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

Water for the WorldDesigning Septic TanksTechnical Note No. SAN. 2.D.3Designing a septic tank involvesselecting a location for the tank,calculating the tank's capacity anddimensions, and determining thenecessary labor, materials, and tools.The products of the design process are(1) a location map, (2) a plan and elevationview of the tank, and (3) adetailed materials list. These itemsshould be given to the constructionforeman before construction begins.Figure 1 Is a sample location map. Theactual location map will be needed bythe construction foreman prior to andduring construction, and will enablehim to locate the system properly.r ------------I Ground slopeThis technical note describes how todesign a septic tank and arrive atthese three end-products. Read theentire technical note before beginningdesign procedures.MaterialsNeededMeasuring tape - To obtain accuratefield Information for a location mapRuler - To draw a location maplocationatLocate the septic tank downhill andleast:15m from the nearest water supply,including the neighbors',3m from the nearest building,3m from the property line.Do not put the septic tank in anarea where rainwater or other surfacewater will flow over or stand on it, orwhere vehicles will drive over it.When looking for a location for theseptic tank, draw a map of the lotshowing actual distances from the septictank to water supplies, dwellings,property lines, vegetation, any othermanmade or prominent geographicalfeatures, and the proposed (or actual)site of the subsurface absorptionsystem (see "Designing SubsurfaceAbsorption Systems," SAN.2.D.l).GeneralDistances shown are minimum distancesII-------_--J /IFigure 1. Sample Location MapDesign InformationStudy Figure 2 while following thesesteps.1. All sewage from the building tobe served should enter the septic tank.This includes both excreta and washwater,but it does not include rainwater,surface water, or subsurfacedrainage.2. The septic tank walls may be madeof poured reinforced concrete, brick,concrete blocks, or stone masonry. Thewalls must be watertight. A 25mm thickinside coating of cement plaster,usually applied in two 12mm coats, Isneeded to make laid-up walls watertight.

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

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For example, suppose the septic tankhas a capacity of 3800 liters. First,calculate the dimensions of the tank asin Worksheet A. Then proceed withsteps (11, (21, (3) above using a formlike Worksheet B for the calculations.This worksheet is only a sample thathas bee.n filled out; you will need toprepare one for your use.1. The volume of the top is the outsidelength times the outside widthtimes the thickness. The outsidedimensions are used because the topmust span the inside dimensions plusthe thicknesses of the walls.The volume of the bottom is thesame as the volume for the top.The volume of the two side walls isthe inside length times the insideheight times the thickness times two.The volume of the end walls is theinside width times the inside heighttimes the thickness times two.The total mixed volume is the sum ofall these volumes, in this example2.lm3.2. Multiply the mixed volume by 1.5to determine the unmixed volume. Inthis example, the unmixed volume is3.15m3.3. Multiply the unmixed volume byeach percentage to find the quantity ofeach material. In this example, thequantities are:cement 0. m3sand 0.9, 3gravel 1.4m3water 0.3m3 (300 liters)Reinforcing material. If the reinforcingmaterial is chain-link fencingor wire mesh, the quantity will beapproximately equal to the combinedsurface area of each slab. Add thearea of the top plus the area of thebottom plus the area of the two sidesplus the area of the two ends. Thearea of the top is the outside lengthtimes the outside width; the area ofthe two ends is the inside width timesthe inside height times two (seeWorksheet B).If the reinforcing material is steelbars, sketch each slab (see Figure 41,draw in the bars using the criteriagiven in Figure 4, and count the bars.Remember, a separate quantity Is neededfor each length of bar.Top View25mmnEnd ViewFigure 4. Steel Rod Placement in Reinforced Concrete SlabL8

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