Septic Tank
Function of
the Septic TankEverything
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.
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."