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-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
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.
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.
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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."