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Design of a Concrete Septic Tank

Tom Raley

If you moved into a home with a septic system, you have probably never had the opportunity to see your septic tank. The most common type of tank is made of concrete and will hold approximately 1,000 gallons. Some homes have a 1,200-gallon tank, but few residences have much larger than that.

The design is very simple, very reliable and critical to the efficient working of your system.

The Construction

The exterior walls of the septic tank are made of concrete, normally 4 inches thick. The concrete is either a minimum of 4,000 or 5,000 PSI concrete. A 1,200-gallon tank can weight as much as 8,000 pounds, so these are not items a homeowner can install on his own. Tank dimensions vary somewhat by manufacturer, but a 1,000 gallon septic tank, usually is 8 feet, 6 inches long by 5 feet, 5 inches tall by 4 feet, 10 inches wide. The tops of some tanks are 5 inches thick for greater strength. Vehicle traffic never should be allowed on top of a septic tank.


Tank designs vary, but each tank has an opening on each end to allow standard 4-inch PVC pipe to enter and exit the tank. The hole on one end is 4 inches to 6 inches higher than the other. The high end is the inlet, and the low end is the take-away pipe. On the top of the tank is an access hatch which can be used to inspect the interior of the tank, and, on some tanks, it is used to clean the tank. Some tanks have additional, smaller access holes that can be used to bring PVC pipe to the surface. This can be very helpful when cleaning out the tank. Without these smaller pipes, cleaning requires finding the tank under ground and digging to find the access hatch.

Interior Of The Tank

Some tanks have two compartments, but most have only one large chamber. At the inlet, there is a baffle to deflect incoming waste toward the bottom of the tank. Without the baffle, the waste could be propelled across the tank and into the take-away pipe. This could clog a field line and result in costly repairs. There is also a baffle on the take-away side of the tank to help prevent solids from entering the take-away pipe. Should your tank ever be exposed while being emptied or cleaned, never allow anyone to enter the tank. The fumes and lack of oxygen can result in rapid suffocation.