**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.

Fig:- Septic Tank |

### Introduction and process of the septic tank

### How to design a septic tank?

- The capacity of the septic tank should be sufficient enough to hold liquid taken 130 liters to 70 liters per member.
- 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.
- 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.
- The width of the septic tank should not be less than 750 mm.
- The length should be provided 2 to 4 times the width of the septic tank.
- 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.
- The minimum capacity of the septic tank should be 1 cubic meter.
- The detention period of septic tanks should be considered to be 24hrs (mostly).
- 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.
- 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.
- 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.
- 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.

**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

I Hope friends, you liked my written article on “**Design of septic tank calculation**” and remains helpful.