Environmental Engineering Reference
In-Depth Information
(5) Depending on the flow velocity of the receiving water, suspended matter is carried along or removed
to the bottom by sedimentation. Bottom (benthic) deposits are laid down in thickness varying from a thin
pollution carpet to heavy sludge banks.
(6) The decomposition of the bottom deposits differs from that in the supernatant waters. In the presence
of oxygen dissolved in the overlying waters, benthic decomposition varies with depth of deposit from
largely aerobic to largely anaerobic.
(7) In the course of time and distance, the energy values of the single charge of biodegradable organic
waste are used up.
(8) The BOD is decreased, and the rate of absorption of oxygen from the atmosphere, which at first
lagged behind the rate of oxygen utilization, falls into step with it and eventually outruns it.
(9) The water becomes clear. Green plants flourish in the sunlight and release oxygen to the water
during photosynthesis.
(10) Other higher aquatic organisms, including game fish, which are notably sensitive to pollution,
reappear and thrive in a balanced equilibrium.
(11) The waters have returned to normal purity. Self-purification is gradually completed.
Figure 9.1 shows the oxygen sag downstream of a waste discharge and its effects on the aquatic
community. The natural purification of polluted waters described by Fair et al. (1971) and shown in
Fig. 9.1 is never fast, and heavily polluted streams may traverse long distances during many days of flow
before a significant degree of purification is accomplished.
The consumption of biodegradable organic matter is done by microorganisms, primarily bacteria,
which break the matter down into simpler organic and inorganic substances. When this decomposition
takes place in an aerobic environment, the process produces non-objectionable, stable end products such
as carbon dioxide (CO
2
), sulfate (SO
4
), orthophosphate (PO
4
), and nitrate (NO
3
). When insufficient oxygen
is available, the resulting anaerobic decomposition is performed by completely different microorganisms.
They produce end products that can be highly objectionable, including hydrogen sulfide (H
2
S), ammonia
(NH
3
), and methane (CH
4
) (Masters, 1991).
Fig. 9.1
Dissolved oxygen sag downstream from a source of biodegradable organic waste and its effect on stream
biota (Davis and Masten, 2004)
Oxygen demand is exerted by three classes of matter: carbonaceous material, oxidizable nitrogen, and
certain chemical reducing compounds. Clark et al. (1977) simply describes the oxygen demand process as
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