Running Waters - Mathematics and Statistics for the Environment - page 566

19.7 OXYGEN SAG (DEOXYGENATION)
Biochemical oxygen demand is the amount of oxygen required to decay or breakdown a certain
amount of organic matter. Measuring the BOD of a stream is one way to determine how polluted it
is. When too much organic waste, such as raw sewage, is added to the stream, all of the available
oxygen will be used up. The high BOD reduces DO levels because they are interrelated. A typi-
cal DO-vs.-time or -distance curve is somewhat spoon-shaped due to the reaeration process. This
spoon-shaped curve, commonly called the oxygen sag curve , is obtained using the Streeter-Phelps
equation (to be discussed later).
Simply stated, an oxygen sag curve is a graph of the measured concentration of DO in water
samples collected (1) upstream from a significant point source of readily degradable organic mate-
rial (i.e., pollution), (2) from the area of the discharge, and (3) from some distance downstream from
the discharge, plotted by sample location. The amount of DO is typically high upstream, diminishes
at and just downstream from the discharge location (causing a sag in the line graph), and returns
to the upstream levels at some distance downstream from the source of pollution or discharge. The
oxygen sag curve is illustrated in Figure 19.1. From the figure, we can see that the percentage of DO
vs. time or distance shows a characteristic sag, which occurs because the organisms breaking down
the wastes use up the DO in the decomposition process. When the wastes are decomposed, recovery
takes place and the DO rate rises again.
Several factors determine the extent of recovery. First of all, the minimum level of dissolved oxygen
found below a sewage outfall depends on the BOD strength and quantity of the waste, as well as other
factors. These other factors include velocity of the stream, stream length, biotic content, and the initial
DO content (Porteous, 1992). The rates of reaeration and deoxygenation determine the amount of DO
in the stream. If there is no reaeration, the DO will reach zero in a short period of time after the initial
discharge of sewage into the stream. But, due to reaeration, the rate of which is influenced directly by
the rate of deoxygenation, there is enough compensation for aerobic decomposition of organic matter.
When the velocity of the stream is too low and the stream is too deep, the DO level may reach zero.
Depletion of oxygen causes a deficit in oxygen, which in turn causes absorption of atmospheric
oxygen at the air-liquid interface. Thorough mixing due to turbulence brings about effective reaera-
tion. A shallow, rapid stream will have a higher rate of reaeration (will be constantly saturated with
oxygen) and will purify itself faster than a deep sluggish one (Smith, 1974).

Key Point: Reoxygenation of a stream is effected through aeration, absorption, and photosynthesis.
Riffles and other natural turbulence in streams enhance aeration and oxygen absorption. Aquatic
plants add oxygen to the water through transpiration. Oxygen production from photosynthesis of

9.5

8.0

6.0

4.0

2.0

Deoxygenation

Reoxygenation

1 4

2 3

Time in Days

Sewage
Outfall

FIGURE 19.1 Oxygen-sag curve. (From Spellman, F.R., Stream Ecology and Self-Purification, , Technomic,
Lancaster, PA, 1996.)