How Exactly do Septic Tanks Work?
The Anatomy of a Septic System
By: Mark J. Donovan
For those of us who own a home and are not on a public
sewage system we have private septic systems. A septic system comprises of a
septic tank and a leaching field that are interconnected together via large
pipes. It is important that a septic system be used and maintained properly
in order to prevent major repair costs. The installation of a new leach
field, for example, could cost upwards of $25,000. Consequently it is
important to understand exactly how a septic tank and leach field work to
ensure that you do not inadvertently abuse it and maintain it properly.
Summarized below is an explanation of how waste water is processed in your
septic system after it exits your home.
Waste water exiting the home first enters a septic tank where the solids in the
waste water are allowed time to settle to the bottom of the tank. The septic
tank is a large, water tight, rectangular concrete tank that is buried into the
ground relatively near the home. It is meant mainly for containing waste solids
and somewhat clarifying waste water prior to sending it on to the leach field
for further cleansing.
In order to separate the waste solids and liquids, a septic tank consists of 2
or more chambers. Waste water entering the septic tank, first flows into the
large chamber. This chamber is twice the size of the other chambers in the
septic tank. It is this chamber where most of the waste solids, or sludge,
settle. The sludge quickly settles due to the fact that there are no currents
flowing in the septic tank.
|While the sludge sits in the bottom of the septic tank anaerobic bacteria
digests and decomposes the sludge. In the process, the bacteria produces methane
and carbon dioxide that needs to be vented from the tank. Sometimes you may
notice a small PVC pipe near the septic tank. This pipe is for venting the gases
from the tank. Similarly you may also notice a couple of larger “Candy Cane” PVC
pipes over the leaching field. Again, they are to vent off methane gas.
As the bacteria breaks down the sludge it eventually becomes stabilized and
ceases to decay any further. At that point, some of the sludge may begin to
float and form a scum at the top of the septic tank.
In the second chamber the waste water is clarified, or purified, further. This
water is then referred to as grey-water. The grey water then exits the septic
the tank to the leach field for final purification. The leaching field is a
network of drainage pipes that are sandwiched between crushed stone, and covered
by topsoil. If the grey-water in the septic tank needs to be pumped up to the
leach field, the grey-water may enter a third chamber where it is then pumped-up
to the leaching field.
Waste water entering the septic tank can either be gravity fed or pumped-up from
the home. It all depends upon the height of the septic tank inlet waste pipe
hole, relative to the height of the waste pipe exiting the home. It is
preferable that a septic tank be gravity fed whenever possible. This eliminates
the need for any type of pump and related concerns of the pump failing. There
has been many a home that has had its waste pump fail and waste water back up
into the home. It’s not pretty.
A septic tank is designed to maximize the time that bacteria and microorganisms
are in contact with the sludge to enable faster digestion of it. Thus inlet and
outlet pipes are placed diagonal to one another. Also, the water is made to flow
from one chamber to the other in vertical positioned pipes. This enables
semi-clarified water to flow from one chamber to the other such that solids and
larger particles are not transferred from one chamber to the other.
The digestion and settling of solid organic matter continues to occur in the
second chamber, however the time the waste water is in this second chamber is
approximately half the time of the first chamber, due mainly to the fact that
the second chamber is half the size of the first.
The grey-water then exits the septic tank where it goes on to the leach field
for final cleansing. If a pump is required to transfer the grey-water to the
leach field, the grey water enters a third chamber where the pump is located for
pumping the waste water up to the leach field.
The frequency of emptying the solids from a septic tank varies with the number
of occupants in the home and the size of the septic tank. The rule-of-thumb is
that 0.05 cubic yards of sludge are produce per person, every year.
Consequently, for the average family of 4, plan to have the septic tank pumped
every 3 to 4 years. You may be able to go one or two years longer, however you
begin to risk the chance of sludge draining into the leach field. Again,
replacing a leach field is a very expensive proposition, so it is wise not to
wait so long to pump your septic tank.
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