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A septic system disposes of wastewater underground. Even properly designed and
installed systems eventually fail; this fact is easy to forget when the system
is buried and operating effectively. A failure has a serious effect, however.
Sewage backs up into the building or gathers on the ground. Animals and people
may become ill. Pollution may enter surface waters and shallow drinking water
supplies. In addition to public health concerns, there is a costly repair
bill. Normal activities are interrupted while the system is uncovered and
repaired. In spite of these concerns, there are ways to extend a system's
operating life and make repairs easier to handle. A good start is knowing how
to maintain the system and use it wisely. A septic system has two basic parts - a septic tank and an underground
disposal field. The septic tank removes most solid wastes from the water.
Heavier solids settle to the bottom of the tank where bacteria break them down
into sludge. Lighter wastes (some solids, grease and oil) form a scum layer on
top of the water. Cleaner water between the two layers flow into the disposal
field. The disposal field filters and treats the wastewater by distributing it
over a large underground area of coarse soil.
Septic Tank
A septic tank is an underground box constructed of reinforced concrete or
plastic. It must be installed at least ten feet from a building foundation, so
repairs or pumping do not weaken the building. Since the tank holds
wastewater, it must be installed at least fifty feet from a residential or
private drinking water well (wells serving several buildings or more than 25
people require larger isolation distances).
Most solids fall out of the wastewater and remain in the septic tank.
Bacteria breaks them down into sludge but never eliminates them completely:
periodic cleaning is needed. A small amount of sludge is stirred up from the
bottom by incoming wastewater; it them moves into the disposal field. This
happens even with the best maintenance program. Over time, the sludge clogs
the soil in the field and reduces it ability to absorb water. If the field is
undersized or already overloaded with water, even a small amount of sludge
will have a great effect. All septic systems eventually fail.
Disposal Field
There are three different types of disposal fields: trench, bed, and dry well.
While these systems are used in different situations, they serve the same
purpose. In each system, wastewater flows through pipes into layers of stone
and coarse soil. The stone holds the water and lets it filter slowly through
the soil. The trench and bed systems have a network of pipes which distribute
the wastewater down the trench or over a rectangular open bed. The dry well is
a bottomless, concrete box with slots in the sides. It looks similar to a
septic tank. Stone and coarse soil surround the tank on the sides and bottom.
Wastewater is distributed first through the bottom of the dry well and later,
as the bottom stone and soil become clogged, through the sides.
The disposal field must be able to treat at least the amount of wastewater
produced, or a septic failure results. A modern system design is based on the
expected water production from people in the house (2 people per bedroom).
However, even an ideal system can be overloaded with water. If the soil is
saturated, absorption stops and sewage backs up in the pipes or flows out onto
the ground. It is important that the field be kept free of unneeded water.
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