Environmental Engineering Reference
In-Depth Information
1991 reflecting the improvements in treatment processes at the 3 large waste water treatment plants
discharging to the Chicago Waterway System. Similarly, for the Passaic River, New Jersey, U.S., SOD
rates measured at Route 46 Bridge near Pine Brook reduced from 12 g/m
2
/day in 1969 to near zero in
1983 (New Jersey Department of Environmental Protection, 1987).
Fig. 9.9
SOD rates measured in the Chicago Deep-Draft Waterway System between 1976 and 1991 (data provided
by the Metropolitan Water Reclamation District of Greater Chicago)
As previously noted, measured values of the SOD rate are expressed in terms of a common temperature
of 20
ć
. Temperature effects on the SOD rate can be approximated in the 10-30
ć
range by
S S
(9.25)
Below 10
ć
the value of
S
B
probably decreases more rapidly than indicated by Eq. (9.25) and approaches
zero in the water temperature range of 0-5
ć
(Thomann, 1972, p.105).
(
)
(
)
u
1.065
T
20
BT
B
20
9.2.3.3
Photosynthesis and Respiration
Photosynthesis is the process by which plants convert water (H
2
O) and carbon dioxide (CO
2
) into glucose
(C
6
H
12
O
6
) and oxygen (O
2
) is released as a by product. Photosynthesis is performed by chlorophyll using
solar energy as follows:
o (9.26)
where 36 ATP is a measure of the photon energy required for the reaction. Photosynthesis releases pure
oxygen into the water as compared to the reaeration process wherein the atmospheric source is only
21% oxygen. Thus, since all saturation values of DO are measured relative to equilibrium with standard
atmospheric conditions, photosynthesis can result in supersaturated oxygen concentrations. Values as high
as 150%-200% of the saturation DO concentration are not uncommon (Thomann, 1972, p. 105).
Although photosynthesis may make considerable amounts of oxygen available, oxygenation by green
plants is confined to (Fair et al., 1971, p. 649):
(1) waters that are clean enough to encourage plant growth and (a) either not so heavily polluted that
green plants die off or (b) sufficiently recovered to reestablish the growth of green plants;
6CO
12H O
36ATP
C H
O
6O
6H O
2
2
6
12
6
2
2
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