Civil Engineering Reference
In-Depth Information
3. Aesthetics (acceptance, taste and odor, and color). Water from streams that do
not receive treated wastewater is usually more aesthetically pleasing than water ob-
tained from streams receiving wastewater. Increasing the separation between outfall
and intake, up to a point at least, probably provides greater public acceptance of the
water. Physical problems of taste, odor, and color may arise from the discharge of
wastewaters to water sources, although these problems can be avoided if proper water
and wastewater treatment are provided.
4. Possible Chronic Effects of Trace Organics (carcinogens, mutagens, tetratogens).
Many water supply sources contain trace organics. Either unreacted or in the form of
their reaction products with chlorine, ozone, or other oxidants, these substances may
have adverse health effects following their long-term ingestion in trace amounts. These
organics may originate in nature or be present in wastewater, which also may contain
synthetic organics not found in nature, and they are discussed in Chapter 3, ''Organic
Compounds in Drinking Water.'' The degree of hazard probably is not affected to any
practical extent by the time of stream travel or storage. Removal of organics by AWT
or upgraded water treatment is more important than separation distance between outfall
and intake.
Organics in Wastewaters
The organic material in municipal wastewater is a mixture of many compounds that
are only partially known. The three broad classes of organics in municipal wastewater
are fats, carbohydrates, and proteins. They are usually considered removable by pri-
mary and secondary biological treatment, although protein is somewhat less readily
removed than fats and carbohydrates. A properly operating biological treatment plant
treating residential wastewater is capable of producing secondary effluent with a sol-
uble COD of 30 to 50 mg / L and a soluble biochemical oxygen demand (BOD) of 1
to 2 mg / L. Data on TOC levels in wastewater indicate that secondary effluent con-
centrations are in the 30 to 50 mg / L range. The results of an extensive study including
activated sludge effluents are summarized in Tables 8-1 and 8-2.
14
Another study
found 60 percent of the organics in secondary effluent had molecular weights less than
700, and 25 percent had apparent molecular weights greater than 5,000.
15
In one study of municipal wastewater, 77 organic compounds were detected in the
primary effluent, and 38 in the secondary effluent. Several compounds found in the
secondary effluent were not present in the primary effluent.
16
The concentrations of
individual compounds in the secondary effluent were estimated to be less than 20
g/
l. It was found in another study that soluble organics are produced in biological treat-
ment that are more refractory to further treatments than are the organics in raw
sewage.
17
It was hypothesized by these same investigators that the residual organics
not removed by activated carbon are intermediate breakdown products of protein, and
that these are most likely proteins that originate from the cell walls of microorganisms
present in biological treatment processes. The fate of organics during chlorination is
of particular concern, and is discussed in Chapter 3.
In addition to the presence of organic compounds contributed directly by waste-
waters, there is also the potential that inorganic nutrients, such as nitrogen and phos-
phorus, can contribute to organics of concern by stimulating algal growths. There is
evidence that chlorophyll, algal biomass, and algal extracellular products can serve as
