Find Here: A good explanation of what septic systems are, how they work, and why they fail, contributed by Lockwood, Dietershagen Associates Licensed Professional Engineers, Clifton Park, NY.

Contents: Frequently Asked Questions - Septic FAQ's

• What Are Septic Systems?
• How do Septic Systems Work?
• What are the Components of a Private Septic System?
• Where are These Components Located?
• Care and Maintenance
• Why Septic Systems Fail
• How Long Should a Septic System Last?
• What are the Signs of a Failing System?
• If I Plan on Repairing, Installing, or Replacing A System, What should I expect?
• If my Family is Growing and I add a New Bedroom, What Should I Expect?
• How Big Should a Septic Tank Be?
• How Big Should the Leach Field Be?
• What is Gray Water?
• Alternative (Other Types of) Systems

What are Septic Systems?

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.

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What are the Components of a Private Septic System?

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 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

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.

Estimated Septic Tank Pumping Frequency In Years (assumes no garbage disposal)

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Why Septic Systems Fail

1. 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.

2. 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.

3. 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".

4. 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.

5. 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?

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What are the Signs of a Failing System?

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?

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?

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 York State 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?

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How Big Should the Leach Field Be?

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                                   Application Rates
minutes per inch                       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 - 60                                             0.45

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 York State).

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What is Gray Water?

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Alternative (Other Types of) Systems

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.