Biological Systems Engineering


Date of this Version



WASTE MANAGEMENT D-3, Home Waste Systems Issued October 1980, 15,000.


Copyright 1980 U.S. Department of Agriculture


Homeowners in areas not served by municipal sewage treatment systems face a problem of providing safe and effective wastewater treatment systems for their homes.

There are several methods currently available for proper treatment of home sewage, including septic tank absorption fields, mounds, lagoons, aerobic treatment units and others. These systems and their advantages and disadvantages are discussed in this NebGuide.

Treatment and disposal of domestic sewage is of concern because of the variety of pathogenic organisms contained in sewage. These include bacteria, viruses, and other microorganisms that come from the digestive tract, respiratory tract, and skin of man. Some of the illnesses caused by these include dysentery, infectious hepatitis and typhoid. It is important to keep these disease-causing organisms from entering underground water supplies or surface waters through improperly designed or installed sewage treatment systems.

Septic Tank-Absorption Field System

Conventionally, the septic tank-absorption field system has proven satisfactory for many areas when properly designed, installed, and maintained. However, conditions do exist where this system is not suitable. Areas of seasonal high groundwater tables, bedrock in close proximity to the soil surface, or soils having very fast or very slow percolation rates are not suited for the septic tank-absorption field system. Other limitations for this system include topography, small lot size and proximity to water supplies used for drinking or recreation.

The typical septic tank-absorption field home sewage treatment system consists of two major components- the septic tank and the absorption field (Figure 1). In the septic tank, solids are separated from the liquid, 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, fine particles, and some chemicals.

Figure 1. Typical septic tank-absorption field home sewage treatment system.

The minimum septic tank liquid capacity for any location is I ,000 gallons (3,800 1). For houses having more than three bedrooms, an additional 250 gallons (950 I) of tank capacity is added per bedroom. Septic tanks must be water tight and constructed from durable materials that resist excessive corrosion, frost damage and cracking or buckling due to settlement or backfilling. Common construction materials include concrete, fiberglass and bitumastic coated steel.

Location of the septic tank is usually determined by the placement of the home plumbing and the topography of the land. Septic tanks should be located at least 15 feet (4.5 m) from foundation walls and at least 50 feet (15 m) from private water supplies or surface waters. The location should be accessible for cleaning but should not be located beneath sidewalks, patios or driveways. Also, consider possible expansion of the house when selecting a site for septic tank placement.