Everything you need to know about Septic Tanks



A subsoil dispensation system shall not be closer than 18m from any source of drinking water such as well to mitigate the possibility of bacterial pollution of water supply. Not less than 6m to the building.

In order to provide the satisfactory disposal of sewage obtained from hotel, septic tank may be adopted.

It is sedimentation tank with extra provision for digestion of the settled sludge by anaerobic decomposition. Septic tank is completely covered and is provided with an air vent pipe. 50mm diameter minimum for escape of gases, on top cage of mosquito proof wire mesh, 2 meter height of building.

Roof of septic tank contains man hole for inspection and cleaning. Septic tank should be constructed of materials which are resistant to corrosion, brick, stone, concrete. The tank is constructed water tight. Direct currents are not estab­lished between the inlet and outlet. This is achieved by using submerged pipe tee or by baffle wall. Level of outlet is 15cm. lower than that of inlet level.

For bigger septic tank C. I. steps (stair) may be provided.

The sludge settles at the bottom of the tank and the oils and greasy matters rising to the top surface of the sewage as scum.

During the detention period sewage is purified and the effluent is taken to soak pits for disposal. Sludge decomposes by the anerobic bacterial to form gases and liquids by the process called sludgedisetion.

It removes 60 to 70% of dissolved matters from the sewage.

For 100 bed hotel, size is 8m x 2.6m. The septic tank will be designed with the following data-

(a) Capacity of septic tank:

A septic tank should be capable of storing the sewage flow during the deten­tion period and an additional volume of sludge for 6 months or more depending upon the periodicity of cleaning. The rate of accumulation of sludge has been recommended as 30 liters/person/year.

A free board of 0.3 m may be provided above the top sewage line in the septic tank. This will help to accommodate the scum in the septic tank.

(b) Inlet and outlet baffles, etc.:

The baffle wall or Tees should extended up to top level of the scum (22cm above the top sewage line) but must stop a little below the bottom of the covering slab (7.5 cm) so as to allow for the free movement of gases.

Inlet should penetrate by 30cm below the top sewage line and the outlet should penetrate to about 40% of the depth of sewage. The outlet invert level should be kept 5 to 7.5 cm below the inlet invert level.

(c) Detention period:

The detention period for septic tanks is 24 hours.

(d) Length to width ratio of septic tank:

Length = (2 to 4) x width

Minimum width = 90 cm

Depth = 1.2 to 1.8 meter minimum

Advantages of septic tank are:

(i) Easily constructed

(ii) Cost is reasonable

(iii) Reduced suspended solids and BDO

(iv) Sludge volume is less

(v) Effluent can be disposed

(vi) Suitable for isolated hospitals, hotels.

Disadvantages of septic tank are:

(i) Size is large

(ii) Periodic cleaning, removal and disposal of sludge remain a tedious problem.

(iii) During ill functioning of the tank, the effluent is very foul, dark and even worse than influent.

(iv) Leakage of gases pollute environment

(v) Working is unpredictable and non uniform.

Miscellaneous methods of sewage treatments are:

(a) Can pools

(b) Chlorination of Sewage

(c) Imhoff tank

(d) Oxidation pond