Get to Know Your Septic Tank
By Roger Machmeier, Ph.D., P.E., Professor Emeritus, University of Minnesota
The bottom of the inlet baffle or sanitary tee should extend below the surface of the liquid at least six inches and not more that 20 percent of the tank liquid depthÑsee Figure 1. So for a tank that has 60 inches of liquid, the baffle or tee should extend at least 6 inches but not more than 20 percent of 60, or no more than 12 inches below the surface of the liquid. If the baffle or tee is not deep enough, the floating scum layer may tend to plug it. If the baffle or 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 as shown in Figure 1. At the top is the floating scum layer
which collects wastes such as soap or detergent scum, cooking fats,
cigarette filters, and any other material that floats. Most of the
material in the scum layer does not 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 too close. See Figure 1 for these
amounts.
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.
An outlet device, baffle or a tee as shown in
Figure 1, 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 unit,
plugging the soil pores. This is why it is a good idea to have access
to the soil treatment unit at a valve box or the first drop box to
inspect the quality of the septic tank effluent. Such inspection could
be a service contracted with a local company. Providing regular
maintenance to homeowners with a service contract should be an
excellent sales opportunity for local service businesses.
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 treatment
system.
In northern climates, lower septic tank
temperatures result in slower bacterial action than further south where
the average soil temperature is higher. The use of cold water
detergents has decreased septic tank temperatures. A septic tank which
is installed relatively shallow will be warmer at least during the
summer months. Many installers in Minnesota are insulating the top and
sides of the septic tanks with several inches of expanded polystyrene.
This increases the tank temperature which is particularly beneficial in
the winter.
A septic tank in a northern climate will need to
have the solids removed more often than a tank further south. This is
primarily because of the cooler temperatures which inhibit bacterial
action and provide less decomposition of the sewage solids.
