Septic Advisor
Help with septic tank problems, odors, backups & repairs.

There are many different types of septic tanks.  The picture shows the top of a single compartment rectangular concrete tank, with three access ports (green cylinders) located over the septic tank inlet, outlet and middle. The most common septic tank variations are discussed below.

Number of Compartments.  Single compartment septic tanks and double compartment tanks are both common for residential septic systems.  See Figure 1.  In a double compartment tank, the first compartment holds two-thirds of the tank volume. The second compartment holds one-third of the tank volume.

...moreTwo compartment tanks  retain  solids and fats, oils & grease better than single compartment tanks. See Blog "Septic Tanks: 1 compartment vs 2."

Material of Construction. Septic tanks are usually made of precast reinforced concrete. Other septic tanks are made of fiberglass or reinforced plastic.  Older systems may have metal septic tanks. Metal tanks often have severe corrosion and safety issues.

...more The fiberglass and plastic septic tanks can be more cost-effective at remote locations, where delivery and installation vehicles cannot readily access the installation location. In high groundwater (or flood prone areas), the fiberglass or plastic tanks are subject to floating (moving upwards) and require ballast and/or tie-downs. Tank flotation can cause significant problems,  that include pipeline breakage, seam seal failure, groundwater flow into tanks, and system flooding.

Minimum liquid depth in septic tanks. Residential septic tanks are often designed with a minimum liquid depth of 4-ft or more. This depth provides adequate storage volume for solids so that maintenance pumping is typically required every two to three years.

...more Shallower tanks are sometimes installed, due to site constraints, such as shallow depth to bedrock. Note to Homeowners: shallower tanks require more frequent pumping. 

Baffle Type.  Concrete septic tanks may have precast concrete baffles or pipeline tee baffles installed at the tank inlet and tank outlet ends.  Plastic tanks may have preformed plastic baffles. Proper baffle placement and integrity are critical for retention of solids, as well as fats, oils and grease ("FOG").  The septic  tank outlet baffle protects your drainfield from plugging by solids and FOG. Our figures generally show the most common configuration for new installations — a plastic tee baffle in a concrete tank. 

The use of septic tank outlet filters is becoming more common, see "Outlet Filters."

Some septic tanks may be modified to provide more advanced treatment, see "Advanced Topics".

For information on locating your septic tank, see "Locating a Septic Tank".

Go to next page in Septic Advisor Tutorial.