Septic Tank Guidelines | Septic Tank | Sewage Treatment

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A septic tank takes raw sewage in, allows the solids to settle (sludge) and allows the remaining liquid to flow into the surrounding soil by means of a soakaway. Scum on the surface is also prevented from leaving the tank. Microorganisms in the anaerobic environment in the tank digest the sludge and scum. The system consists of several stages, supply to the tank, the tank itself and the soak field. Septic tanks take sewage (grey water - washing and household waste and black water - sewage from latrines,) but not rainwater. Sludge volume is reduced by microbial action but still needs periodic emptying. Septic tanks provide partial treatment of wastewater. The soakfield provides secondary treatment in the form of subsoil infiltration. Septic tanks are suitable for conditions where the wastewater can drain away and be absorbed into the soil without contaminating ground water where it is extracted. Sealed solid waste storage is an option if soil is unsuitable or the water table is too high. Cesspits are another option.
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  Septic Tank Guidelines Septic tanks are suitable for conditions where the wastewater can drain away and be absorbed into the soil without contaminating ground water where it is extracted. Sealed solid waste storage is an option if soil is unsuitable or the water table is too high. Cesspits are another option. Septic Tanks  A septic tank takes raw sewage in, allows the solids to settle (sludge) and allows the remaining liquid to flow into the surrounding soil by means of a soakaway. Scum on the surface is also prevented from leaving the tank. Microorganisms in the anaerobic environment in the tank digest the sludge and scum. The system consists of several stages, supply to the tank, the tank itself and the soak field. Septic tanks take sewage (grey water - washing and household waste and black water - sewage from latrines,) but not rainwater. Sludge volume is reduced by microbial action but still needs periodic emptying. Septic tanks provide partial treatment of wastewater. The soakfield provides secondary treatment in the form of subsoil infiltration. Designing and making a system OXFAM Technical Briefs – Septic Tanks and Drainfields 1 Initial Design 1.Choose a suitable location. This should bedownhill from the source of sewage. At least 15mfrom the nearest water supply. This is a minimumand should be more if the ground is rocky andfissures could take the outflow further. It shouldbe at least 3m from the nearest building. Avoidareas where rainwater would stand or flow overthe tank or vehicles could drive over it.2.Draw a plan showing the septic tank and distancesto dwellings, property lines, wells, water sourcesand any other prominent manmade or naturalfeatures. Show the ground slope.3.Calculate the volume of the tank. The volumeconsists of two components. Sludge storage andLiquid retention volume.4.Calculate how much sewage will enter the tank in24 hours (Daily Flow). This depends on thecontext. If the water supply is known then sewagecan be taken as 90%.e.g. DF=.9 x DWS DF - Daily Flow DWS – Water supply. If monthly water supply is 1800 Litres DWS = 18000 / 30 = 600 litres per day. DF = 60 x 0.9 = 540 Litres 5. Decide on a retention time (RT) of 1 to 3 days. Choose a long retention time to minimize cleaning frequency (running costs). Choose a short retention time to minimise tank size and initial cost. Larger tanks allow the sewage more time to digest and thus reduce the load on the drainage system. Smaller tanks (< 6m3) need longer retention times due to increased turbulence (i.e. 2 or 3 days.)  6. Multiply the daily flow by the retention time in days.  i.e. 540 litres daily flow X 2 days = 1080 litres capacity  . Note : If all the flow takes place during an   8 hour period this must be multiplied by 3 (24/3) This could be the case in a school or institutional setting. (e.g. 3240 litres)   7.The volume needed for sludge accumulation in warmclimates is:B = P x N x S x F P = Number of people using the system. B = sludge storage capacity in litres. N = the number of years between sludge emptying. S = rate of sludge and scum accumulation i.e. 25 litres per person per year for tanks receiving WC waste only, and 40 litres per person per year for tanks receiving WC waste and sullage. (Ref C.) As a rule of thumb 2/3 of storage volume is for sludge and 1/3 scum. F = Sludge Digestion Factor (=1 in warm conditions.) See Appendix 2. e.g. for 100 people, emptying the tank every two years in a warm climate. B=100 x 2 x 40 x 1 = 8000 litres. 8.Total Volume TV = B + ( DF x RT)e.g. TV = 8000 + ( 540 x 1.5 ) = 8810 Litres  A minimum volume of 1.3m 3  is suggested to minimize turbulence. 9.Total Length (3W) should be two or three times thewidth (W). (Ref. B)So for a 9500 Litre tank W = 1.8 m 3W = 5.4 m Depth = 1.2 m    Fig 1. Septic Tank.   Septic Tank Checklist.  Your design should:  Contain wastewater long enough for maximum removal of suspended solids.  Prevent suspended solids from being discharged into the soakfield with the effluent.  Provide enough sludge and scum storage space.  Ensure no blockages are likely.   Allow adequate ventilation for gases. Groundwater Protection Permeability Test.   Septic tanks are only suitable where soil is permeable enough to allow the soak field to absorb the outflow from the tank, but not so permeable as to cause problems of pollution of the groundwater down stream. The Two Meter Rule. This says that if there is 2 metres of fine sand or loam separating the drainfield and the ground water then virtually all pathogens will be removed. (Ref. A.) This must be true all year round. Water is safe after travelling for ten days. So water can be extracted at least 15m away from a soakaway if the soil is fine. Limestone or fissured rock allows pathogens to travel much further. In unsuitable areas a mound soakaway should be used so that there is at least 2m of soil between the bottom of the soakaways and the water table. These can absorb 5 litres per m2 per day. Water goes into the atmosphere by evapotranspiration by means of plants such as alfalfa. Sewer Line 1.The sewer pipe carries the sewage to the septic tank.The line should be at least 15 m away from the source(latrine block, etc.) and down hill from any nearby wellor spring, it should be water-tight joints and a uniformslope (minimum 2%). The line should be as straight aspossible. 2. The pipe should be made of non-corrosive,hardwearing material (i.e. vitrified clay, concrete,plastic or cast iron.) 3. For latrine blocks individual traps or access points foreach latrine should be considered to avoid blockagesin the sewer line. Septic Tank Construction 1.The walls of the tank can be made of poured,reinforced concrete, stone masonry, brick or concreteblocks. The tank should be made water tight with a25mm coating of cement plaster, applied in two coats,in order to avoid infiltration around the tank andmaintain and anaerobic space. For small tanks thefloor need not be reinforced. The space between thewalls and the side of the hole need to be filled withgravel or suchlike. The base should be at least 15cmthick. 2. The tank should be divided into two compartments.The first should be twice as big as the second. Thereis hole in the separating wall which allows liquid toflow through but not scum or sludge. Max velocitythrough holes is 0.1m/sec to minimise turbulence. Fig 2. Basic Tank Dimensions. 3. Inlet and outlet pipes consist of T pipes. On the outletthis is to avoid scum or solids going into the soakfield.On the inlet this is to reduce turbulence. The base canslope down towards the inlet in a large tank to allowmore sludge to be stored. The outlet on a larger tank can be a weir design. 2W W W (0.6m) min. 1.2m min. 1.5m ideal 3W max. inlet sludgescum to soakfield 2WW 40% liquid depthBaffle OXFAM Technical Briefs Septic Tanks and Drainfields. 2  Fig 3. Weir design. 4.If the tank will be below the ground water level atany time the weight of the empty tank should begreater than the weight of water displaced,otherwise the tank may float.5.Ventilation – The inlet waste water pipe should beventilated above head height in order to allow thegasses produced in the tank to escape.6.The roof of the tank can be made of removablesections with lifting handles (easy access) or asolid, reinforced concrete roof with round accessholes (min diameter 0.6m) (cannot fall into tank).These provide access to the tank for desludging,checking levels and maintenance. Soak Field Design 1.A soakfield consists of a series of 15-30m longtrenches with open jointed 100mm diameter pipeslaid on rocks, broken bricks or gravel. Thetrenches should be narrow and deep. They shouldbe arranged in series so that each trench overflowsinto the next one. This means that the trencheswill either be fully used or not and avoids a crustforming on the sides. Trenches should be 2mapart. The length of the trench can be calculatedusing the formula below. Note the base area of the trench rapidly becomes blocked so cannot beincluded. If the septic tank is not working well theinfiltration rate will be lower because the trenchwill get clogged with solids.L=(N x Q) / 2 x D x IL = Length in metres.N = Number of users.Q = Wastewater flow in litres per person per day.D = Effective depth in meters.I = Infiltration rate in litres per m2 per day.(see table 1.) Fig 4. Soakfield trench section. 2.The top of the pipe should be laid about 5cm underthe building paper/ Straw.3.The bottom of the trench should be above the watertable. This may mean a mound has to be made.4.Pipes can be made porous by making them out of concrete without sand, not sealing the joints or, in thecase of plastic pipes cutting slots or holes in them (atleast 6mm).5.The slope of the trenches should be6.Other options are soak pits (small systems), ponds,reed beds or evapotranspiration mounds (high groundwater). Fig 5. Soakfield. 15cm 15cm 45cm outlet 0.3m TopsoilBuilders  paper or straw layer.Porous or loose jointed  pipe1.5 - 2 m Gravel or stones. 0.3-1.0 m From Septic Tank 2m min.Slope OXFAM Technical Briefs Septic Tanks and Drainfields. 3  Fig 6. Line joining trenches in Soakfield. InletFirst ½ m Lined Boulders support  pit walls. Table 1. WHO Suggested Infiltration Capacities (Ref. C.) Type of soil Infiltration Capacity. (L / m² per day) (SIR) Coarse / medium sand  50 Fine sand, loamy sand 33 Sandy loam, loam 25 Porous silty clay / porous silty clay loam 20 Compact silty loam, compact silty clay loam and non-expansive clay 10 Expansive clay <10 Soak Pit Design. 1.In areas where the ground water level below 4m asoak pit can be used. These can be lined (like awell) or unlined and filled with rocks.2.The area of the soak pit does not include the base.WA = DF / SIR e.g. WA = 540 L / 50 L/m2 = 10.8 m2WA =Wall AreaDF = Daily FlowSIR = Soil infiltration rate (See table 1).3.Calculate pit dimensions below inlet pipe.D = WA / π  x PDe.g. D = 10.8 / 3.14 x 1 = 3.4mD= depth in metersPD = pit diameter in meters. π   = 3.144.Add depth of inlet pipe or 0.5m whichever is thehighest.TD = D + IPD TD = Total Depth D = Depth IPD = Inlet pipe depth e.g. TD = 3.4m +0.6m = 4 m So for this example the ground water depth should be at least 6m. Overflow line Fig 7. Unlined Soakpit.   Maintenance 1.Organise a maintenance system with schedule(Appendix 1.) and a manual.2.Do not overload the system – this will cause cloggingof the drainfield.3.Measure sludge and scum levels regularly and emptywhen needed. Check baffles.4.Do not put strong or hazardous chemicals into thesystem or use disinfectant. OXFAM Technical Briefs Septic Tanks and Drainfields. 4
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