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Septic Tanks
A Septic System  is a small wastewater treatment system designed to dispose of household, biological sanitary waste.  Wastewater from the home flows into the septic tank who's primary purpose is to separate solids (which settle to the bottom of the tank as sludge) from the wastewater before they reach the drain (leach) field.  The lighter waste particles (such as hair or grease)  form a type of scum which accumulates at the top of the tank until purged. 

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Up to 50 % of the solids retained in the bottom of the tank decompose while the rest accumulate in the tank and require periodic pumping for their removal.  After the solids have been removed, the effluent or clarified wastewater, is transported to a distribution box which splits the effluent into a series of pipes set in trenches which are filled with gravel (called the drain or leach field).

These distribution pipes have holes that allow the wastewater to seep through the gravel and into the soil which acts as a natural filter eliminating many of the bacteria that cause diseases. Microorganisms in the soil break down many of the impurities before the filtered water flows back into the groundwater.

Tank Diagram    

According to the U.S. Census Bureau, 25% of US homes have septic tanks which treat and dispose of effluage on site.  Most systems should last between 25-35 years if maintained properly.  Improperly functioning systems can cause sewage backups, smelly pools of sludge in the yard, excessive green growth, and/or contaminate well/water supplies.

Drain Field Diagram


Health Effects

Septic tanks are usually designed to hold enough sludge for up to three years of normal operation.  When the sludge level increases beyond its designed capacity, sewage has less time to settle before leaving the tank allowing more solids to escape into the absorption field.  Sludge infiltration into the soil absorption field can result in system system failure. If sufficient ground area is not available for repair/replacement of the drain field, the home could be rendered inhabitable. To prevent this, the tank must be periodically pumped (the material pumped is known as septage) to remove the sludge buildup. 

Improper maintenance by homeowners is the most common reason for system failure.  

  • Faulty systems can lead to high levels of coliform bacteria and nitrates in  drinking water. Common problems associated with faulty systems include contaminated wells when the septic system is located to close or uphill from the water supply. 


  • To work properly, the system should be properly installed by a licensed contractor following local health department regulations.

  • The system should be regularly inspected and pumped** about every three years or more depending on soil conditions.

  • Water conservation practices tends to prolong the life of the system.  For example, repair leaky faucets, fill the sink to do dishes instead of running water continuously and do not leave the water running while brushing your teeth. Excessive water in the drain field prevents the soil from naturally cleansing wastewater.  

  • Do not divert roof drains, sump pumps, footing drains, etc. into the system. 

  • Never pour products such as gasoline, motor oil, or other products not normally used with water down the drain.

  • It is a good idea to "space out"  wash-loads to increase the time over which harmful soaps and detergents are introduced to the system.  Your system will have a much easier time treating 100 gallons of wastewater over 6 hours than it would if all 100 gallons were dumped all at once. 

The frequency of pumping will depend on: 

  1. the capacity of the septic tank

  2. the volume of wastewater (also related to the size of the household)

  3. the amount of solid material (such as produced by a garbage disposal) in the wastewater 

Estimated Pumping Frequencies (in years) For Year-round Residences (Calculated to provide a minimum of 24 hours of wastewater retention assuming 50 percent digestion of the retained solids).

 Tank           Household size (number of people)
 size  1    2    3    4    5    6    7    8    9   10
      ---------------Years between pumping--------------
500* 5.8  2.6  1.5  1.0  0.7  0.4  0.3  0.2  0.1   - 
 750* 9.1  4.2  2.6  1.8  1.3  1.0  0.7  0.6  0.4  0.3
 900 11.0  5.2  3.3  2.3  1.7  1.3  1.0  0.8  0.7  0.5
1000 12.4  5.9  3.7  2.6  2.0  1.5  1.2  1.0  0.8  0.7
1250 15.6  7.5  4.8  3.4  2.6  2.0  1.7  1.4  1.2  1.0
1500 18.9  9.1  5.9  4.2  3.3  2.6  2.1  1.8  1.5  1.3
1750 22.1 10.7  6.9  5.0  3.9  3.1  2.6  2.2  1.9  1.6
2000 25.4 12.4  8.0  5.9  4.5  3.7  3.1  2.6  2.2  2.0
2250 28.6 14.0  9.1  6.7  5.2  4.2  3.5  3.0  2.6  2.3
 2500 34.9 15.6 10.2  7.5  5.9  4.8  4.0  3.5  3.0  2.6

If you have just moved into a home and do not know the tank capacity, it is wise to have the tank pumped and inspected. In order to extract all the material from the tank, the top scum layer must be first broken up.  The sludge layer is then mixed with the liquid portion of the tank by alternately pumping liquid from the tank and  re-injecting it into the bottom of the septic tank. The tank should be pumped through the central manhole and not through the baffle ports.  If the baffle ports are damaged, this could eventually lead to the destruction of the leach field.  The baffles prevent the floating scum from exiting the tank. If pumping is too infrequent, (even if the tank is not completely clogged with solids) the reduced liquid volume in the septic tank cuts settlement time forcing small floating solids prematurely into the drain field. 

Exposure Guidelines

Decomposing septic tank waste produces toxic gases which can kill a human in minutes. When servicing a tank, be sure the area is well ventilated and never go into a septic tank to retrieve someone who has fallen in without a self-contained breathing apparatus (SCBA). If this occurs and a SCBA is unavailable, call emergency services and place a fan at the top of the tank to supply fresh air. 



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