What are they and how do they work?
If you are like most people, you might not know much about your septic
system this will help you understand what a septic system is and how it
The Septic system itself is made up of two parts:
1. The Septic Tank (see diagram below)
- Newer tanks are usually constructed of cement, or polyurethane, but
older systems can be made from anything from wood to metal.
- All household waste (i.e. toilet / kitchen / laundry / bathing) is
carried out to your septic system usually by a 3 or 4 pipe called the
- All the organic solids (i.e. waste / food / paper ) will float to
the surface and form a layer commonly referred to as The Crust or
*The bacteria culture in the septic tank will break down these solids
into gasses and liquids.
- Inorganic solids (i.e. plastic / dirt / other inert materials) and
the by-products of the bacteria culture digestion all will settle to the
bottom of the tank and is called Sludge.
- Only relatively clear water should exist between the crust and
sludge this is called Leachate. This clear water is the only material
that should be allowed into the leach area.
- Both the crust and sludge materials should not be allowed to flow
into the leach area at any time. If this is allowed to occur, it can clog
soil pores in the leach area and cause septic systems to fail.
- The three factors that can cause solids to travel into the leach
area are missing outlet tee, bacterial deficiency, and failure to
maintenance pump your septic tank on a regular basis as per Current
The diagram above is of a standard
cement tank with a normal water level, two cement lids, a baffle that is
cement and intact, and inlet and outlet Tees intact. Most septics will
resemble this model in design.
How do they work?
A septic system is a highly efficient, self-contained, underground
wastewater treatment system. Because septic systems treat and dispose of
household wastewater onsite, they are often more economical than
centralized sewer systems in rural areas where lot sizes are larger and
houses are spaced widely apart.
Septic systems are also simple in design, which make them generally
less expensive to install and maintain. And by using natural processes to
treat the wastewater onsite, usually in a homeowner's backyard, septic
systems don't require the installation of miles of sewer lines, making
them less disruptive to the environment.
A septic system consists of two main parts a septic tank and a
drainfield. The septic tank is a watertight box, usually made of concrete
with an inlet and outlet pipe. Wastewater flows from the home to the
septic tank through the sewer pipe. The septic tank treats the wastewater
naturally by holding it in the tank long enough for solids and liquids to
separate. The wastewater forms three layers inside the tank. Solids
lighter than water (such as paper, fecal matter, greases, and oils) float
to the top forming a layer of scum. Solids heavier than water settle at
the bottom of the tank forming a layer of sludge. This leaves a middle
layer of partially clarified wastewater.
The layers of sludge and scum remain in the septic tank where bacteria
found naturally in the wastewater work to break the solids down. The
sludge and scum that cannot be broken down are retained in the tank until
the tank is pumped.
The layer of clarified liquid flows from the septic tank to the
drainfield or to a distribution device, which helps to uniformly
distribute the wastewater in the drainfield. A standard drainfield (also
known as a leachfield, disposal field, or a soil absorption system) is a
series of trenches lined with gravel and buried one to three feet below
the ground surface. Perforated pipes or drain tiles run through the
trenches to distribute the wastewater. The drainfield treats the
wastewater by allowing it to slowly trickle from the pipes out into the
gravel and down through the soil. The gravel and soil act as biological
More Detailed information:
How many people would buy an expensive car and never change the oil?
How many would never remove the ashes from a wood stove or fireplace?
When the car breaks down or the stove is so full of ashes that no more
wood can be put in, we don't blame the car or the stove. But when a
septic system fails for lack of proper maintenance, the blame is often
incorrectly placed on that "no-good" septic tank.
A septic tank is the essential first part of an onsite sewage
treatment system; and it's both very simple and very complex. Raw sewage
flows into the tank from the house sewer. The solids separate from the
liquid and stay in the tank. The liquid flowing out is called septic tank
effluent. A septic tank is watertight so that when 5 gallons of sewage
enters from the house, 5 gallons of effluent must flow out of the tank.
Bacteria that do not use oxygen (from the air) grow in the tank. These
bacteria are called anaerobic and the by-products of their activity are
methane and hydrogen sulfide gas, plus other substances having an odor.
Hence, the word "septic" has been applied to this tank. The septic tank
might also be described as a settling tank where the sewage solids are
stored while the bacteria decompose them and reduce their volume. [The
volume is never reduced to zero, so a residue remains.] It is this
residue that must be cleaned out of the tank when the volume becomes too
Sewage flows to the septic tank through the house sewer. This pipe
must have the proper slope; not too steep so that the liquids run away
from the solids and not too flat so that the solids settle out in the
sewer pipe. A grade of from one to two inches in eight feet is used. This
is a slope of one to two percent. A one percent slope is a one-foot drop
in a hundred feet of pipe.
The house sewer pipe should be smooth on the inside so that sewage
won't catch and start a blockage. Toilet paper might hang up on a rough
spot at a pipe joint causing a continuing problem of sewer pipe plugging
every so often. If the homeowner notices that the toilet isn't flushing
as fast as it used to, or the floor drain is backing up when the clothes
washer discharges, the problem may be a partially plugged house sewer
pipe. There are also many other causes but this is one place to start
The house sewer is the inlet pipe to the septic tank. The bottom
(invert) of this inlet pipe should be two to three inches higher than the
invert of the outlet pipe of the septic tank. As the sewage reaches the
tank it drops into the liquid in the tank with a downward flow direction.
This drop tends to move the sewage into the depth of the tank using a
sanitary tee. The purpose of this sanitary tee is to prevent the floating
solids, called the scum layer, from building up and plugging the end of
the sewer pipe.
The bottom of the sanitary tee should extend below the surface of the
liquid to the middle of the liquid depth. If the tee is not deep enough,
the floating scum layer may tend to plug it. If the tee extends too deep,
the downward flow may cause agitation in the tank and result in more
solids carried out with the effluent.
There are three distinct zones or layers in a septic tank. At the top
is the floating scum layer which collects wastes such as fecal matter,
toilet paper, soap or detergent scum, cooking fats, cigarette filters,
and any other material that floats. Most of the material in the scum
layer does decompose under the bacterial action in a septic tank. At the
bottom of the tank is the sludge layer which consists of decomposing and
partially decomposed solids which sink to the bottom of the tank. The
decomposition process continually goes on in the sludge layer. Some
solids can't make up their mind whether they will sink or float and may
remain in the clear zone between the scum and sludge layers until such
time as they are carried out through the outlet baffle and pipe. These
items have the same density as water so will tend to neither sink or
float. Inorganic items such as plastic film, condoms, etc., can cause
serious plugging problems in a septic tank and should never be flushed
down the toilet.
The center zone is called the clear zone which is liquid that contains
suspended solids and bacteria. It is important that the tank have a deep
clear zone. As the scum and sludge layers become thicker, the depth of
the clear zone decreases. Then the speed at which liquid flows through
the tank speeds up and begins to carry some of the solids out of the
tank. It is these solids that begin to plug the pores in the soil. The
tank needs cleaning whenever the bottom of the scum layer gets too close
to the bottom of the outlet device or the top of the sludge layer gets
The sewage flowing into the tank determines the nature of the bacteria
in the tank. Some families of bacteria do better with one type of sewage,
others do better with different sewage. No two septic tanks operate
exactly the same. The nature of the raw sewage is different because of
the eating and living habits of the family. The amount of water use is
different so that the dilution of sewage is different. Different home
cleaning products are used, as are varying amounts of other household
chemicals and cleaners. Tank temperatures differ depending on type of
water use, depth that the tank is buried, tank insulation, etc. All of
these factors affect the bacterial population and the intensity of
bacterial action. Bacteria are always present in sewage, however. If
there is food and the proper temperature, they will grow and multiply.
Septic tank bacterial action starts by itself and continues as long as
solids are deposited in the tank.
A sanitary tee definitely is needed on every septic tank. The bottom
of this device should extend into the liquid a distance equal to 40
percent of the liquid depth. This is the location determined by a number
of tests to provide for the clearest effluent to leave the septic tank.
If there is no outlet device or if it falls off or is removed, then the
scum layer will flow out of the tank and into the soil treatment area,
plugging the soil pores. This is why it is a good idea to have access to
the outlet side of the septic tank to inspect the quality of the septic
Septic tank effluent is usually cloudy and contains suspended solids
and pathogens (disease-causing bacteria and viruses). No, it is not good
enough to drink, regardless of what you may have heard at one time. A
half cup of effluent will likely contain a million or more bacteria and
as many viruses. Suspended solids have a concentration of 45 to 65 mg/l
(milligrams per liter). These are the solids that won't settle out, and
cause the cloudy color. Since only about half of the oxygen-demanding
material in the raw sewage has been removed in the septic tank, the
effluent must have further bacterial action for treatment. This treatment
takes place in the soil.
Risers or lid extensions can be installed over the inlet and outlet
lids. Their basic purpose is to provide access for removing obstructions
and determining the amount of sludge in the bottom of the tank or the
amount of scum floating on top of the tank, and pumping the septic
Water softener discharges are often blamed for the malfunction of the
septic tank. A slight saline solution is good for bacterial growth. The
agar culture used to grow bacteria in the laboratory is saline. However,
too much salt will be detrimental to bacterial growth. A properly
functioning water softener will not add much sodium chloride to the
recharge waste water. As the softener gets older and particularly if
there is iron in the water, the efficiency may drop unless the resin
beads are kept clean. Resin beads that are iron-fouled cannot take on the
same amount of salt and the unused salt goes into the waste water. As the
beads become more and more fouled, the frequency between softener
recharges is more often in order to get soft water, and more and more of
the salt is wasted. This means that more salt gets into the septic tank
and may be too strong for the bacteria to grow.
Another problem is the extra water that the water softener recharge
adds to the septic system. A system that is just big enough to take the
daily sewage flows may experience back-ups when a softener is added. This
extra water cannot be handled by the system which is now too small. The
softener gets blamed for the sewage system failure. Remember that each
septic system has a maximum daily capacity. When more liquid flows in
than the system can handle, back-ups or surfacing will occur.
Septic tanks should be constructed of good quality materials, which
are not subject to excessive decomposition or deterioration. The tank
material must be strong enough to withstand earth pressures and not
collapse when the tank is cleaned and pumped. The septic tank, tank
cover, and manhole extensions must be water tight to prevent leakage or
infiltration of ground water.
Remember that the septic tank is a settling tank which collects and
stores sewage solids. When the storage gets full, the tank must be
cleaned and pumped.