Septic Tank Information
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.
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.
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.
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.
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).
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.
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.
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."