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

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2Top ViewOutsidelength-1TInlet (1 OOmm sewer pipe)Raw sewage fromdwelling,,Effluent toabsorptionsystemSide and end walls:100-l 50mm reinforcedconcrete or 200mm brick masonry with 25mminside finish of cement plasterSide View Top:JOOmm reinforced concreteRaw sewage &Inlet 7” exends -into liquid minimum150mm. Maximumis depth of outlet ‘TFigure 2. Septic TankFlow line (same elevationT.5c ..‘. -‘; .,:.-.-..:. , ‘..-C-r :,.,.:.e.es::,.. .:.: :’: ‘, >.,:.:‘;‘: ,2.-..::.. .-.._.z:r,:’: 269: ._ +-fi into liquid 360-600mm,....;. a (40% of liquid depth)J sg: . .;.-’ 35..): .:. ::.... . : *‘.: .*.. r. . _ ,., ..,rl*,‘,.. .. y , : :. ‘r: ‘- ‘-., .:: .: : : y.:,: ‘; :.. . .. ,.f .,.... y:.Bottom: lOO-150mm reinforced concreteor 200mm brick masonry with 25mminside finish of cement plasterb Effluent3. The floor of the septic tank 5. Predesigned, prefabricated tanksshould be made of reinforced concrete may be available. They are made of100-150mm thick and should rest on a fiberglass, precast reinforcedbed of gravel or sand 75mm thick.4. The tank must have a strong,watertight top, usually made of reinforcedconcrete. The top should bedesigned in 300mm wide sections, witheach section as long as the septic tankis wide and equipped with a handholdnear each end. One or two sectionsover the outlet end can be removed toinspect the tank. All or most of thesections can be removed to clean thetank.concrete, or steel. The most Importantconsideration with these tanks isselecting the proper capacity (see nextpage, "Steps in Design").6. The recommended liquid depth Inthe tank is 1.2m, however, it may be asshallow as l.lm or as deep as 1.8m.7. The length of the tank is two tothree times its width (see Table 1).
8. The bottom of the inlet pipe (thepipe entering the septic tank, not the"T" fitting) Is 300mm below the top ofthe tank. The bottom of the outletpipe (the pipe leaving the septic tank,not the rtT" fitting) is 75mm below thebottom of the Inlet pipe, or 375mmbelow the top of the tank (see Figure2).9. The Inlet and outlet pipes arefitted with open "T" sewer pipe fittings.10. The sewer pipe from the source ofsewage to the septic tank and from theseptic tank to the subsurface absorptionfield is made of vitrified clay,concrete, special plastic, or othernoncorrosive material, and Is usually1OOmm In diameter.11. After the tank has been constructedand installed, the space betweenthe septic tank's outside wallsand the earth sides of the hole shouldbe carefully filled and the septic tankshould be covered with dirt to gradelevel or above.Table 1. Suggested Septic Tank DimensionsVolumeliters1100150019002800380047005700760095001100013000150001900023000260003000038000Lennth. Width, Liquidinside- insld Ie Depthmeters meters meters1.51.81.82.32.73.43.24.34.45.24.94.95.96.27.37.08.50.70.90.91.11.21.21.51.51.81.81.82.22.22.42.42.42.41.10.91.21.21.21.21.21.21.21.21.51.51.51.51.51.81.8\Steps in DesignRefer to Worksheet A in followingthese steps. This is only a samplethat has been filled out; you will needto prepare one for your use.1. Determine how much sewage willenter the septic tank during each24-hour period (see "Estimating Sewageor Washwater Flows," SAN.2.P.2).2. Determine the desired retentiontime. It should be a minimum of oneday and a maximum of three days. Whendetermining retention time, considerthe following:(a) A longer retention timerequires a larger tank, and thus ahigher initial cost. However, a largertank needs to be cleaned less oftenthan a smaller one. Larger tanks alsotreat the sewage more, which Increasesthe life of the subsurface absorptionsystem.(b) A shorter retention timerequires a smaller tank, and thus reducesthe initial cost.3. Determine the capacity of thetank. Do this by multiplying the dailyflow by the retention time (In days).For example:Suppose that the estimated dailyflow of sewage is 1500 liters, andthe desired retention time Is 2.5days. The tank capacity Is 1500liters a day times 2.5 days: thetank capacity equals 3800 liters.4. Is the type of building thesystem is being designed for a familydwelling or public building? If it Isa family dwelling, go to step 5. If itis a public building, go to step 4a.4a. Septic tanks for certain types ofpublic buildings, such as schools,stores, and factories, must be largeenough to receive sewage when all ormost of the sewage flow takes placewithin a few hours. For suchbuildings, first determine the normalsize of the septic tank (steps 1, 2,and 3). In this example, the flow is3800 liters a day.3
Page 1: Water for the WorldDesigning Septic
Page 8 and 9: For example, suppose the septic tan

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