by Arlene Miles

Created on: March 14, 2011   Last Updated: March 25, 2012

Sewer septic tanks must be designed to treat and dispose of a specified amount of wastewater produced by a building. Because the tank is one of two major components along with the leach field in designing a septic system, minimal size and depth are important in the overall design. Hazardous chemical and large amounts of grease should never enter a septic tank. It should also be pumped periodically to avoid buildup of sludge and scum.

• Tank Capacity

Local ordinances vary, but most governing bodies overseeing regulating installation of septic systems require a minimum capacity of 1,000 gallons. Proper capacity, however, is determined by dwelling’s size, which takes into account the number of bedrooms and if any devices such as whirlpool baths are present. Septic tanks should provide retention for at least 24 hours. The 1,000-gallon capacity is considered minimum for three-bedroom houses, with each additional bedroom and whirlpool requiring an extra capacity of 250 gallons.

• Sizing and Configuration

The University of Nebraska recommends 42 to 78 inches for tank depth. At least four feet must separate inlet and outlet baffles, which help ensure settling of solids and venting of gases. Tanks that have a minimum 3,000-gallon capacity have at least two compartments, although smaller units may also have the same configuration. Multi-unit tanks help reduce the chance of sludge entering the drainfield. When three or more tanks are used, the first tank must hold one half of total capacity. Inlet baffles prevent scum from building and entering the sewer pipe, while outlet baffles prevent scum from flowing into the effluent treatment system.

• Placement

If possible, septic tanks should be placed in such a way that gravity will aid in the movement of fluids through the system. When this arrangement is not possible, pumps must be installed to move solids and liquids from the dwelling into the tank. The unit should be placed at least 100 feet from a drinking water source, 50 to 75 feet from a body of water and 10 feet from water lines. 

• Installation

Concrete, fiber-reinforced plastic, high-density plastic or fiberglass may be used to construct a septic tank. The structure must be able to withstand pressure from the soil above it when empty

Excavation of a hole for the tank should be deep enough to allow two to three feet of soil to cover it. Concrete or asphalt, however, should not be placed on top of the soil layer. A layer of sand or fine gravel up to six inches deep is often placed at the bottom of the excavated pit. Tanks installed in areas with high groundwater are either anchored or filled with a ballast layer to prevent flotation when empty.

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