The primary purpose of the tank is to separate the solids and greases from the liquid waste. The septic tank contains anaerobic bacteria (these work without oxygen) which partially digests the solids and greases. Some undigested solids are left in the bottom of the septic tank. These solids must be pumped from the tank on a regular basis. This pumping prevents solids from being carried into the drainfield and preserves the effective capacity of the tank. The solids in the form of sludge settle on the bottom of the tank. Grease, scum, and hair float to the top. A typical septic tank holds 1,000 gallons of liquid, is watertight, and is usually made of concrete, but can be made of fiberglass or plastic. The tank has internal baffles at the inlet and outlet which controls flow patterns.
The soil absorption system or drainfield is the most important part of the septic system. It is also the most expensive part of the system to replace if it fails. Care should be taken to help ensure that this part of the system is not mistreated.
In Volume 6 No 3 of Pipeline, a publication of the Small Flows Clearing House, advantages of septic systems are outlined. Septic systems are simple and effective for wastewater treatment, are less expensive to operate than centralized treatment facilities, provide wastewater treatment in areas where it would not otherwise be available, and when functioning properly can help to replenish groundwater. However, unofficial data from Pennsylvania Septic Management Association indicates that approximately one-third of all septic systems are failing if failing is defined as water standing in the laterals. Septic system maintenance and preventative maintenance can help ensure years of good operation.
Septic tank effluent or black water passes from the septic tank to the drainfield and the laterals where the water is transferred to the ground. The septic tank is no more than a separation system which separates the solids and grease from liquid and allows the liquid to pass to the soil absorption system. The only thing that can generally go wrong with a septic tank is deterioration of the inlet and outlet baffles. These should be inspected each time the tank is pumped. Unless there is heavy usage of the septic system, the septic tank should generally be pumped at least every three years so that the baffles can be inspected. This is a small price to pay to help protect the drainfield. The tank may need to be pumped more frequently if the system is heavily used.
As black water flows into the stone layer, a biomat is formed. This is an organic layer where bacteria action and filtration takes place. The aerobic or top side of the biomat is where much of the waste digestion occurs. Aerobic bacteria are very efficient digesting the remaining waste in the black water. The biomat may grow and thicken until water flow is restricted and the system starts to slow. This may be due to several reasons including high water usage and large amounts of organic waste passing through the septic tank. As this happens, the drainfield turns more and more anaerobic. Anaerobic bacteria are 15-20 times less efficient that aerobic bacteria in digesting waste. As the system turns anaerobic, the water absorption slows which helps make the system more anaerobic.
The biomat is generally black in color. This is due to the formation of sulfides. This is a natural process in typical septic systems. These sulfides are formed from the anaerobic bacteria in the septic tank and the anaerobic side of the biomat and metals in the soil and water. Metal sulfides are insoluble and absorb available oxygen in the soil. The results are less efficient absorption of water and a clogging of the drainfield.