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Everything
that goes down any of the drains in the house (toilets,
showers, sinks, laundry machines) travels first to the
septic tank. The septic tank is a large-volume, watertight
tank which provides initial treatment of the household
wastewater by intercepting solids and settle able organic
matter before disposal of the wastewater (effluent) to the
drain field.
Function of the
Septic Tank
How Long
Liquids Must Remain In Tank
Solids Storage
Anaerobic Decomposition
Flow Into And Out
Of The Tank
Effluent Filter
Flow Buffering
Microbes in Septic Tanks Digest, Dissolve, and Gasify
Complex Organic Wastes.
FUNCTION OF THE
SEPTIC TANK
While relatively simple in construction and operation, the
septic tank provides a number of important functions through
a complex interaction of physical and biological processes.
The essential functions of the septic tank are to: receive
all wastewater from the house separate solids from the
wastewater flow cause reduction and decomposition of
accumulated solids provide storage for the separated solids
(sludge and scum) pass the clarified wastewater (effluent)
out to the drain field for final treatment and disposal.
Primary Treatment
As stated, the main function of the septic tank is to remove
solids from the wastewater and provide a clarified effluent
for disposal to the drain field. The septic tank provides a
relatively quiescent body of water where the wastewater is
retained long enough to let the solids separate by both
settling and flotation. This process is often called primary
treatment and results in three products: scum, sludge, and
effluent.
Scum: Substances lighter than water (oil, grease, fats)
float to the top,
where they form a scum layer. This scum layer floats on top
of the water surface in the tank. Aerobic bacteria work at
digesting floating solids.
Sludge: The "sinkable" solids (soil, grit, bones,
unconsumed food particles)
settle to the bottom of the tank and form a sludge layer.
The sludge is denser than water and fluid in nature, so it
forms a flat layer along the tank bottom. Underwater
anaerobic bacteria consume organic materials in the sludge,
giving off gases in the process and then, as they die off,
become part of the sludge.
Effluent: Effluent is the clarified wastewater left over
after the scum has floated to the top and the sludge has
settled to the bottom. It is the clarified liquid between
scum and sludge. It flows through the septic tank outlet
into the drain field.
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HOW LONG
LIQUIDS MUST REMAIN IN TANK
Effective volume: The floating scum layer on top and the
sludge layer on the bottom take up a certain amount of the
total volume in the tank. The effective volume is the liquid
volume in the clear space between the scum and sludge
layers. This is where the active solids separation occurs as
the wastewater sits in the tank.
Retention time: In order for adequate separation of
solids to occur, the wastewater needs to sit long enough in
the quiescent conditions of the tank. The time the water
spends in the tank, on its way from inlet to outlet, is
known as the retention time. The retention time is a
function of the effective volume and the daily household
wastewater flow rate:
Retention Time (days) = Effective Volume (gallons)/Flow
Rate (gallons per day)
A common design rule is for a tank to provide a minimum
retention time of at least 24 hours, during which one-half
to two-thirds of the tank volume is taken up by sludge and
scum storage. Note that this is a minimum retention time,
under conditions with a lot of accumulated solids in the
tank. Under ordinary conditions (i.e., with routine
maintenance pumping) a tank should be able to provide two to
three days of retention time. As sludge and scum accumulate
and take up more volume in the tank, the effective volume is
gradually reduced, which results in a reduced retention
time. If this process continues unchecked-if the accumulated
solids are not cleaned out (pumped) often enough-wastewater
will not spend enough time in the tank for adequate
separation of solids, and solids may flow out of the tank
with the effluent into the drain field. This can result in
clogged pipes and gravel in the drain field, one of the most
common causes of septic system failure.
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SOLIDS STORAGE
In order to avoid frequent removal of accumulated solids,
the septic tank is (hopefully) designed with ample volume so
that sludge and scum can be stored in the tank for an
extended period of time. A general design rule is that
one-half to two-thirds of the tank volume is reserved for
sludge and scum accumulation. A properly designed and used
septic system should have the capacity to store solids for
about five years or more. However, the rate of solids
accumulation varies greatly from one household to another,
and actual storage time can only be determined by routine
septic tank inspections.
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ANAEROBIC DECOMPOSITION
While fresh solids are continually added to the scum and
sludge layers, anaerobic bacteria (bacteria that live
without oxygen) consume the organic material in the solids.
The by-products of this decomposition are soluble compounds,
which are carried away in the liquid effluent, and various
gases, which are vented out of the tank via the inlet pipe
that ties into the house plumbing air vent system. Anaerobic
decomposition results in a slow reduction of the volume of
accumulated solids in the septic tank. This occurs primarily
in the sludge layer but also, to a lesser degree, in the
scum layer. The volume of the sludge layer is also reduced
by compaction of the older, underlying sludge. While a
certain amount of volume reduction occurs over time, sludge
and scum layers gradually build up in the tank and
eventually must be pumped out.
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FLOW INTO AND OUT
OF THE TANK
The inlet and outlet ports of the tank are generally
equipped with devices such as baffles, concrete tees, or in
more recent years, sanitary tees (T-shaped pipes with one
short and one long leg).
Inlets
The inlet device dissipates the energy of the incoming flow
and deflects it downwards. The vertical leg of the tee
extends below the liquid surface well into the clear space
below the scum layer. This prevents disturbance of the
floating scum layer and reduces disruptive turbulence caused
by incoming flows. The inlet device also is supposed to
prevent short-circuiting of flows across the water surface
directly to the outlet. The upper leg of the inlet should
extend well above the liquid surface in order to prevent
floating scum from backing up into, and possibly plugging,
the main inlet pipe. The open top of the inlet tee allows
venting of gases out of the tank through the inlet pipe and
fresh air vents of the household plumbing.
Outlets
The outlet device is designed to retain the scum layer
within the tank. A sanitary tee can be used with the lower
leg extending below the scum layer. The elevation of the
outlet port should be 2 to 3 inches below the elevation of
the inlet port. This prevents backwater and stranding of
solids in the main inlet pipe during momentary rises in the
tank liquid level caused by surges of incoming
wastewater.Typical inlet/outlet tees
Gas Deflection Baffle
Gases are produced by the natural digestion of sludge at
the bottom of the tank, and particles of sludge can be
carried upward by these rising gases. Some tanks have a gas
deflection baffle, which prevents gas bubbles (to which
solid particles often adhere) from leaving the tank by
deflecting them away from the outlet and preventing them
from entering the drain field.
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THE EFFLUENT FILTER
In newer systems, there is often an effluent filter: one of
the significant improvements in septic tank design in
decades. They range from 4 to 18 inches in diameter. As we
have described, the most serious problem with septic systems
is the migration of solids, grease, or oil into the drain
field, and the filter is effective in preventing this. A
filter restricts and limits passage of suspended solids into
the effluent. Solids in a filtered system's effluent
discharge are significantly less than those produced in a
non-screened system.
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FLOW BUFFERING
The septic tank also provides a buffering of flows between
the house and the drain field. Large surges from the
household, such as toilet flushing or washing machine
drainage, are dampened by the septic tank so that the flows
leaving the tank and entering the drain field are at
substantially lower flow rates and extend over a longer
period of time than the incoming surges.
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MICROBES IN SEPTIC TANKS DIGEST, DISSOLVE, AND GASIFY
COMPLEX ORGANIC WASTES
In 1907, W. P. Dunbar conducted tests on the decomposition
of vegetable and animal matter in septic tanks. He stated,
"The author has investigated the subject by suspending in
septic tanks a large number of solid organic substances,
such as cooked vegetables, cabbages, turnips, potatoes,
peas, beans, bread, various forms of cellulose, flesh in the
form of dead bodies of animals, skinned and unskinned,
various kinds of fat, bones, cartilage, etc., and has shown
that many of these substances are almost completely
dissolved in from three to four weeks. They first presented
a swollen appearance, and increased in weight. The turnips
had holes on the surface, which gradually became deeper. The
edges of the cabbage leaves looked as though they had been
bitten, and similar signs of decomposition were visible in
the case of other substances. Of the skinned animals, the
skeleton alone remained after a short time; with the
unskinned animals the process lasted rather longer. At this
stage I will only point out that the experiments were so
arranged that no portion of the substances could be washed
away; their disappearance was therefore due to solution and
gasification."
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