Design of septic tank calculation – with Numerical for 20 user

Hi, friend you came here to get ideas and learn that, Design of septic tank calculation and process of construction. Well, you are on the right way i tell you all detail about the septic tank. So, before moving on the design of a septic tank you should have some basic knowledge of septic tank. Let us get started.
What is a septic tank, how to design septic tank in a proper way?
Fig:- Septic Tank

Introduction and process of the septic tank

Septic tank is a rectangular chamber generally constructed below the ground as a primary sedimentation tank to carry disposal of night soil for large time (12 hrs to 36 hrs) as compared to ordinary sedimentation tank carry only for less than 2 hrs.
Septic tank is constructed either with a concrete or brick wall containing two or more than two chambers. This is mostly used in the ruler and urban areas where there is no proper management of the sewer system (drainage system). It can be constructed in domestic as well as commercial building and designed according to people’s reach or living in that building.
For proper functioning of septic tank water should be sufficient as the night soil from latrine has to reach the septic tank by flowing.
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Septic tank should have the capacity to store the sewage for a long hour because, during that period, the sludge decomposes by the biological action of anaerobic bacteria and solid waste settle down at the bottom of the septic tank as sludge. After that surface water from the tank should have to throw or discharged out which is known as effluent.
The settled sludge that can be removed after 2 or 3 years according to design and to capacity septic tank made. At most septic tank the effluent of the septic tank is disposed by absorption in the ground through soak pit.
Otherwise, there is a municipal drainage system provided to that area. You may be confused about soak pit let me make you clear about it. Soak pit is dug underground in a circular shape with dry bricks or stone having a gap between their joints through which the surface water of septic tank (effluent) can easily be soaked into the soil.
The top portion of the soak pit should be closed by RCC Slab and its diameter should be greater than 90 cm, and 150 cm depth. It should not be below 2 meters from groundwater.
Keep in mind that the cleansing agent like bleaching powder, phenyl, etc should not be used because it kills the growth of bacteria and retards the biological decomposition inside the septic tank which may cause not proper working of the septic tank.

How to design a septic tank?

Here i have listed the pointwise requirement to design a proper septic tank. So, read it carefully.
  1. The capacity of the septic tank should be sufficient enough to hold liquid taken 130 liters to 70 liters per member.
  2. For the best design of the septic tank, the mortar used at the wall of the septic tank should be used of thickness more than 20 cm and concrete used below the septic tank should be of M15 or M20 grade.
  3. The outside, inside faces of the wall and top floor of the septic tank should be plastered with a thickness greater than 12mm of 1:3 or 1:4 grade of mortar. The concrete should be prepared waterproof and all the inside corners of the septic tank should be rounded giving slope toward the center base of the septic tank for better extraction of a slug at the time of removing slug. Also, the floor of the septic tank should be given a little slope (1:15) toward the inlet side.
  4. The width of the septic tank should not be less than 750 mm.
  5. The length should be provided 2 to 4 times the width of the septic tank.
  6. The depth of the septic tank should be kept 1000 mm to 3000 mm (+ 300 mm to 500 mm according to water level. But should not exceed 1800mm (+ 450 mm) for freeboard.
  7. The minimum capacity of the septic tank should be 1 cubic meter.
  8. The detention period of septic tanks should be considered to be 24hrs (mostly).
  9. For inlet pipe, an elbow or T pipe of 100mm diameter should be deep below the liquid at the depth of 250mm to 600mm and for outlet pipe at the depth of 200mm to 500mm.
  10. The inlet baffle wall should be at (1/5) the distance of the length of the wall. The baffle wall should be generally enlarged 150 mm above and 400 mm to 700 mm to scum level.
  11. The roof of the septic tanks should be covered with RCC slab of thickness 75 mm to 100 mm. If circular manholes are made then diameter should be 500mm but if rectangular manholes are made then its dimension should be 600X450 mm for inspection and desludging.
  12. The outlet pipes for ventilation purposes may be of cast iron or asbestos should be kept  2-meter minimum above the ground level and diameter should be of 50 mm to 100 mm. The topper open part of this pipe should be covered with net or cowl to prevent the mosquito from entering through it.
Further, let if you have to design a septic tank for 20 users then how will you do the design of septic tank calculation?
Let us, do it.Step1:- Calculate the liquid capacity of the tank.
Let take @120 liters per user as i have told you to consider above.
= 0.12 X 20 = 2.4 meter cube
[Since, 1000 liters = 1-meter cube of water]

Step2:- Assume the liquid depth of the tank.
Let it be 1.3metersrs.
Therefore, the floor area will become,
=2.4/1.3
=1.85 meter square
[ Volume of tank= Area X height]

Step3:- Assume the length of the septic tank
Let it be 2.5 times the breadth,
Then, L = 2.5 X B
Where B is the breadth of the tank.
Or, L X B = 1.85 meter square
Or, 2.5B X B = 1.85
Or, B = Sqrt(1.85/2.5) = 0.86 meter

Hence, the dimension of the tank will be (2.5 X 0.86 X 1.3) m

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