2.Design of On-site Sanitation systems.pptx

Lecture note

1. Design of On-Site Sanitation Systems

2. Basic Design Principles of VIP latrine The vent pipe should have: an internal diameter of at least 110mm to a maximum of 150mm and reach more than 300mm above the highest point of the toilet  superstructure . It can be made out of PVC, bricks or iron pipes . The mesh size of the fly   screen  must be large enough to prevent clogging with dust and allow air to circulate freely To construct the pit and choose the location for the VIP, the same rules as for the  pit latrine  must be considered. Pits design life of 15 to 20 years is perfectly reasonable.

3. Basic Design Principles of VIP latrine The depth of the pit is at least 2m , but usually more than 3m . The depth is usually limited by the  groundwater  table or rocky underground.  Lining  the pit prevents it from collapsing and provides support to the  s uperstructure. Because of the static properties, a round pit with a diameter of more than 1.5m guarantees a stable construction and avoids collapse. A horizontal distance of 30m between the pit and a water source is recommended to limit exposure to contamination. In densely populated areas with many pit latrines , the risk of a groundwater  contamination is even higher .

4. Basic Design Principles of VIP latrine

5. Pit sizing for latrines To size pits or tanks , it is important to determine: the rate at which sludge (including faeces, urine and anal cleansing material) will accumulate , and the rate at which effluent will infiltrate in the surrounding ground. The top 0.5m of a pit should not be filled: to allow for safe backfilling and to prevent splashing, unpleasant sights and increased odour and fly nuisance. The approximate m 3 size of the pit can be calculated as a function of the following equation: and  

6. Pit sizing for latrines V: Pit volume ( m 3 ) N: Number of users S: Sludge accumulation rate (liters/cap/year ) is listed in the table below D: Pit life (years )_ Design period/life F: Infiltration area ( m 2 ); (water depth = F/pit circumference) W: Amount of water used for flushing (liters/cap/day ) I : Infiltration rates ( liters/m 2 /day ) Sand 40 Sandy loam 25 Silt loam 20 Clay loam 8 Clay unsuitable

7. Pit sizing for latrines Sludge Accumulation Rates Waste deposited and conditions Sludge accumulation rate, S (liters per capita per year) Waste retained in wet environments where degradable anal cleaning materials are used 40 Waste retained in water where non-degradable anal cleaning materials are used 60 Waste retained in dry conditions where degradable anal cleaning materials are used 60 Waste retained in dry conditions where non-degradable anal cleaning materials are used 90 The term ‘ wastes retained in water ’ when applied to a pit latrine means that wastes are in a section of the pit below the water-table. In emergency situations (rapid accumulation), these rates should be multiplied by 150 − 200%

8. Pit sizing for latrines This method assumes that liquid waste is absorbed by the surrounding soil. If liquid remains in the pit it will fill far more rapidly. It should also be noted that soil pores become clogged with time, thus reducing or even stopping infiltration . Pits should therefore be oversized rather than under-sized, especially where soil infiltration rates are relatively low .   A = pit-base area (m 2 )

9. 9 The pour-flush squatting pan consist of a steep bottom slope and a water seal trap. From there, faeces and flushing water is directed to one of two leach pits . An inspection chamber containing a Y junction is normally built between the pits and the pan so that the excrete can be channeled into either pit. Each pit is designed to last for about 3 years before it gets filled . 2. Design Principles of Twin-Pit Pour-Flush Toilets Pour-flush Toilet ( Twin -pit Model )

10. 10 2 . Design Principles of Twin-Pit Pour-Flush Toilets The  water seal  at the bottom of the pour-flush toilet or pan should have a slope of 25 to 30°.   Water seals  should be made out of plastic or ceramic to prevent clogs and to make cleaning easier (concrete may clog more easily if it is rough or textured). The optimal depth of the water seal is approximately 2cm to minimize the water required to flush the excreta. The trap should be approximately 7cm in diameter.

11. Pits location If site conditions do not permit this layout, the pits can be placed on the side or even in front of the pan. Toilets can be constructed inside the house , while the pits can be situated outside the house . To remain accessible, pits should be constructed in open ground. But if no space is available, they can also be constructed below the toilet. Pits should not be situated in drainage lines or the paths of storm water drains to prevent cross-contamination . The pits should not be located in depressions where water is likely to collect. The pits should be constructed over 1m from any structural foundation as leachate can negatively impact structural supports.

12. 1 st pit 2 nd pit urine-collection

13. As pathogens move through unsaturated soil, they die off. The degree of faecal organism removal varies with Soil type; Distance travelled; Moisture Hydro-geological conditions of sites where leach pits are to be located are prerequisites. If hydro-geological conditions are not known, minimal distance of 30 water sources should always be maintained! No risk in alluvial soils (silt mixed with fine sand) exists and where the pit bottom is at least 2m above the maximum ground-water level. Virus and bacteria can travel hundreds of meters in saturated conditions. There is a risk for groundwater pollution where Water table is high Crack and fissures in bedrock allow short- cutted flow Treatment Efficiency

14. Design features of septic tank As septic tank is a settling digestion tank , its rational design is based on the following three functions it is expected to perform: Sedimentation to remove the maximum possible amounts of suspended solids from the sewage Digestion of settled solid resulting in a much reduced volume of dense, digested sludge, and Storage of sludge and scum accumulating in between successive cleanings thereby preventing their escape Hence, the tank should be capable of storing the sewage flow during the detention period and additional volume of sludge for 6 months to 3 years depending on the periodicity of cleaning

15. Cont …. Detention period : A detention period of 24 to 48 hours based on the average daily flow of sewage can be used. Inlet and out let baffles: The baffles or tees should extend of about 20cm above the top sewage line. Inlet should penetrate by about 30cm below the top sewage line and outlet should penetrate of about 40% of the depth of the sewage. The outlet invert level should be kept 5 to 7.5cm below the inlet invert level

16. Cont …. Length to width ratio: Septic tanks are usually rectangular with their length at about 2 to 3 times the width ( L = 2B to 3B ) The width should not be less than 90cm The total depth of the tank generally ranges between 1.2 to 1.8m Sludge withdrawal : The rate of accumulation of sludge has been recommended as 30l/c/year ( 0.082 l/c/day ) Sludge is withdrawn from the septic tank either half yearly or yearly . For small domestic tanks, de- sludging may be done at least once in 6 months to 3 years .

17. Septic tank dimensions Inside total height  1.5m  Height of the liquid  1.2m   Ratio L/W  2  Thickness of floor & ceiling  150mm Thickness of the walls  200m   Min. height between inlet and outlet pipes 75mm   Table 1: Minimal dimensions of a septic tank for isolated dwellings

18. Septic tank plan view

19. Sectional Dimension of Septic Tank Schematic of conventional septic tank Inspection opening 150 mm diameter Inlet At least 75 mm At least 20mm Access opening near side wall at least 600 mm diameter Inspection opening 150 mm diameter Liquid level 20% of Liquid depth Water line 40% of Liquid depth Scum clear space (75 mm, minimum) Sludge clear space (300 mm, minimum) 20% of Liquid depth (150 mm, minimum) Scum Clear space Sludge Sludge Outlet 40% of Liquid depth Liquid depth First compartment 2/3 length second compartment 1/3 length Total length equals two to 3*width

20. Septic tank dimensions Average  sewage wastewater flow m³/day   Minimum septic tank size in  m 3  of  effective  capacity needed    0 ‐ 1.9   3.4 2.3 ‐ 2.65  4.5 3.0 ‐ 3.4   5.7 3.9 ‐ 4.7   7.2 7.6 ‐ 9.5   12.1 17.0 ‐ 18.9   22 Table 2: Minimum capacity of septic tank based on the average daily flow Table 3: Minimum capacity of septic tank based on the number of bedrooms Number of  bedrooms   Septic  tank   size  ( m 3 )   0 ‐ 2    2.8 3   3.8 4   4.5 5 ‐ 6    5.7

21. Septic tank dimensions Table 4: Suggested septic tank dimensions Volume m 3 Length, inside in m Width, inside in m Liquid depth in m 1.1 1.5 0.7 0.9 1.5 1.8 0.9 1.1 1.9 1.8 0.9 1.2 2.8 2.3 1.1 1.2 3.8 2.7 1.2 1.2 4.7 3.4 1.2 1.2 5.7 3.2 1.5 1.2 7.6 4.3 1.5 1.2 9.5 4.4 1.8 1.2 11 5.2 1.8 1.2 13 4.9 1.8 1.5 15 4.9 2.2 1.5 19 5.9 2.2 1.5 23 6.2 2.4 1.5 26 7.3 2.4 1.5 30 7 2.4 1.8 38 8.5 2.4 1.8

22. Guidelines for installation For selecting a site to install the septic tank, these guidelines should be followed. Stay at least 30.5m from drinking water sources, 15.25m from streams or ponds and 3m from water lines. Slope drain fields away from houses, buildings and the water supply. Keep drain fields unshaded and free from trees and shrubbery. Allow sufficient space to enlarge the drain field if it should become necessary. Keep septic tanks or drain fields uncovered by driveways Locate septic tanks and drain fields away from drainage areas and waterways . Never use an open flame or matches to inspect a septic tank. Sewer gases may explode violently.

23. Guidelines for installation

24. Effluent disposal in septic tank The effluent coming out of the tank will be se`ptic and foul. Final purification of the effluent and the removal/ death of pathogen can be done by the following methods: Soil absorption systems Upflow filters Biological filters Soil absorption systems: Seepage pit or soak pit Dispersion trenches

25. Effluent disposal in septic tank Soak pits or dispersion trenches can be adopted in all porous soils where: percolation rate is below 30 minute per cm and the depth of water table is 1.8m or more from the bottom level of the drain field. Percolation rate of soil or ground is the time in minutes required for seepage of water through that ground by 1cm . Incase, sufficient porous ground is not available, the effluent of the septic tank shall be subjected to a secondary treatment: biological filter up-flow anaerobic filter

26. Effluent disposal in septic tank The total subsurface soil area required for the soak pits or dispersion trenches shall be worked out on the basis of: the maximum allowable rate of effluent application which is given by the following empirical relation:  Where: Q – maximum rate of effluent application in l/d/m 2 of leaching surface t – standard percolation rate for the soil in minutes per cm In calculating the effective leaching area required, only area of trench bottom in the case of dispersion trenches (0.5 to 1m deep and 0.3 to 0.9m wide) and effective side wall area below the inlet level for soak pits should be taken into account  

27. Disposal in Soak pits A soak pit is a circular covered pit through which the effluent is allowed to be soaked or absorbed into the surrounding soil. The depth of a soak pit should be between 1.5 and 4m . The soak pit may either be filled with stone aggregate or may be kept empty . When the soak pit is empty, the pit is lined with brick , stone or concrete blocks with dry open joints. When the soak pit is filled with stone or brick aggregate, no lining is required except for a top masonry ring constructed to prevent damage by flooding

28. Disposal in adsorption trenches

29. Disposal in adsorption trenches

30. Disposal in adsorption trenches

31. Example Design a septic tank for the following data: Number of people = 100 Sewage/capita/day = 120 liters De- sludging period = 1 years Length: Width = 3:1 What would be the size of its soak pit if the effluent from this septic tank is to be discharged in it. Assume percolation rate through the soak pit to be 1250l/m 3 /d Solution Quantity of sewage produced per day = 12000 liters/day Assuming the detention period to be 24 hours, The quantity of sewage produced during the detention period, i.e the capacity of the tank

32. Example Assuming the rate of sludge deposit as 30 liters/capita/year and with the given 1 year period of cleaning, The quantity of sludge deposited Total required capacity of the tank Assuming the depth of the tank as 1.5m, the cross-sectional area of the tank Using L:W as 3:1  

33. Example Therefore the dimension of the tank will be 0.3m is for the free board . Hence use a tank of size 5.5m*1.8m*1.8m D esign of soak pit The soak pit can be designed by assuming the percolating capacity of the media, as 1250 l/m 3 /d Sewage outflow = 12000l/d Percolation rate = 1250l/m 3 /d Volume of filter media required for the soak pit If the depth of the soak pit is taken as 2m, The area of the soak pit required  

34. Exercise Estimate the size of a septic tank (length to width ratio of 2.25, liquid depth of 2m with 0.3m freeboard), de- sludging interval in years and the total trench area of the percolation field for a small colony of 300 people. Assume water supply of 100 l/c/d, wastewater flow at 80% of water consumption, sludge production of 0.04m 3 /c/y and the retention time of 3 days at start up. De- sludging is done when the tank is one-third full of sludge. A percolation test indicated an allowable hydraulic loading of 100 l/m 2 /d. Design the absorption field system for the disposal of septic tank effluent for a population of 100 persons with sewage flow rate of 135 l/c/d. The percolation rate for the percolation test carried out at the site of the absorption field may be taken as 3 minutes.

35. Design Considerations of Imhoff Tank Sedimentation chamber Detention period : 2 to 4 hours (usually 2 hours) Flowing velocity should not be more than 0.3m/minute Surface loading should not exceed 30,000 liters/m 2 of plant area per day In case of effluent coming from activated sludge plant the surface loading can be adopted to 45,000 l/m 2 /d Length of tank should preferably not exceed 30m or so . Length to the width ratio varies between 3 to 5 Depth of the chamber should be kept shallow as far as possible. Practically a total depth of 9 to 11m has been found to be satisfactory. Depth of SC 3 to 3.5m . The free board should be provided may be about 45 cm

36. Design Considerations of Imhoff Tank Digestion chamber It is divided into a number of interconnected compartments . The bottom of each digestion compartment is made up in the form of an inverted cone or hopper with side sloping 1:1 , so as to concentrate the sludge at the bottom of the hopper. The digested sludge from the bottom of the hoppers is removed periodically (after 30 to 45 days , depending upon the temperature of sludge) through the cast-iron de- sludging pipes provided in each compartment. The chamber is generally designed for a minimum capacity of 57 liters per capita In warmer climates the capacity may be reduced to about 35 to 40 liters per capita , where shorter periods between the sludge withdrawals are possible

37. Design Considerations of Imhoff Tank Gas vent or Scum chamber It is provided above the digestion chamber and alongside the sedimentation chamber to take care of the gases escaping to the surface. The surface area of the scum chamber should be about 25 to 30% of the area of horizontal projection of the digestion chamber. The width of vent should be 60cm or more the scum and sludge must be maintained at least 45cm above and below the slot respectively. to prevent the particles of the sludge or scum from entering into the sedimentation chamber from the digestion chamber

38. Example Design an imhoff tank to treat the sewage from a small town with 30,000 population. The rate of sewage may be assumed as 150 l/c/d. Make suitable assumptions, wherever needed. Solution Design of Sedimentation Chamber The sewage discharge per day Assuming a detention period of sewage in the sedimentation chamber as 2 hours, The volume of sewage entering in two hours, i.e. the capacity of the sedimentation chamber  

39. Cont’d… Assume an effective depth of 2.2 m (effective depth includes part of the bottom sloping walls of the chamber) and a width of 4.3 m (say). The length of the sedimentation chamber This length is too large for a single tank. So let us adopt two tank units, each of length 20 m and width 4.3 m ; then Which is in the range of 3 to 5 and, therefore, it is satisfactory. The discharge passing through each unit  

40. Example Check for velocity Length of tank = Velocity * Detention time 20m = Velocity in m/min * (2 * 60 min.) Check for Surface Loading Hence, the dimensions chosen can be accepted. . . .