Septic Systems and Their Maintenance
Table of Contents
The septic-tank-soil-treatment system
(also called a septic system) is an effective, long-standing method for
collecting, treating, and disposing of sewage from rural and suburban
homes. Septic systems are used in every county in North Carolina: more
Septic systems are used when sewage treatment
plants are not accessible. They safely treat and dispose of wastewaters
produced in the bathroom, kitchen, and laundry. These wastewaters may
contain disease-causing germs and pollutants that must be treated to protect
human health and the environment. Although septic systems are usually
a permanent solution to wastewater treatment and disposal, they sometimes
serve as a temporary solution until sewer lines
There are a number of different septic systems,
each with its own design. The conventional system is the one most commonly
used in North Carolina (
The septic tank is a watertight concrete
The tank is connected to the drainfield by
a buried pipe. A typical drainfield consists of two to five trenches excavated
into the subsoil. In many systems, a distribution box or a flow divider
helps move wastewater to each trench. In most conventional septic systems,
the trenches are
The drainfield has also been called the nitrification field or the soil absorption field. The sole purpose of the drainfield is to deliver wastewater to the soil. The soil purifies the wastewater by removing the germs and chemicals before they reach the groundwater or any adjacent surface waters such as rivers, lakes, and estuaries.
All of the wastewaters from the home should flow into the septic tank. Even waters from the shower, bathtub, and washing machine can contain disease-causing germs or environmental pollutants. As wastewater flows into the tank, the heavier solid materials settle to the bottom (forming a sludge layer), the lighter greases and fats float to the top (forming a scum layer), and the liquid (sewage effluent) flows out of the tank. An outlet baffle (or a sanitary tee at the outlet end) prevents solids from flowing out with the liquids. The tank's primary purpose is to retain the solids while releasing sewage effluent to the drainfield.
The real treatment of the wastewater occurs in the soil beneath the drainfield. Sewage effluent flows out of the tank as a cloudy liquid that still contains many disease-causing germs and environmental pollutants. Effluent flows into the perforated pipe in the trenches, passes through the holes in the pipe, and then trickles down through the gravel to the soil. As effluent enters and flows through the soil, many of the bacteria that can cause diseases are filtered out. Some of the smaller germs, such as viruses, are adsorbed by the soil until they are destroyed. The soil can also retain certain chemicals, including phosphorus and some forms of nitrogen.
Unlike a sewer system, which discharges treated
wastewater into a body of water, the septic system depends on the soil
around the home to treat and dispose of sewage effluent (
In some cases where the soils do not adequately absorb the wastewater, the toilets and sinks might not drain freely. If the soil can absorb the effluent but not treat it, the sewage may contaminate the groundwater.
Gently sloping, thick, permeable soils with
deep water tables make the best sites. The soil should be a uniform brown,
yellow, or bright red, and it should not have spots of gray, which often
indicate that it is excessively wet. The soil texture should be neither
too sandy nor too clayey, and it should have good aggregation, or structure
(that is, a handful of the soil should easily break apart into small aggregates).
Avoid areas that have rock close to the surface, very sticky clays, or
soil layers that restrict the downward flow
There are more than
Usually, the drainfield can fit within the
front yard or the backyard of a typical 1-acre homesite. The precise area
requirements will depend upon the kinds of soils at the homesite, the
size of the house (the number of bedrooms), and the topography of the
lot. A site with clayey, slowly permeable soils needs a larger drainfield
to absorb the sewage effluent than does a site with sandy, permeable soils.
Adequate land area must be available to isolate the entire septic system
from any nearby wells, springs, streams, lakes, or other bodies of water.
There also must be enough area to install a replacement system in case
it is ever needed. This replacement area must meet the same soil and site
requirements as the
State law requires that soils be evaluated
by the local health department and that an improvement permit be issued
before house construction begins or the septic system is installed. Also,
the installation must be approved by the health department before electrical
service can be permanently connected to the home and the septic system
Both the septic tank and the drainfield must be properly maintained. With conscientious maintenance, the system should work correctly for many years. Such maintenance begins with water use and waste disposal habits. Since your family will determine which materials enter the system, you should establish rules for proper use and maintenance.
The suggestions outlined in the box will save you anguish and money when applied to most conventional systems. More sophisticated systems require additional maintenance, possibly at much greater cost. Also, recent rule changes now require owners of some alternative septic systems and community septic systems to hire a certified operator to maintain their systems. For more information about these requirements, contact your local health department.
After a few years, the solids that accumulate
in the septic tank should be pumped out and disposed of at an approved
location. If not removed, these solids will eventually overflow, accumulate
in the drainfield, and clog the pores (openings) in
This blockage severely damages the drainfield.
While some clogging of soil pores slowly occurs even in a properly functioning
system, excess solids from a poorly maintained tank can completely close
all soil pores so that no wastewater can flow into the soil. The sewage
effluent will then either back up into the house or flow across the ground
surface over the drainfield. If this happens, you may need to construct
a new drainfield on a different part of your lot. Pumping the septic tank
after the soil drainfield has become completely clogged will not rejuvenate
the system. It will provide only a few days reprieve until the tank fills
The frequency with which you will need to
pump depends on three variables: the size of your tank, the volume of
your wastewater, and the solids content of your wastewater. If you are
unsure about when to have the tank pumped, observe the yearly rate of
solids accumulation. The tank should be pumped if the sludge layer has
built up to within
If the septic system is not used very often (as in an infrequently used vacation home with a correctly sized tank), it will probably not need to be pumped as frequently as indicated in the table. If you use a garbage disposal, the tank may need to be pumped more frequently. After a few inspections, you should be able to adjust the schedule according to the rate at which solids accumulate.
Make sure you are aware of the types and
amounts of extra waste materials that are poured down the drain. Limiting
the use of your garbage disposal will minimize the flow of excess solids
to your tank. Garbage disposals usually double the amount of solids added
Do not pour cooking greases, oils, and fats down the drain. Grease hardens in the septic tank and accumulates until it clogs the inlet or outlet. Grease poured down the drain with hot water may flow through the septic tank and clog soil pores completely.
Pesticides, paints, paint thinners, solvents, disinfectants, poisons, and other household chemicals should not be dumped down the drain into a septic system because they may kill soil microorganisms that help purify the sewage. Also, some organic chemicals will flow untreated through the septic tank and the soil, thus contaminating the underlying groundwater.
No. These products include biologically based
materials (bacteria, enzymes, and yeast), inorganic chemicals (acids and
bases), or organic chemicals (including solvents). They do not reduce
the need for regular pumping of the septic tank. Some of these products
contain organic chemicals and may even damage the drainfield or contaminate
the groundwater and
Yes. The drainfield does not have an unlimited
capacity. The more water your family uses, the greater the likelihood
of problems with the
Water conservation practices can help reduce
the amount of wastewater generated in the home. Periodically check your
plumbing for leaky faucets and toilets. Uncorrected leaks can more than
double the amount of water you use. Many soils that can absorb the
Be sure that foundation drains, roof waters,
gutter waters, and surface waters from driveways and other paved areas
do not flow over the septic tank or the drainfield. Careful landscaping
can help direct excess surface waters away from
Other types of septic systems are sometimes
used on sites where the soil is not suited to a conventional system. Where
soils are too wet or too shallow for the conventional system, the drainfield
might be placed very close to the ground surface in the upper layers of
the soil. In some wet soils, artificial drainage around the septic system
lowers the level of the shallow water table. On some clayey soils that
have a thick sandy surface, the low-pressure pipe (LPP) system provides
an alternative. A wrapped pipe
On some soils that are not deep enough to provide adequate treatment of the sewage effluent, it may be possible to use an aerobic treatment unit (ATU) to supplement the soil's treatment capacity.
The septic system is an efficient, inexpensive,
and convenient method for treating and disposing of household wastewater.
Because not all soils are suited for conventional systems, a comprehensive
soil and site investigation must be performed before you purchase
Septic systems will adequately absorb and
purify wastewater if they are properly maintained. A few precautions can
save you anguish and money. Reducing water use, avoiding grease, pumping
the tank periodically, and properly landscaping the yard to keep surface
water away from the tank and drainfield are inexpensive precautions that
can help assure your system a long life. The Cooperative Extension Service