### HOW TO DESIGN SEPTIC TANK FOR RESIDENTIAL COLONY

Ex:1
Design a septic tank for a small residential colony having a population of 800 persons. the rate of supply is 150 litres per head per day design also the soak wall if the effluence for the septic tank is to be discharged in if.

The quantity of water supply

=per capita demand x population

=800 x 150

= 120000 litres / day

Assuming that the 80 % of water supply apply appears as sewage

The quantity of sewage produced = 120000 x 0.8

= 96000 lit / day

Assuming the detention period  of 24 hours the quantity of sewage prodused during this detention period is         = (96000 x 24)/24
=96000 litres
Assuming the rate of deposited sludge as 40 lit/capita/year and assuming the period of cleaning is one year

The volume of sludge deposition x population x period of cleaning

=40 x 800 x 1

= 32000 litres

Total capacity of tank = capacity of sewage + capacity of sludge

= 96000 + 32000

= 128000 litres

= 128 litres

Assuming depth of tank = 1.6 m

Area = volume of tank / depth

= 128/1.6

= 80 m 2

The ratio  length to width is taken as 1:2 to 1 :4

So we take L : B = 1 : 4

L= 4B

=L XB

= 4B x B

= 4 B2

4B= 80 m2

B= 80/4

= 4.5m

Length of tank  L = 4 B

L = 4x 4.5

L= 18 m

Assume free board  = 0.2 m

The over all depth    = 1.6+ 0.2
= 1.8m
Size of tank              =18 x 4.5 x1.8

DESIGN OF SOAK PIT
Assuming the percolating capacity of filter media of soak pit as 1260 litres /m3 / day
Volume required for soak pit

= volume of tank / percolating capacity of filter media
= 128 m3/1.26 m3x m3 x day
= 101.6 m 3

If the depth of soak pit is 3.5 m

Area of soak pit   =   volume of soak pit / depth of soak pit
=   101.6 / 3.5
=  29 m2
D  =  6m

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