Septic tank is a water-tight masonry tank into which household sewage is admitted for treatment. Excreta and liquid wastes from individual houses, groups of houses or institutions can be let into it.
The design of a septic tank is as follows:
1. It may contain 1 or 2 chambers depending on requirement.
2. Minimum capacity is 500 gallons Length is twice the breadth. Depth is 5 to 7 feet. Liquid depth is 4 feet. Air space between cover and level of liquid must be 1 feet.
3. Bottom should be sloping towards inlet. This facilitates retention of solids.
4. Inlet and outlet pipes must be submerged.
5. Cover must be made of concrete and it must have a manhole.
Mechanism of action
Two stages are involved in the purification of sewage.
1. Anaerobic oxidation: It takes place inside the septic tank. Solids steed down and form sludge. Lighter solids, geese and fat rise to the top and form scum. By anaerobic oxidation, the sludge is reduced in volume. Also it is made stable and inoffensive.
2. Aerobic oxidation: It occurs outside the septic tank. The effluent which comes out through the outlet pipe percolates in the sub-soil. The aerobic bacteria in the soil attack the organic matter which is converted to nitrates, carbon dioxide and water.
1. Use of soap water and disinfectants should be avoided. They are injurious to the bacteria in the septic tank.
2 The contents of the septic tank should be bailed out every year.
3. Newly built septic tank should be seeded with ripe sludge drawn from an old tank. This will provide the bacteria which is required for the decomposition process.