**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