Biological Systems Engineering


Date of this Version



WASTE MANAGEMENT D-2, Home Waste Systems Issued June 1979, 15,000


A properly designed, installed ao1d maintained septic tank system is an approved method of sewage treatment for private residences in rural or urban areas where public sewage treatment systems are not available. The components of a septic system are the house sewer, the septic tank and the absorption field, as illustrated in Figure 1. Sewage flows to the septic tank through the house sewer. In the septic tank, the solids are separated from the liquids, undergo anaerobic digestion and are stored as sludge at the bottom of the tank. The liquid (septic tank effluent) flows to the absorption field where it percolates into the soil. The soil acts as a final treatment by removing bacteria, pathogens and fine particles.

All household wastes and normal amounts of cleaning materials, including soaps, detergents, bleaches, drain cleaners and other mild chemical preparations, can be tolerated by a septic system. Roof drains and foundation drains should not be connected to the septic tank system. These sources of clear water only overload the septic system and may cause premature failure of the absorption field.

Septic Tank Location

The site of a septic tank is usually determined by the location of major bathroom and kitchen plumbing within the home and by the slope of the land. Locate septic tanks at least 15 feet (4.5 m) from the foundation walls and approximately straight out from the point where the house sewer pipe goes through the wall. Table 1 gives minimum distances between the septic tank system and water lines, property lines and surface water supplies.

Locate the septic tank out of high traffic areas where excessive loads may damage the tank top. Keep clear of areas subject to flooding, ponding or surface drainage from surrounding areas. The location should be accessible to a driveway or other acceptable route so a tank truck can be driven close enough to pump out the sludge and clean the tank. Do not locate septic tanks under sidewalks or patios where the tank is inaccessible for pumping. Consider the possibility of future expansion and locations for additions, such as sidewalks, patios, garages and storage buildings, before selecting the tank site.

Sizing and Septic Tank Construction

The minimum septic tank size for any installation is 1,000 gallons (3,800 I). For more than 3 bedrooms, add an additional capacity of 250 gallons (950 I) per bedroom. Garbag3 disposals add an extra load to the system, requiring about 25 percent additional tank capacity. Minimum tank capacities are listed in Table 2.

Septic tanks must be watertight and constructed of sound and durable materials that are not subject to excessive corrosion, frost damage, cracking or buckling due to settlement or back filling. Common construction materials include concrete, fiberglass and steel. Minimum liquid depth of the tank should be 4 feet (1.2 m). The inlet should not be less than 2 inches (5 cm) above the liquid level. Many septic tank shapes are satisfactory and the shape may depend on the type of material used. Figure 2 illustrates a typical septic tank with sanitary tees used as baffles on the septic tank inlet and outlet.