Design the dimensions of the septic tank for a small colony of 150 persons provided with an assumed water supply from the municipal head works at a rate of 120 litres per person per day. Assume any data you may need?

Step 1:-Number of users for the septic tank=150 persons

Step 2:-Tank Capacity

Consider the whole quantity of water to be as a sewage

∴Flow of sewage per day=150*120=1800 litre

The detention period varies from 12 to 72 hours

Detention period in common practice is used to be 24 hours.

Here assume a detention period of 20 hours.

∴septic tank capacity=$\frac{1800*20}{24}=1500 litre$

Step 3:-Total tank capacity:-

Note:-

The space for sludge is kept usually at the rate of 15 to 45 litre per capita per year

Assumed that the tank to be designed is cleaned every year=150*15=2250 litre

∴Total tank capacity=1500+2250=17250 litre

Step 4:-Total tank capacity for future expansion

Total tank capacity is formed to be 17250 litres and consider total tank capacity for future expansion as 20,000 litre or $20m^3$

Provide two units of septic tank each having the capacity of $(\frac{20}{2})=10m^3$

Step 5:-Plan area of the septic tank

Assume the depth of liquid as 1.5m

∴Plan area of the septic tank=$\frac{10}{1.5}=6.66=7m^2$

Step 6:-Width of the tank:-

Note that according to design aspect, the ratio of length to width is about 2 to 4

Let the ratio of length to width of tank to be 2

$width of tank=\frac{Plan area of the tank}{ratio of length to width}^\frac{1}{2}$

$=\frac{7}{2}^\frac{1}{2}=1.87=1.9m$

Step 7:-Length of the tank

Plan area of tank=length * width

7=length*1.9

$length=\frac{7}{1.9}=3.684=4m$

length=4m

Step 8:-Design dimension of tank

According to design aspect, the freeboard should be about 400 mm to 600 mm

Here assume the freeboard as 500 mm i.e. 0.5m

Here total depth of the tank = (depth of tank+freeboard) =(1.5+0.5)=2m

Here provide two septic tank, each of having dimensions as 4m X 1.9m X 2m