Environmental Engineering Reference
In-Depth Information
The scale of decentralized reuse in the United States probably falls within the
range of one hundred to two thousand homes. A smaller population base typically
cannot support the technology and operational requirements to treat wastewater to
minimum reuse standards. A larger population base will support a more centralized
treatment and reuse model.
Few things are as unglamorous and prosaic as sewers. Nevertheless, the presence
of working sewers is fundamental to civilization. They merit attention here because
the type of collection systems (sewers) often defines decentralized treatment.
Typically, the largest expense for a centralized reuse system is not treatment; it is
collection. Traditional gravity sewer systems are expensive to construct and maintain
because of deep excavations, large pipes, and the need for lift stations to periodically
pump sewage up to higher elevations once sewer pipes get too deep. Decentralized
treatment systems use smaller diameter pipes in shallower excavations, thereby real-
izing large cost savings.
Small-gravity sewers are cost-effective for buildings in close proximity to one
another, such as a small town or cluster development. In these situations, a simple
collection net of 200 mm pipe may convey wastewater to a local treatment system
without additional pumping. Where frost is not an issue, excavations for pipe may be
shallow, cutting costs even more. This type of sewer must be laid at a constant slope
to maintain a scouring velocity to prevent buildup of solids in the pipe. Some sites
are ill suited for this type of construction. Conventional sewers also will convey all
sewage solids directly to the local treatment system. Because decentralized treat-
ment is done near the people producing wastewater, fresh sewage solids present odor
problems that must be carefully managed to avoid adverse public reactions.
Septic tank effluent gravity sewers (STEGs) are common in decentralized appli-
cations (FigureĀ  19.1). In this system, interceptor (septic) tanks take waste from a
building and provide primary treatment. The liquid fraction of the sewage then dis-
charges out of the septic tank in a small-diameter pipe.
Sewer pipes in STEG systems need only convey the liquid fraction of wastewater
because large particles settle out in the interceptor tanks. Several benefits immedi-
ately follow. Pipes can be small diameter (50-100 mm) because there are no large
solids to plug flow. Pipes can be laid with a variable (inflective) grade in which no
elevation of the sewer pipe is above a discharge elevation from an interceptor tank.
Inflective grading can substantially facilitate sewer construction. There is no need
for large volumes of flushing water to keep pipes open as in conventional gravity
sewers because there are no large solids to settle in pipes. This feature promotes
lower water use.
Additionally, the interceptor tanks that make these hydraulic advantages possible
also provide primary treatment of wastewater by settling and anaerobic digestion.
Primary treatment lowers the cost of the treatment system.
Other collection system types are more energy intensive, but are often popular
in the United States. Pressure sewers use grinder pumps or pumps in septic tanks
(STEP system) to convey sewage. The advantage of pressure sewers is that they may
follow virtually any gradient and can be made from small-diameter, flexible pipe
that is cheap and easy to place in the ground in comparison to conventional gravity
sewers. Vacuum sewers are also used in decentralized applications because of the
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