Sewer-Septic Tank Hybrid Promises Savings

by Richard J. Otis, Partner; Ayres Associates, Madison, WI,
Karen Sirotiak, Editor; Ayres Associates, Madison, WI,

Serial Information: Civil Engineering—ASCE, 1987, Vol. 57, Issue 8, Pg. 74-76

Document Type: Feature article


Until recently the only options for sewage were the septic tank/leach field or conventional sewers. In some cases a hybrid of the two, called the septic tank/effluent drain, is less costly and environmentally better. It consists of a septic tank at each house plus a small diameter plastic pipe that carries only clear effluent to a central sewage plant. This article describes the development of this system in Australia and includes case histories of its applications in Wisconsin, Alabama, and Ohio. Additional savings may be possible beyond those achieved in early U.S. installations. Two possible ways to cut costs are cleanouts instead of manholes, and using a positive grade or variable negative grade on gravity sewers in certain situations. Laboratory studies suggest that design criteria could be changed to permit drains as small as 2 in. and minimum flow velocities in depressed sections of 0.5 fps during peak flows.

Subject Headings: Sewers | Hybrid methods

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