Designing Septic Tanks (USAID) - The Water, Sanitation and Hygiene

Sewer pipe. The pipe must be ofnoncorrosive, durable material such ascast-iron soil pipe, vitrified clay,concrete, bitumlnized fiber, asbestoscement, or plastic. The trench linefor the pipe should be as straight aspossible from the dwelling to the septictank and from the tank to theabsorption field. Avoid bends. Planthe pipe line so that it falls evenlyand smoothly from the dwelling to thetank with a slope of from 1 in 50 to 1In 100, and from the tank to the fieldwith a slope of 1 in 100. Figure 5shows the slope of these pipes. Thetotal length of pipe needed will be thedistance along the trench line from thedwelling to the tank and from the tankto the absorption field (see "DesigningSubsurface Absorption Systems,"SAN.2.D.l). Sewer pipe is generally1OOmm in diameter.When labor requirements, materials,(Including concrete mix, reinforcingmaterials, and sewer pipe), and toolshave been decided, prepare a materialslist and give it to the constructionforeman.ImportantConsiderations1. The location of the septic tankis vital. (Review section onlocation.) Provide the constructionforeman with a map similar to Figure 1,showing actual distances from the septictank to dwellings, water supplies,roads, and other man-made or prominentfeatures.2. The entire system must bedesigned so that sewage moves by gravityflow from the building, throughthe septic tank, and to the subsurfaceabsorption system. As Figure 5indicates:(a) The septic tank inlet must belower than the sewer line;(b) The septic tank outlet must belower than the inlet, usually 75mmlower;(c) The distribution box, If thereis one, must be lower than the septictank outlet;(d) The subsurface absorptionsystem must be lower than the distributionbox.All of these elevations must bechecked both on a drawing and In thefield prior to and during construction.(see "Designing Subsurface AbsorptionSystems," SAN.2.D.l).Give the construction foreman alocation map and plan view of the septictank containing all necessarydimensions and a detailed materialslist with quantities of all materialsneeded.lgure 5. Slope and Elevation Checkpoints