Oxygen Demand Tests for Organic
Wastes
G.   A.   RHAME
Water Pollution Control Engineer
Water Pollution  Control Authority
Columbia, South Carolina
The purpose of this discussion is to consider the field of oxygen
demand tests for organic waste and outline a method of co-ordinating
the various testing procedures with each other and with field conditions.
The entire problem of organic waste disposal revolves around the
question of what happens to the stream which receives the waste—
treatment plants are merely a means of controlling the influence of
the waste on the stream. Waste characteristics which affect receiving
streams may be roughly broken down into three main divisions: toxic
effects, oxygen consuming effects and bacteriological effects. This
discussion will be limited to the problem of oxygen consumption, but
keeping always in mind the possibility of this being interfered with
by toxic effects.
The purpose of oxygen demand tests is to furnish information
which can be used to predict the behavior of wastes in a receiving
stream and to regulate their discharge thereto. For many years the
5-day biochemichal oxygen demand has been used as a principle means
of estimating the effect of organic wastes on the receiving stream and
treatment plant. As this characteristic has been widely studied and
used, some serious weaknesses have become apparent. Most serious
of these is the fact that the testing procedure gives only a fraction
of the total oxygen demand of the waste and there is no way of
knowing the value of this fraction. A mathematical formulation of
the oxygen consumption curve of the B.O.D. reaction has been developed from which it is theoretically possible to calculate the total
demand from the 5-day demand, but this will generally apply only
to the first-stage or carbonaceous portion of the reaction. Its use
is further hampered by the fact that the rate of oxygen consumption
must be assumed to be a constant when actual experience shows that
it will vary considerably from sample to sample of the same material.
A number of studies have been made which show that in an
attempt to make a critical evaluation of the effect of an organic waste
on the receiving stream one of the decisive factors is the total first-
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