Design of Septic Tank and Soak Pit

The water demand is assumed to be 135 lpcd . The water demand depends upon the type of housing or community. For rural areas the water demand will be low while for urban areas the water demand will be high.

The population governs the design of septic tank. Assume the population as 40 numbers for a group housing.

All the water doesn’t get converted into sewerage. Consider 80% of water demands gets converted into sewerage.

• Water demand = 135 lpcd
• Population = 40
• Sewage discharge = 80 % of water demand

\Large Total\;Water\;Demand= 40*135 =5400\:\frac{liters}{day}=225\:\frac{liters}{hours}

\Large Total\;Sewage\;Demand= 0.8*225\:\frac{liters}{hour}=180\:\frac{liters}{hour}

\Large Sewage\;Volume= 180*24 =4320\:liters

Assume the sludge disposal rate as 30 liter /population / year and desludging period as 1 year

• Sludge disposal rate = 30 l/poo/year
• Desludging period = 1 year

\Large Total\;Sludge\;Volume= Sludge\;Disposal\;Rate\;*Population\;\\*Desludging\;Period

\Large Total\;Sewage\;Demand= 30*40*1 = 1200 liters

\Large Total\;Volume\;of\;Tank= 4320+1200 = 5520\:liters=5.52\:m^{3}

Assume the depth of tank as 1.2 m and 0.3 m free board . Assume the L/B ratio as 2.5

• Height = 1.2 m
• Freeboard = 0.3 m
• L/B ratio = 2.5

\Large Surface\;Area= \frac{5.52}{1.2} = 4.6\:m

\Large Breadth= \sqrt\frac{4.6}{2.5} = 1.36\: m

\Large Length= 2.5*1.36 = 3.4\:m

• LENGTH OF SEPTIC TANK = L = 3.4 M
• BREADTH OF SEPTIC TANK = B = 1.36 M
• HEIGHT OF SEPTIC TANK = H = 1.5 M
• VOLUME OF SEPTIC TANK = V = 6.94 M³