Methods for Operators of Wastewater
Treatment Plants to Determine BOD Parameters
OWEN SLETTEN, Public Health Engineer
Division of Environmental Engineering
Nebraska State Department of Health
Lincoln, Nebraska
INTRODUCTION
The rate of biochemical oxidation of organic matter is a parameter of great
interest and importance to the sanitary engineer, and as the writer shall endeavor to
prove, also the the operators of the wastewater treatment plant. Streeter and Phelps
(1) introduced the concept that the rate of biochemical oxidation is proportional to
the concentration of unoxidized substance, measured in terms of oxidizability. This
is a first-order reaction, which may be written y = L(l-10"'ct), where y is the BOD at
time t, and L corresponds to the theoretical ultimate BOD. Many methods for
calculation of BOD parameters have been developed by other investigators, such as
Young and Clark, (2) Theriault, (3) Thomas, (4) Fair, (5) Wiegand, (6) Fujimoto, (7)
Moore, (8) Sheehy, (9) and Navone (10). This investigator presents procedures which
are quite unlike those developed by these scientists but hopefully of practical use
even to the wastewater treatment plant operator who is not well grounded in
mathematics.
The 5-day BOD is a widely-used parameter to assess wastes in water and sewage
but it is the contention of the writer that the velocity of it's reaction, commonly
termed the "k" parameter, is the really significant element. It is his thesis that
routine determination of the k parameter can be effectively used by sewage plant
operators as one of the ways of measuring efficiency of treatment of sewage or
wastewater. The k parameter is a measure of the rate of progression of the
biochemical oxidation. A high k rate is held to mean a rapid rate of waste
decomposition. If this idea is valid, the operator should try to achieve a high k rate,
since this is an indication of efficient treatment. In short, he should modify or
control the operation of the treatment facility to get as high a k rate as possible.
THE NATURE OF THE BOD REACTION
Ideally, the BOD curve plotted on semilog graph paper, produces a smooth
asymptotic line, the lower end of which approaches but never reaches the y-axis at
time zero, while the upper end gradually approaches an imaginary line, parallel with
the x-axis, which is equivalent to "L", the ultimate BOD.
Figure 1 illustrates such an asymptotic curve, where L is equivalent to 100
mg/1, k is 0.12, and Ty2 (half-life) is 2.50 days. Half-life is the length in days, at any
time during the BOD progression, required for half of the remaining waste to be
oxidized. For example, if a given waste is 50 percent "destroyed" in the biochemical
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