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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 settleable organic matter before disposal
of the wastewater (effluent) to the drain field.
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.
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.
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.
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.
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.
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.
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.
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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