Septic
Systems for Waste Water Disposal
on-line
version of American Ground Water Trust's
Consumer Awareness Information Pamphlet #4
- Introduction
- What
Is The Purpose of Waste Water Disposal Systems?
- How Do Septic Tanks
Work?
- How Do Aeration
Systems Work?
- Which
System - Septic Tank or Aeration?
- What
Is The Purpose of a Soil Absorption System?
- How
Does a Soil Absorption System Work?
- What
are the Siting Requirements for a Soil Absorption System?
- How
Can Septic System Failure Be Prevented?
- Helping Your
System Serve You
- Where To Go
For More Information
- American Ground
Water Trust
INTRODUCTION
This pamphlet provides basic information for homeowners about
septic system waste disposal. One-third of all American homes
use on-site waste water treatment systems. When they are
properly designed, installed and maintained, septic tanks
and similar on-site disposal systems are effective
from an engineering perspective, economic for home
owners and friendly to the environment. Because septic
systems are out of sight, many homeowners rarely think about
them. All states have septic system regulations that are intended
to protect ground water. Most on-site waste water systems
are used for homes that also have their water supply from
a well.
In some rural and suburban areas, sewer lines would have
to stretch great distances to connect homes with a centralized
treatment plant, making the connection to such systems impractical.
Fortunately, in many rural areas, natural soils can treat
waste water as thoroughly and safely using an on-site disposal
system, as can be achieved by municipal sewage treatment systems.
WHAT
IS THE PURPOSE OF WASTE WATER DISPOSAL SYSTEMS?
Disposal systems remove waste water (effluent) from the home,
separate contaminants from the waste water, and return effluent
to the soil. Each American produces an average of about 50
gallons (about 200 liters) of waste water per day from water
use in the kitchen, baths, showers, toilets and laundry. This
waste water includes suspended solids, dissolved organic and
inorganic materials, and microorganisms such as viruses and
bacteria. An on-site waste water treatment system treats this
waste on the property where the waste water originated.
There are many different types of septic system design. The
most frequently used design for single family homes has two
parts: waste water storage and treatment tank, and the soil
absorption and filter part (leach field).
The waste water tank separates out the large solids, and
the soil filter removes the fine solids and allows natural
biological and biochemical processes to destroy accompanying
bacteria. The only maintenance required by a properly operating
system is the removal of accumulated solids from the waste
water tank every few years.
A waste water tank may be a septic tank (anaerobic) or an
aerobic tank (with an air agitator). In both, the large solids
are separated and partially decomposed by bacteria. Bacteria
also digest some of the solids and grease. The digestive (septic)
processes release a smelly gas that moves back through the
sewer pipes and is discharged by a vent through the roof of
the house. The liquid out-flow from the waste water tank (effluent)
contains disease-causing bacteria and nutrients and must have
further treatment through a soil absorption system. The laws
of most states and counties prohibit the direct discharge
of septic system effluent onto the ground surface or into
surface waters.
HOW
DO SEPTIC TANKS WORK?
Eighty-five percent of U.S. on-site waste water disposal
systems are septic systems. They treat waste water by allowing
anaerobic bacteria (those that live in the absence of air)
to digest organic materials, while allowing scum and sludge
materials to separate from the waste water. Wastes that the
bacteria cannot digest are filtered and removed from the effluent
in a soil absorption system.
Septic tanks may have one or several chambers where solids
are separated from waste water. The biological action of bacteria
compacts the heavier solids causing them to settle at the
bottom of the chamber; lightweight compounds such as waxes
and grease drift to the surface. Any indigestible solids must
be periodically removed from the tank. Even after the septic
tank has separated sludge and other solids, its clarified
effluent is not yet purified. For example, the effluent is
likely to contain high levels of bacteria and phosphorus,
which must be removed or inactivated before reaching the water
table.
Make sure you know the exact location of your septic system.
Check your phone book to find septic system pumping companies.
The Cost of the service will vary (usually between $100 and
$200) according to the volume pumped and the distance of your
home from the disposal site.
HOW
DO AERATION SYSTEMS WORK?
Aeration systems are "cousins" to septic tanks;
they use bacteria that live only in the presence of air. The
purification process in an aeration system is generally similar
to a septic tank in its initial stages. Following treatment
in the first chamber, incoming waste water forces effluent
into a second chamber, through a pipe equipped with a filter
or baffle. Next, fine bubbles of air are blown into the effluent,
encouraging the growth of aerobic bacteria which feed on the
organic nutrients and decompose them. The partially treated
effluent then flows into a settling chamber. Bacteria fall
to the bottom of the chamber, where a sloping floor returns
them to the first chamber to continue biodegradation.
WHICH
SYSTEM - SEPTIC TANK OR AERATION?
Local builders and waste water specialists are usually familiar
with which system is most appropriate for a home. Aeration
systems can purify waste water much more thoroughly than septic
tanks. Sometimes they can remove as much as 90 percent of
organic material and suspended solids. However, aeration systems
have disadvantages too; they require servicing and maintenance
more frequently than septic tanks. An aeration system uses
electricity to operate the mixing mechanism and therefore
is vulnerable to power failures. Aeration tanks are usually
more expensive to purchase and install than septic tanks.
Deciding which system is right for you will depend on several
factors: the soil and slope of the site, proximity to environmental
features such as wetlands and streams, proximity to roads
and buildings, the projected daily and peak waste water volume,
your budget, and state and local building codes and regulations
in your area.
WHAT
IS THE PURPOSE OF A SOIL ABSORPTION SYSTEM?
Although septic and aeration tanks remove many pollutants
from waste water, further treatment is required after the
effluent leaves the tank. Nitrogen compounds, suspended solids,
organic/inorganic materials, bacteria, and viruses still must
be reduced before the effluent is considered purified.
There are two similar words, adsorption and absorption,
that need to be explained. "Adsorption" refers
to the process by which pollutants are attracted to and held
on the surfaces of soil molecules, thus immobilizing them.
"Absorption" is a more general word used
to describe the way in which pollutants are removed from effluent
as it percolates through the soil. Soil absorption
systems remove most of the suspended solids by filtration
and reduce the contaminants by adsorption and microbial
degradation.
The action of microbes consumes or transforms nutrients and
makes them harmless. If the volume of soil underlying a soil
absorption system is great enough, all but an insignificant
proportion of the pollutants (except for the nitrogen compounds)
can be removed before the waste water reaches the water table.
HOW
DOES A SOIL ABSORPTION SYSTEM WORK?
Effluent flows or is pumped from the waste water tank into
a network of porous pipes located in trenches covered with
soil and turf. The bottoms of the trenches must be level throughout
their lengths, so they usually should follow ground contours.
The soil absorption system is often described as the leach
field.
Shallow trenches do a better job of treating waste water
than do deep ones. Twelve inches of soil backfill over the
porous pipes in the trench is usually enough to prevent freezing,
even during harsh winters. The placement of gravel surrounding
the pipes promotes even distribution of the effluent. Local
builders and septic system installers will know the recommended
depths and code requirements for your area.
WHAT
ARE THE SITING REQUIREMENTS FOR A SOIL ABSORPTION SYSTEM?
The siting requirements for a soil absorption system depend
mainly on the waste water flow rate and volume, and the site
conditions that affect the soil's ability to absorb, treat,
and dispose of septic tank effluent. Waste water disposal
via septic systems should not create a public health hazard
or contaminate surface or ground water. A relatively small
leach field (drain field) size may be used on sites that (1)
have stable, nearly level to gently sloping land surface not
subject to flooding, (2) have at least six feet of well-drained
permeable soil, free of coarse fragments, and (3) occur above
the maximum expected level of ground water.
A larger leach field size may be needed when there is sloping
ground, a thin soil layer, low soil permeability, or ground
water levels close to the surface.
The two principal criteria for determining leach field size
are: the soil percolation rate and the waste water loading
rate. Ideally there will be an additional adjacent area available
for the installation of a repair system in the event of treatment
and disposal system failure. This replacement area should
be kept free of development, traffic, or soil modification.
However, in practice, a septic system replacement area is
rarely needed and is not usually required by building code.
Final selection of an area for a leach field depends on the
location of natural landscape features and proximity to pipelines,
roads, buildings, etc. The distance that fecal organisms will
travel laterally through soil depends mainly on gradient,
hydraulic conductivity, and the occurrence of soil layers
or bedrock that restrict vertical movement of effluent. A
safe distance should be maintained between the disposal site
and ground water supply sources, lakes, streams, tile drains,
and natural or cut slopes where seepage may occur.
Some natural dilution of potential contaminants results from
the precipitation (rain and snow-melt) that occurs on the
actual leach field surface. Even greater dilution takes place
as the drainage from the leach field mixes with the naturally
occurring ground water. In most cases, the area of most leach
fields is very tiny in comparison with the area of natural
recharge and volume of water stored in surrounding geologic
formations.
Plumes of nitrate above the drinking water standard level
of 10 parts per million have been known to spread laterally
hundreds of feet from leach fields over a period of 10 to
20 years. Buildings and roads should be set back from leach
fields so that they will not interfere with proper operation
of the system. Problems may occur if heavy trucks drive over
the leach field, resulting in excessive compaction of soil
and broken drainage pipes.
Minimum building lot size for zoning is often presented for
planning purposes as a matter of public health safety. This
is based on an acreage figure for maximum density of septic
systems. Most local zoning decisions relating to septic density
safety are made using sound science that takes into account
all the geological and topographical variables. There are
instances, however, when concern over minimum lot size, while
justified by proponents as being in the interest of public
health, may in fact be used as a convenient "one-size-fits-all"
means to achieve a no-growth or a large lot-size zoning objective.
Many states mandate minimum separation distances between
natural and man-made features; others do not have comprehensive
requirements. Typically suggested minimum distances appear
below.
Distance
(ft) |
Feature |
10-25 |
Water mains |
10-20 |
Building foundations |
20 |
Curtain drain
up slope from disposal area |
25 |
Property boundary |
25 |
Escarpments
or man-made cuts that do not intersect restrictive
layers |
50 |
Escarpments
or man-made cuts that do intersect restrictive
layers |
50 |
Curtain drain
down slope from disposal area |
50-100 |
Springs, perennial
streams, or constructed ground water interceptors
up slope from disposal area |
100 |
Water supply
wells |
Critical factors in the siting of a soil absorption system
are depth to the water table and soil texture. A leach field
should not be installed where the water table would be less
than three feet below the bottom of the trench.
Extremely coarse soil, such as sand and gravel, cannot filter
out the fine solids and bacteria from waste water tank effluent.
However, coarse soil can be mixed with finer grained soil
to get the right texture, or the required soil type can be
purchased and trucked to the site.
Extremely fine soil, such as clay, may be too tight to allow
much waste water to pass through it. In addition, clay soils
frequently have perched water tables (lenses of ground water
that form periodically due to poor drainage). Therefore, such
soils are not suitable for a soil treatment system. It is
possible to overcome the problem of poorly drained soils and
a high water table by installing the whole leach field system
in a constructed mound.
It may be necessary to install a pump in order to raise the
effluent from the septic tank to the mound. There are many
simple engineering solutions for waste water disposal. The
Smallflows Clearinghouse in West Virginia (see page 13) provides
technical information about waste water treatment options
for homes and small communities.
HOW
CAN SEPTIC SYSTEM FAILURE BE PREVENTED?
There are two main factors that cause problems: overload
and lack of maintenance. An overloaded (or under-designed)
septic system can occur if, for example, an original two bedroom
home has subsequent bedroom and bathroom additions, but no
increase in septic system capability. With overload, waste
water flows to the leach field before there has been time
for natural biological process to settle out solid waste in
the septic tank. As a result, particles can clog up the holes
in the drain-field pipes and build up extra pressure on the
holes that remain open. The increased flow through fewer drain
holes produces more liquid than the soils of the leach field
can naturally treat. The result can be contaminants moving
to the ground water, rise of waste water to the surface, or
both. In the case of subsurface overload, the situation may
go unnoticed for years.
Lack of maintenance can also lead to problems even if the
system is not overloaded. If the solid material is not periodically
removed from a septic tank there could be insufficient room
for solid matter to settle out. In such cases, solid material
may also clog parts of the drain field. If the septic system
is both overloaded and the septic tank full with solids, then
there can be a real risk of environmental degradation, such
as nitrates moving to aquifers.
Reduced production of waste water helps prevent system overload.
Most Americans can cut water consumption 15 to 20 percent
without discomfort. (For water conservation tips, see the
Trust's pamphlet #7, "Water Conservation In Your Home.")
If your septic system is the right size and properly maintained,
it should not give you any problems. If it is undersized and
you have a house full of friends and relatives, then just
go easy on the use of water. Household water use peaks just
after your family awakens, and again at bedtime. Try to avoid
using more water than necessary during these peak periods.
The following practices can also help prevent septic system
problems:
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Make sure that outdoor drains
from roofs or driveways do not discharge into the system.
Although most state and local codes prohibit such discharge,
it is not strictly monitored. Drain water should be discharged
into a drainage ditch or surface water source.
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Be conservative with your
use of water. Each gallon of water used must be treated
and disposed of. Repair all leaky fixtures and reduce
the amount of water used in doing laundry, bathing, and
toilet flushing.
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CAUTION: The diversion of so-called grey water (water
from laundry, showers and baths) away from the septic system
for use on lawns, gardens and trees needs to be undertaken
with caution. Local plumbing codes may forbid the practice
and your septic system will need a certain amount of dilution
water in order to work efficiently. Consult local experts
before altering your home's waste water system.
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Wash only full loads in the
washer and spread the washing out during the week to avoid
overloading the waste water system in a single day.
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Each bath or shower uses up
to 30 gallons of water. Filling the tub not quite so full
and using a low-flow shower head will reduce water use.
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Routinely check the toilet
float valve to be sure that it isn't sticking and the
water running continuously. Don't use the toilet to dispose
of tissue paper or cigarette butts.
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The salt used in water softeners
will not harm septic tank action although it could eventually
lead to a slight increase in ground water sodium levels.
Check the local codes and regulations that apply to water
conditioning equipment discharge.
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A biological "starter"
is not needed for bacterial action to begin in a new septic
system.
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Only in exceptional cases,
and on the advice of an expert, should additives be used.
Be aware of telemarketing calls that claim that "Product
X" added regularly to your water will save you money.
Most additives are of no benefit and some may do great
harm if they cause the sludge and scum to be flushed out
into the drain field where it may block drain holes.
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Household chemicals labeled
"septic friendly" are available in most stores;
however, normal amounts of regular detergents, bleaches,
drain cleaners, toilet bowl deodorizers, and other household
chemicals can be safely used without harming the bacterial
action in the septic tank.
|
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Don't deposit coffee grounds,
cooking fats, disposable diapers, or similar materials
into the septic tank. Avoid dumping grease down the drain.
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Remove the sludge and scum
by pumping the septic tank. (Every one to three years
for a 1000-gallon tank serving a typical four person household).
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If you must use a garbage
disposal unit, you may need to remove the septic tank
solids every year or more often. Ground garbage will frequently
find its way out of the septic tank and clog the soil
treatment system. It is better to compost, incinerate,
or throw out garbage with the trash.
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Do not use your septic tank
to dispose of organic compounds such as acetone alcohol,
naptha, motor oils, or dry cleaning fluids. Do not use
organic chemicals to clean your septic system. The addition
of organics can lead to serious ground water contamination.
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If any part of your on-site disposal system does fail,
you should contact a qualified contractor immediately.
HELPING
YOUR SYSTEM SERVE YOU
To ensure proper system design, location, and construction,
consult a qualified contractor. Discuss requirements for your
on-site waste water disposal system before construction begins.
Many water well contractors also install septic tanks, and
can offer complete service by installing your water and waste
disposal systems at the same time.
Remember that your septic tank or aeration system operates
according to standard engineering principles. It is not a
mysterious machine that works best when left to itself. With
proper use and maintenance, your waste water disposal system
can offer a reliable, inexpensive alternative to centralized
waste water treatment.
The amortized cost of a properly designed and adequately
maintained privately owned on-site waste water system is usually
considerably less than regular monthly charges for district
systems. In many areas the natural purification and recycling
of water back into the ground water system represents very
efficient water conservation.
WHERE
TO GO FOR MORE INFORMATION
Local building inspectors, agricultural extension offices
and state environmental departments will have information
about specific septic system requirements. A one-stop-shop
of information about small community waste water systems is:
National Smaliflows Clearinghouse West Virginia University,
P0 Box 6064 Morgantown, West Virginia 26506 Telephone 800-624-8301
Information in this
pamphlet is provided in good faith to inform the public
about ground water and ground water related issues.
In all cases, the Trust urges consumers to contact local
experts, and where appropriate,
to refer to local codes, rules, regulations and laws. |
Trust
Pamphlets
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America's Priceless Ground
Water Resource - An introduction for the homeowner
to the occurrence of ground water in the hydrologic system.
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Water Conservation In Your
Home - Offers practical suggestions for conserving
water resources and reducing the use of energy.
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When You Need A Water Well
- Guidance on making good economic and environmental decisions
about water wells and choosing a professional well contractor.
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Rural
Drinking Water - Private Wells or Public Water Supply
- Water supply decisions should be based on properly planned
cost-benefit studies. This pamphlet offers a checklist
guide for citizens.
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Bacteria
And Water Wells - Describes the natural occurrence
of bacteria beneath the ground, how wells may be affected
and how to fix them.
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Ground Water - A Source
of Wonder - The basic geology of ground water and
related issues for middle school student science programs.
|
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