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336 Chapter 10 Introduction to Wastewater Systems
head continuously downhill, except where pumping stations lift flows through force mains
into higher lying conduits. Pumping is used to either avoid the costly construction of deep
conduits in flat country or bad ground or to transfer wastewaters from low-lying subareas
to main drainage schemes. Gravity sewers are not intended to flow under pressure.
However, some system designs provide comminutors or grinders and pumps for individual
or groups of buildings for the purpose of discharging nonclogging wastewaters through
pressurized pipes.
Hydraulically, gravity sewers are designed as open channels, flowing partly full or, at
most, just filled. Vitrified-clay and plastic pipes are generally the material of choice for
small sewers and concrete or plastic reinforced with fiberglass pipes for large ones.
Pressurized sewers are smaller in size such that force mains flow full. They are generally
laid parallel to the ground surface in a similar fashion to water lines.
In well-watered regions of the globe, the collected wastewaters are normally dis-
charged into nearby receiving bodies of water after suitable treatment. This is referred to as
disposal by dilution, but natural purification as well as physical dispersion of pollutants is
involved. In semiarid regions, terminal discharge may be onto land for groundwater
recharge or for reuse in irrigation and industry. Treatment before disposal aims at removal
of unsightly and putrescible matters, stabilization of degradable substances, elimination of
organics, removal of nutrients and minerals, and destruction of disease-producing organ-
isms all in suitable degree. Conservation of water and protection of its quality for other
uses are the important considerations.

10.1 SOURCES OF WASTEWATERS

Sanitary wastewater is the spent waters supplied to the community. Domestic wastewater
is the spent water from the kitchen, bathroom, lavatory, toilet, and laundry. To the mineral
and organic matter in the water supplied to the community is added a burden of human ex-
crement, paper, soap, dirt, food wastes, and other substances. Some of the waste matters
remain in suspension, while others go into solution or become so finely divided that they
acquire the properties of colloidal (dispersed, ultramicroscopic) particles. Many of the
waste substances are organic and serve as food for saprophytic microorganisms, that is, or-
ganisms living on dead organic matter. Because of this, domestic wastewater is unstable,
biodegradable, or putrescible.
Here and there, and from time to time, intestinal pathogens reach domestic and other
wastewaters. Accordingly, it is prudent to consider such wastewaters suspect at all times.
The carbon, nitrogen, and phosphorus in most wastewaters are good plant nutrients.
They, as well as the nutrients in natural runoff, add to the eutrophication (from Greek eu,
meaning “well,” and trophein, “to nourish”) of receiving waters and may produce
massive algal blooms, especially in lakes. As pollution continues, the depth of heavily
eutrophic lakes may be reduced by the benthal buildup of dead cells and other plant
debris. In this way, lakes may be turned into bogs in the course of time and eventually
completely obliterated.
Industrial wastewaters vary in composition with industrial operations. Some are rela-
tively clean rinse waters; others are heavily laden with organic or mineral matter, or with
corrosive, poisonous, flammable, or explosive substances. Some are so objectionable that
they should not be admitted to the public sewerage system; others contain so little and such
unobjectionable waste matters that it is safe to discharge them into storm drains or directly
to natural bodies of water. Fats, lime, hair, and fibers adhere to sewers and clog them; acids