Septic Tank and its Design | Building Construction

What is a septic tank? and why we use it?

The tank which Stores a waste water of House or building is called Septic Tank. Septic Tank is installed where there is no proper drainage system. In order store, the Wastage or sewage for 10-30 Days Septic tank is constructed and the same is designed.

This tank is usually installed below 1.5m-1.8m from ground level. Waste water coming from sewer Pipes of the house are connected to Septic tank.

Wastewater flows into the tank at one end and leaves the tank on the other end as shown in the figure. The septic tank is drained out with the help of suck out the outlet. The municipal Department suck out the wastage from Septic tank.

In the early 1990’s, the tank is constructed without bottom slab below the tank. Due to seepage from Septic tank, the waste water is mixed with Fresh Water sump. To avoid this, the septic tank is cased up with the bottom slab.

All waste water from the bath, toilet, kitchen flows into the tank. Heavy Solids in waste water is settled at bottom of the tank. Due to the presence of anaerobic bacteria in waste water Heavy solids are reduced to sludge and gases. Generally,  if the density of solids which is more than the water is settled at the bottom of tank, if density of solids which is less than the water floats. The settled Solids is called as Sludge and floating solids is called as scum.

Septic tank and its design, (Maintenance)

Septic Tank Maintenance:-

Remove the sludge when it reaches up to the height of 3″ below the suck out the pipe. Due to the presence of Waste gases, there might be a chance of smell coming out from the pit. Periodic effluent is added to the septic tank to avoid the smell.

The design of Septic Tank:-

Septic tank Consists of Top Slab, Bottom Slab with PCC Bed, Shear walls around the tank and a top cover (Opening) to check the level of Waste water. Below are the septic tank design calculations.

How to design Septic System?

For clear understanding here we designing a septic tank for 8 people in a house which get filled in 10 Days.

The average waste generated from each human / Day including kitchen waste = 95 to 150 liters/Day. 

The average waste generated from each human includes
  • Bathing & toilet = 85 litres/day
  • Cooking = 1 litres/day
  • Cleaning house = 2 liters/day
  • Washing utensils & clothes = 6 liters/day
  • Others = 1 litres/day

The values may change as per location and atmospheric conditions.


Total Members in house = 8 (Assume)

Total Waste Generated by House = 8 x 130 = 1040 lts/Day [Considering 130Lts]

The Total waste generated by house in 10 days = 10 x 1040 = 10,400lts

Remember , 1m3 = 1000 litres (Standard)

So, For 10,400lts = 10.4 m3

How to find Septic tank Size:

In order to calculate the Septic tank size know about the standard length and width ratio of Septic tank.

The length and width ratio of Septic tank is 4:1 or 2:1

The depth or height of Septic tank is in between 1.5m- 1.8m

So the area of Septic tank = 10.4/1.8 = 5.78m2

Considering Length and width ratio = 4:1

Length of Septic tank = (4/5) x 5.78 = 4.62m

Breadth or Width of Septic tank =  (1/5) x 5.78 = 1.15m
(Approx 10.4 m3)

Length, Breadth & Depth of Septic tank = 4.6m x 1.15m x 1.8mSEPTIC TANK DESIGN

Automatic Calculator For Septic Tank Size Calculation:-

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