide equalization. These units resemble either sedimentation or mixing tanks which are known
to have, in principle, quite different flow patterns. Since two general types of
tanks with different flow characteristics are used in an attempt to accomplish the
same purpose, it would appear that some attention should be focused on evaluating
these units.
The objective of this work was to develop a rational approach to the design of
equalization tanks for continuous flow treatment.
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