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350 Chapter 10 Introduction to Wastewater Systems

Apron
High water
Combined sewer Low water

Dry-weather outlet

(a) Outfall of combined sewer into a stream.

Terminal chamber
or pumping station
Mean high tide

Mean low tide

Multiple outlets

(b) Outfall with multiple nozzles in tidal estuary.

Figure 10.11 Wastewater Outfalls

the receiving water close to its original state of cleanliness. However, the enrichment of
lakes, ponds, and impounding reservoirs with plant nutrients (eutrophication) and the re-
sulting ecological changes in receiving waters of this kind are quite another thing. In a
sense, natural purification is the prototype of biological treatment. Inherent metabolic ac-
tivities remain aerobic as long as the rate of oxygen supply is not outbalanced by the rate
of oxygen demand. The turbulence of streams usually keeps their running waters aerobic.
However, their benthal environment may become anaerobic, because oxygen diffuses into
bottom deposits only slowly. Anaerobic conditions are most likely to appear in deep stand-
ing waters in which stratification makes for stagnation and consequently poor oxygen
transport to the low water strata. Heavily polluted streams may become, for a time at least,
black, unsightly, septic, and malodorous bodies of water in which the normal aerobic,
clean-water flora and fauna have given way to a different, generally less acceptable, as-
semblage of living things.
A rule of thumb formulated by engineers for large American rivers and giving a
rough estimate of the amount of dilution that would be required in the absence of treat-
ment if combined wastewater from 1,000 persons was to be discharged without much
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nuisance is (a) for swift streams, 2.5 ft /s (0.071 m /s) of diluting water; (b) for normal
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streams, 6 ft /s (0.17 m /s); and (c) for sluggish streams, 10 ft /s (0.28 m /s). For do-
mestic wastewater, treated wastewater, and industrial wastes, equivalent populations
should be substituted. Combined wastewater averages about 40% stronger than domes-
tic wastewater in an industrial area. Industrial wastewaters may be weaker or stronger.