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SEPTIC TANK
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-
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
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-
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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-
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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-
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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-
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-
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-
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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."
Dear Marlin, I received your emails and the inspection report, thank your very much.
Your reports are very detailed and the pictures came out exceptionally clear. I
really appreciate your services. After I get settled, I will probably contact you
to get a name of a handy man in the area. Thanks again.
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