|Septic Tank and
Drainfield Operation and Maintenance
by Michael P. Vogel, Ed.D., MSU Extension Service
Housing specialist and Gretchen L. Rupp, P.E., former MSU Extension
Service Environmental Engineer/Specialist
Households not served by public sewers usually depend on a septic
system to dispose of wastewater. There are many different types of septic
systems designed to fit a wide range of soil and site conditions. These
include mound systems, sand filter systems and pressure distribution
systems. This MontGuide should help you understand the operation and
maintenance of a conventional gravity-flow septic system.
A conventional septic system consists of two main parts: the septic
tank and the soil drainfield (also referred to as a leachfield, absorption
bed or absorption field). At the head of the drainfield a distribution box
or a manifold distributes wastewater to several absorption trenches. Some
locations require that newly installed drainfields include a designated
replacement areashould the existing septic system need an addition, repair
or replacement, the replacement area can then be used.
How the system works
Figure 1: Septic System
Courtesy National Small Flows Clearinghouse
The septic tank. A septic tank is a
large, underground, watertight container, typically about 9 feet long, 4-5
feet wide and 5 feet tall that is connected to the home's sewer line.
While typically designed with a 1,000-gallon liquid capacity, the size of
the tank is legally determined by the number of bedrooms in the home.
(Septic tanks come under the legal supervision of counties in Montana.)
Septic tanks may be rectangular or cylindrical and may be made of
concrete, fiberglass or polyethylene.
Raw waste water from the bathroom, kitchen and laundry room flows into
the tank where the solids separate from the liquid. Light solids, such as
soap suds and fat, float to the top and form a scum layer. This layer
remains on top and gradually thickens until you have the tank cleaned. The
liquid waste goes into the drainfield, while the heavier solids settle to
the bottom of the tank where they are gradually decomposed by bacteria.
But some non-decomposed solids remain, forming a sludge layer that
eventually must be pumped out.
Septic tanks may have one or two compartments. Two-compartment tanks do
a better job of settling solids and are required in some areas for new
installations. Tees or baffles at the tank's inlet pipe slow the incoming
wastes and reduce disturbance of the settled sludge. A tee or baffle at
the outlet keeps the solids or scum in the tank. All tanks should have
accessible covers for checking the condition of the baffles and for
pumping both compartments.
Figure 2: A Two-Compartment Septic Tank
Courtesy National Small Flows Clearinghouse
Figure 3 Wastewater Treatment and Disposal In
The Drainfield. Further treatment of wastewater occurs in the soil
beneath the drainfield. The drainfield consists of long underground
perforated pipes or tiles connected to the septic tank. The network of
pipes is laid in gravel-filled trenches (2-3 feet wide), or beds (over 3
feet wide) in the soil. Liquid waste or effluent flows out of the tank and
is evenly distributed into the soil through the piping system. The soil
below the drain-field provides the final treatment and disposal of the
septic tank effluent. After the effluent has passed into the soil, most of
it percolates downward and outward, eventually entering the groundwater. A
small percentage is taken up by plants through their roots, or evaporates
from the soil.
Courtesy North Carolina Extension Service
The soil filters the effluent as it passes through the pore spaces.
Chemical and biological processes treat the effluent before it reaches
groundwater, or a restrictive layer, such as hardpan, bedrock, or clay
soils. These processes work best where the soil is somewhat dry and
permeable, and contains plenty of oxygen for several feet below the drain
field. The size and type of drainfield depends on the estimated daily
wastewater flow and soil conditions.
Tips for using your septic system
Even a properly designed and installed septic system cannot treat
wastewater if the tank is not used and maintained properly. Here are a few
tips for installing and using your septic system:
Water overload occurs when the drainfield is flooded with more water than
it can effectively absorb, reducing the ability of the system to drain
wastes and filter sewage before it reaches groundwater. It also increases
the risk that effluent will pool on the ground surface and run off into
surface water or down nearby water well casings. Typical indoor water use
is about 50 gallons per day for each person in the family. Water-saving
devices such as low-flow shower heads, faucet aerators, toilet dams or
low-flow toilets can greatly reduce water flow into the system. Strategies
such as taking short showers, spreading out laundry loads over the week
and never allowing rain water from downspouts to enter the septic system
will also help.
- For future maintenance and to avoid deep root planting and other
damaging activities in the drainfield area, make an accurate diagram
showing the location of your tank, drainfield and replacement area.
- Keep a record of pumping, inspection, and other maintenance. Include
name, address and phone numbers for installers and pumpers.
- To simplify tank access for inspection and maintenance, install a
watertight concrete riser over the septic tank.
- The area over the drainfield should be left undisturbed, with only a
mowed grass cover. Roots from nearby trees or shrubs may clog and
damage your drain lines.
- Keep automobiles and heavy equipment off the drainfield.
- Do not plan any building additions, pools, driveways, or other
construction work near the septic tank, drainfield or the replacement
- Do not put too much water into the septic system.
- Do not flush non-biodegradable materials such as plastics,
disposable diapers, sanitary napkins and applicatorsthey rapidly fill
up the tank and will clog the system.
- Restrict the use of your kitchen garbage disposalit increases the
amount of solids in the tank, making them slower to decompose.
- Do not pour grease or cooking oils down the sink drain because they
solidify and clog the soil absorption field.
- Don't allow paints, motor oil, pesticides, fertilizers or
disinfectants to get into your septic system. They can pass directly
through the septic system and contaminate groundwater. These chemicals
can also kill the microorganisms which decompose wastes and can damage
the soil in the drainfield.
- Do not use caustic drain openers for a clogged drain. Instead use
boiling water or a drain snake to free up clogs. Clean your toilet,
sinks, shower and tubs with a mild detergent or baking soda rather
than the stronger and potentially system-damaging commercial bathroom
- If a water softener is used in the home, the salt recharge solution
should not be allowed to enter the system if the predominant soils in
the drainfield are very fine textured and drainage is very slow. In
these situations, sodium in the softener recharge solution may damage
soil structure in the drainfield and plug the system. If you have a
water softener, the size of the absorption field must be increased to
accomodate the additional flow.
Figure 4 Septic Drainfield
Courtesy Washington State University
How will I know when to pump the tank?
The frequency with which you will need to pump depends on three variables:
the size of your tank, the number of people in the household contributing
to the volume of your wastewater, and the volume of solids in your
wastewater. If you are unsure about when to have the tank pumped, observe
the yearly rate of solids accumulation in the septic tank. (See the
MontGuide MT 9403 "Septic
Tank Inspection and Trouble-Shooting.") The solids should be
pumped out of the septic tank by a licensed septic contractor. Most county
health departments recommend that the accumulated solids in the bottom of
the septic tank be pumped out every three to five years although if the
tank is large and the household is small a tank can function longer
without requiring pumping (see Table 1).
Table 1. Estimated Septic Tank Pumping
Frequencies in Years
||Household Size (number of people)
* Your local health department may be able to tell you the size
of your tank.
What is septic system failure?
A septic system should effectively accept liquid wastes from your house
and prevent biological and nutrient contaminants from getting into your
well or nearby lakes and streams. Anytime these things do not happen, the
system is failing.
For example, when waste backs up in your backyard, the system has
obviously failed. If significant amounts of biological or nutrient
contaminants reach your well or surface waters, the system is also
failing, even though it may appear to be working just fine.
Why septic systems fail
Most septic systems are designed to have a lifetime of 20 to 30 years,
under the best conditions. However, many septic systems will fail before
this time. Eventually, the soil around the absorption field becomes
clogged with organic material, making the system unusable.
Many other factors can cause the system to fail well before the end of
its "design" lifetime. Pipes blocked by roots, soils saturated
by storm water, crushed tile, improper location, poor original design or
poor installation can all lead to major problems.
But by far the most common reason for early failure is improper
maintenance by homeowners. When a system is poorly maintained and not
pumped out on a regular basis, sludge (solid material) builds up inside
the septic tank, then flows into the absorption field, clogging it beyond
How to know if your system is failing
These symptoms tell you that you have a serious problem:
- Sewage backup in your drains or toilets. This is often a black
liquid with a disagreeable odor.
- Slow flushing of your toilets. Many of the drains in your house will
drain much slower than usual, despite the use of plungers or drain
- Surface flow of wastewater. Sometimes you will notice liquid seeping
along the surface of the ground near your septic system. It may or may
not have much of an odor associated with it.
- Lush green grass over the absorption field, even during dry weather.
Often, this indicates that an excessive amount of liquid from your
system is moving up through the soil, instead of downward, as it
should. While some upward movement of liquid from the absorption field
is good, too much could indicate major problems.
- The presence of nitrates or bacteria in your drinking water well.
This indicates that liquid from the system may be flowing into the
well through the ground or over the surface. Water tests available
from your local health department will indicate if you have this
- Buildup of aquatic weeds or algae in lakes or ponds adjacent to your
home. This may indicate that nutrient-rich septic system waste is
leaching into the surface water. This may lead to both inconvenience
and possible health problems.
- Unpleasant odors around your house. Often, an improperly vented
plumbing system or a failing septic system causes a buildup of
disagreeable odors around the house.
Could an additive help my system?
A number of products are marketed with the pledge that they can keep
septic systems operating smoothly, correct system upsets, or do away with
the need to pump the tank periodically. Chemical additives are strong
acids or alkalis, or organic solvents. Biological additives are cultures
of harmless bacteria, plus waste-digesting enzymes. These sometimes
contain yeast cultures.
Although some manufacturers of additives have test data showing how
their products perform, there has been almost no independent testing of
these products in full-sized septic systems. The information that exists
does not show improved long-term performance in systems where additives
have been used. If a system is not being misused by the homeowner, these
products are unlikely to pose a benefit. The amount of material added with
each dose of product is very small compared to the biological material
already present and working in the tank.
Occasionally a system suffers an upset, when the septic tank bacteria
are harmed or destroyed. This can happen if the home is vacant for a long
period and the tank receives no fresh wastewater, or if strong cleaning
agents are flushed down the drain. After a few days of normal use, the
biological system in the tank will re-establish itself. In this situation
the biological additives may help speed the recovery of the septic tank.
Every septic tank needs to be pumped periodically, because all
wastewater contains inert matter that cannot be degraded in the tank. No
additive can do away with this need.
Could an additive harm my system?
The biological additives are unlikely to be harmful. The chemical
additives could definitely harm your system. These products have the
potential to sterilize your system temporarily. The resulting passage of
raw sewage into the drainfield will hasten its failure. The acid and
alkali products can corrode the plumbing and the tank. The organic
solvents pass through the system unchanged. They can then infiltrate into
the groundwater, creating a chemical plume that endangers nearby wells.
For information on evaluating a septic system when selling or
purchasing property, inspecting solids levels in a septic tank and septic
system trouble-shooting, see MontGuide 9403," Septic
Tanks: Inspecting and Trouble-Shooting." Required design features
are set forth in circular WQB-6, "Standards for Individual Sewage
Systems," published by the Water Quality Division, Montana Department
of Environmental Quality.
Household Equipment, F-4 (Miscellaneous) Revised June 1996