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What are Septic Systems?
Septic systems are considered to be on-site systems designed to safely dispose
of biological sanitary waste. "Gray water", such as laundry waste, is
part of the waste system, but it may not result in what is referred to as
"biological" waste. We will address "gray water" waste as it
impacts the design of a septic system.
How do Septic Systems Work?
Basically a
septic system provides a "holding Tank" where natural bacterial action
decomposes human waste products into environmentally acceptable components - the
major end-components being water, mixed with some other components that are not
readily consumed by the bacterial action, gases, and undigested solids. The end
products, except the undigested solids, are then discharged to the on-site
environment.
What are the Components of a Private Septic System?
The things that are most obvious are the things seen every day - the sinks,
toilets, and pipes in a normal house. What are not visible are the things that
are underground; the things that are underground, and the ground itself, greatly
impact how a septic system works. The individual parts of the system are the
septic tank, a distribution box, and a leach field. Bacterial action takes place
in the septic tank where the end products are mainly water, gases, and
undigested material, called sludge that sinks to the bottom of the tank and scum
that floats to the top of the tank. The septic tank contains baffles that
prevent any scum that floats to the surface and sludge that settles to the
bottom from passing out of the tank. The gases that are generated vent to the
atmosphere via the plumbing vent system. From the septic tank, the segregated
and relatively clear liquid flows into a small distribution box where it is then
metered out to several perforated pipes. These perforated pipes then deliver the
liquid to a large soil surface area, called a leach field, or absorption field,
for absorption. The soil also acts as a filter to remove any small amounts of
solids that may be carried along with the liquid. The sludge in the bottom of
the tank must be periodically pumped out and properly disposed of.
There are other kinds of systems for special situations, but
the septic tank and leach field is the most widely used system in our area. The
following discussion concentrates on this type of system.
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Where are These Components Located?
The concrete, or sometimes steel, septic tank is buried in the ground, usually a
minimum of 10 feet from the house. The top of the tank is usually about one foot
below the soil surface so it can be periodically opened for inspection and
pumping. If you do not know for sure where the tank is located, the first step
is to locate where the house sewer pipe leaves the house. In a house with a
basement, this is where the pipe passes through the wall. Locating the exit
point may be more difficult for a house with no basement. If the pipe exit can
be found, the tank normally begins about 10 feet from the house outside wall and
in line with the house sewer pipe. If the soil is not frozen, you can usually
find the tank by pushing a slender metal rod into the ground until it hits the
buried tank. You can buy a metal rod about 1/8 inch in diameter for a few
dollars at most hardware stores. Be careful when probing for the tank and avoid
hammering the metal rod into the ground - you could break a sewer pipe. The
distribution box is much smaller than the septic tank and is usual found about
20 feet from the house. It too is usually only about one foot below the ground.
Again, you can probe the soil carefully to locate the distribution box with a
slender metal rod.
From the distribution box, several pipes direct liquid to a
series of pipes in trenches called laterals. The pipes in the trenches have
holes in them to allow the liquid to be evenly distributed within the trench. To
keep the pipes from being blocked with soil and to provide a space for water to
be stored while it is being absorbed by the soil, the pipes are laid in a bed of
crushed stone. Above the stone is a soil filter (usually one or two layers of
what is called untreated building paper). Above the soil filter is top soil in
which grass is planted.
Equally important is WHERE THE COMPONENTS SHOULD NOT BE. If
there are wells, either yours or a neighbor's, the leach field must be a minimum
of 100 feet from the location of the well. In some areas, the well is not
allowed to be down-slope from the leach field. If there is a stream or pond, the
leach field must also be a minimum of 100 feet from the mean high water mark.
Normally, no part of the system should be within 10 feet of a property line. In
some areas and in unusual conditions, minimum distances may be greater than
those noted here. In addition, no part of the system should be under a porch or
driveway and you should not drive heavy vehicles (including automobiles) over
the system lest the system be damaged.
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Care and Maintenance
Conventional septic systems are not entirely care free. The undigested solids
(sludge) in the bottom of the septic tank should be pumped out every two to four
years, depending on usage and tank size. If the sludge is not removed
periodically, it will eventually carry over into the leach field and cause the
field to fail. A well designed system can handle a reasonable amount of normal
household chemicals such as drain cleaners, laundry detergent and bleach;
excessive usage can be detrimental. You should avoid putting in chemicals that
are toxic to the bacteria, such as paint thinner, solvents, insecticides, etc.
Cooking fats and grease should also be avoided. If a garbage disposal is used,
more frequent tank pumping may be needed.
Depending on the size of the tank and your location, plan on a
cost of about $200 each time the tank is pumped. When the tank is pumped, your
service person should also check the tank baffles for possible damage; ask them
to do this inspection before you contract with them. While the tank is open, the
service technician can also run some water from a hose into the distribution box
to get an indication that the leach field is also still functioning; ask if the
company offers this service.
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Why Septic Systems Fail
If the liquid effluent cannot soak into the soil surrounding the leach field,
sewage may back up into the system and overflow into the house or puddle on the
surface of the ground. There are several possible causes for this problem.
- Poor Soil Conditions; Faulty Design or Installation A
leaching system placed in unsuitable soil, a system that is too small for
the house it serves, or an improperly constructed system may lead to early
failure.
- Soil Clogging If sludge or scum is allowed to escape into
the distribution box and from there into the leach field, the soil will
quickly become clogged. If this happens, the liquid will no longer soak, or
percolate, into the soil. This condition can be caused by broken baffles in
the septic tank that allow sludge or scum to escape. Failure to have the
tank pumped can also lead to a situation where the sludge and scum overwhelm
the baffles.
- High Water Table During wet, or abnormally wet, seasons
groundwater may rise into the leach field and force sewage upward to the
ground surface. This condition may mean the system has to be re-installed at
a higher level. It may also be possible to intercept the high groundwater
with a series of drains around the system called "curtain drains".
- Roots The roots of trees and bushes planted too close to
the system can sometimes enter and block the pipes of the system. Removal of
the plants and clearing the pipes of the roots is usually required.
- Physical Damage Trucks or heavy equipment passing over the
system can damage pipes and joints to the point of rendering the system
inoperable. You should be aware of the location of the system and direct
traffic to avoid such damage.
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How Long Should a Septic System Last?
You can expect a conventional septic system, such as that being described here,
to last about 30 years. Some systems last much longer and some systems can fail
earlier for reasons like those noted above. Other things can also affect the
life of a septic system. For example, a system may have been providing
satisfactory service for a previous owner for many years, only to fail shortly
after you have bought the house. If the previous owners were a working couple
with no children, the system was probably not heavily used; if yours is a family
of six, the added load could push a marginal system over the edge and into
failure. Contents
What are the Signs of a Failing System?
Sewage backup into the home is one possible sign of a failing system. However,
backup can also be simply the result of a blockage somewhere between the house
and the septic tank (this is relatively easy to fix). Another possible sign of
failure is a smell of sewage outside the house. If this smell is more noticeable
after a lot of water has been put into the system - multiple showers or several
loads of laundry (if the laundry waste discharges into the septic system), for
example - this may be an indication that the leach field is failing. The smell
may also be accompanied by a "spongy" feeling in some areas of the
leach field, near the distribution box, or near the septic tank. The
"spongy" feel may be caused by water and waste being pushed to or near
ground level. If ponding water is also seen, this is called
"breakthrough" and is an almost positive indication of failure of one
or more parts of the system. This smell, however, can also originate at the
plumbing vent. In either case, further investigation is warranted. If you see
such signs, a dye test may confirm your suspicions. For this test, a special
strong dye is put in the system - usually by flushing it down the toilet. A
significant amount of water is then washed into the system. If there is
"breakthrough", the dye will become visible on the ground surface. If
the dye is seen on the surface, this would be a very strong indication that the
system has failed. Your Home Inspector, a licensed professional engineer, or a
septic system contractor can usually perform this test for you if it is needed.
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If I Plan on Repairing, Installing, or Replacing A System,
What should I expect?
There are two major factors involved in adding a new system or repairing or
replacing an existing one. The first is the cost; the second is the
inconvenience of possibly not being able to use the existing system while a
replacement is being installed. For new construction, the second factor is not
usually a major consideration. Repair or replacement cost will obviously depend
on what has to be repaired and/or replaced. If the repair does not involve the
leach field, the cost may be high, but it will probably not be exorbitant. The
least expensive repair will be associated with a broken pipe between the
distribution box and the house. The cost for this type of repair is in the order
of several hundred dollars. If only a septic tank needs to be replaced - and the
leach field is still undamaged - the cost will be in the order of about $1500 to
$2500. If a new leach field is needed, and there is room for such an
installation, you should plan on spending an additional $2000 to $3000 for a
typical home. If there is not sufficient room for a new leach field, the
existing field, including the clogged soil, must be removed and a completely new
system must be installed. Such an effort can easily exceed $10,000.
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If my Family is Growing and I add a New Bedroom, What
Should I Expect?
Septic systems are designed to dispose of household biological waste. The amount
of waste to be handled depends on a number of factors. Among these are the
number of people living in the house and their lifestyle. After many years of
experience, a major guideline in determining the size and capability of a septic
system has been correlated to the number of bedrooms in a house. The number of
bedrooms typically determines the number of people generating waste and hence
the amount of waste that must be handled. If your family is growing and a new
bedroom is needed, then the load on the septic system is also increased. If the
septic system capability does not keep up with the increased demand, system
failure may occur. So, how do you determine the septic system needs for your
growing family? The following sections deal mainly with sizing a septic system
so that it can adequately perform the desired function. Your design professional
can handle the actual testing and number-crunching - but we have provided some
standard guidelines developed by New Jersey to aid you in discussing your
options with your contractor. You may not need all this information, but it
could help in making your decisions.
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How Big Should a Septic Tank Be?
Septic tanks are sized according to the amount of liquid waste they must
process. This is done by counting the number of bedrooms. In New Jersey the
minimum size tank that can be installed now is 1000 gallons for a 1, 2, or 3
bedroom house. For each bedroom after 3, add 250 gallons to the size of the
tank. If a garbage grinder is in the kitchen sink, it counts as an additional
bedroom. Contents
How Big Should the Leach Field Be?
Determining the required size of a leach field is a bit more complicated. The
first thing to consider is the nature of the soil in which the leach field is to
be constructed. Because water has to be absorbed in the soil, we need to know
how fast it can be absorbed. This is called the percolation rate and is
expressed as the time it takes for water in a test hole to decrease in level by
one inch (minutes/inch). We must also know the type of soil and whether seasonal
changes in the natural level of groundwater will interfere with the satisfactory
operation of the system. Seasonal groundwater must be more than four feet from
the bottom of the leach field trenches. Judgments regarding the soil conditions
and percolation rates are best left to a professional. If the soil percolates
very fast (less than one minute per inch) or very slow (greater than 60 minutes
per inch) it will not be possible to install a standard leach field in the
existing soil. We must now determine the amount of water that has to be absorbed
each day. As with the septic tank sizing, there are also "rules of
thumb" that can be used to find out how much water must be absorbed each
day for each bedroom in the house (expressed as gallons per day per bedroom).
For older houses (built before 1979) we must allow 150 gallons per day (gpd) per
bedroom. For houses where the toilets are limited to no more than 3.5 gallons
per flush and the faucets and showerheads are limited to 3 gallons per minute or
less, we must allow 130 gpd per bedroom. For houses with water-saving toilets
that use only one gallon per flush we allow 90 gpd per bedroom. The required
flow rate is found by multiplying the appropriate flow by the number of bedrooms
(in this case, we do not have to count a garbage disposal as a bedroom).
Knowing the rate at which water can be absorbed by the soil
(the percolation rate) and the flow rate (in gallons per day), we can use the
following table to calculate how many square feet of absorption field is needed.
| Absorption Percolation minutes per inch |
Application Rates
Gallons per Day per Square Foot |
| 1 - 5 |
1.2 |
| 6 - 7 |
1.0 |
| 8 - 10 |
0.9 |
| 11 - 15 |
0.8 |
| 16 - 20 |
0.7 |
| 21 - 30 |
0.6 |
| 31 - 45 |
0.5 |
| 46 - 50 |
0.45 |
Soil with a percolation rate less than 1 minute per inch or
more than 60 minutes per inch is unsuitable for a conventional system. Required
Area (square feet) = Flow Rate (gallons per day) / Application Rate (gallons per
day per square foot)
Now that we know the number of square feet of absorption field
that is needed, we can divide by the width of each trench to see how many feet
of trench is required. The normal trench width is two feet.
Let's do a sample calculation to see how this works. Assume
you are buying a 3-bedroom house that was built in 1971. The leach field has
failed and a new one must be installed. You have had a percolation test
performed and the design professional has determined that the soil is suitable,
the groundwater conditions are acceptable, and the percolation rate is 32
minutes per inch. How big an absorption field will be needed?
Since the house was built before 1979, the flow rate is 3
bedrooms times 150 gallons per day per bedroom, or 450 gallons per day. From the
table above, the application rate is 0.5 gallons per day per square foot for a
percolation rate of 32 minutes per inch. The required trench area is then 450
gallons per day divided by 0.5 gallons per day per square foot. You will need
900 square feet of absorption area. If the absorption trenches are 2 feet wide,
you will need a total of 450 feet of absorption trench. Most health codes limit
the length of any one trench (called a lateral) to no more than 60 feet, the
minimum number of laterals is 450 feet divided by 60 feet per lateral, or 7.5
laterals. Where property conditions permit, it is best to keep the laterals the
same length, so your design professional may specify 8 laterals, each 60 feet
long. But what if there is only room on the property for laterals that are 45
feet long. In this case, you would need 10 laterals, or trenches. In addition to
the area needed for the leach field, you should also allow room for possible
expansion (50% expansion area is required in New Jersey State).
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What is Gray Water?
Gray water is usually water from a laundry system, perhaps the effluent from a
sump pump, the foundation footing drains, roof runoff, and sometimes shower
drains. This water usually does not contain human waste products and does not
need to be digested like human waste. The disposal requirements for this type of
water are less stringent than those for human waste. If there is a space problem
on your site, it may be possible to segregate the gray waste from the human
waste and minimize the size of the system needed for control of the human waste.
Your design professional (Licensed Engineer or Registered Architect) can advise
you of your options in this area. Contents
Alternative (Other Types of) Systems
The system discussed above is a conventional system installed in the soil that
exists on the site. Where the site conditions do not lend themselves to
installing this type of system, there are alternatives. For example, if ground
water or percolation rates are unsuitable, it may be possible to install what is
called a "mound" system. In a mound system, a suitable soil is placed
above the unsuitable soil. A conventional system is then installed in the mound.
There are some additional requirements for this type of design.
If there is not enough room for a conventional leach field, it
may be possible to install one or more cesspools, or seepage pits. These units
are usually round, require less open ground, and are deeper than a conventional
leach field. Again, there are specific requirements for these systems.
Conventional, mound, and seepage pit systems all work by what
is called anerobic bacterial action. This means the bacteria work without
oxygen. Some systems are designed to be aerobic - meaning the bacteria need
oxygen (air); There are also hybrid systems that use a combination of anerobic
and aerobic sections.
Your design professional will advise you if one of the
non-conventional systems is best for your needs.
Closing Comment
We hope the preceding has helped you in understanding what a private
conventional sewage disposal system is and what the maintenance and replacement
costs might be. Some of the material presented may be more technical than you
expected - but we hope it will be useful and informative.
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