Septic tanks offer an alternative to municipal sewer systems. When sewage and wastewater products are ejected from the home, they pass through the septic tank. The tank stores solid waste and vents gaseous exhaust through a plumbing vent system. Liquid wastewater passes through the tank for processing, then slowly exits into nearby drainage areas. You can choose among several different types of septic tanks, depending on your yard layout and soil characteristics.
Gravity Tanks
Gravity septic tanks are the most widely used and most economical systems on the market. As waste passes into the system, solids naturally separate and pass to the bottom of the tank due to the effects of gravity. Liquid waste and sludge stay along the top, forming a material known as effluent. The effluent exits through the opposite side of the tank from where it enters, where it flows into drainage trenches and is slowly absorbed into the soil.
These systems work best in yards where there is enough room to position the tank and drainage trenches as needed to maximize the effects of gravity on flow and drainage.
Sand Filter Tanks
A sand filter septic tank system contains two septic tanks connected by a network of drainage pipes. The first tank works just like in a gravity system, where solids settle to the bottom and liquids stay on top of the tank. As the effluent exits the tank, it passes into a separate tank, called a pump chamber. The pump chamber uses a sand filter to recirculate the effluent back through the first tank and break it down even further. Some larger solid particles in the effluent will break down, while others settle to the bottom of the tank. Once the effluent exits after recirculating, it can be absorbed into the soil much more quickly than it could initially. This type of tank is designed for areas with heavy soil, which makes it hard to dig large drainage areas, and for areas with soil that absorbs poorly.
Mound System Tanks
The mound system tank are designed for homes with small or shallow yards. They contain two tanks, including a traditional gravity tank and a separate holding tank. As the effluent exits the first tank, it settles into the holding tank for storage. A pump within this tank pumps the effluent out into a drainage trench at a slow enough rate to allow it to be completely absorbed. By controlling the flow of effluent, homeowners can ensure the material will be fully absorbed without saturating the soil. These systems get their names from the built-up mounds of earth that are generally used to hold the storage tank and drainage trenches.
Aerobic Treatment Tanks
According to North Carolina State University, aerobic treatment tanks are the most expensive and complex of all septic tanks and must be inspected four times a year for safety. These systems use two tanks: an initial aeration chamber and a holding chamber. Oxygen is pumped into the aeration chamber, where it mixes with wastewater and helps to break down solid materials more quickly. All waste products then pass to the second chamber, where the remaining solids settle to the bottom of the tank. The effluent from an aerobic treatment tank is considered very "clean" as it exits this tank and passes into the drainage field. These systems are used in environmentally sensitive areas where traditional septic tanks could cause harm to local ecosystems or waterways.
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