Septic Tank of the House

A Septic tank in a building is a device to deal with the sewage from water closets in areas where a municipal sewerage system has not been installed. A Building Septic tank may be constructed by an individual or by a group of family’s Septic tank is a watertight single-storied, underground tank in which sewage is retained sufficiently long to permit sedimentation of suspended solids and partial digestion of settled sludge by anaerobic bacterial action. The Septic tank of the building is generally of rectangular shape having a roof. It has usually two chambers, separated from each other by a baffle wall. The first chamber in which sewage first enters is called grit or screen chamber and the other is called an anaerobic chamber. Inlet pipe and outlet pipe are all fitted but both should be of such design that while entering or exit there is very little disturbance in the sewage in an anaerobic chamber. In the grit chamber, sand, grit, etc. settle down before the sewage is led to the anaerobic chamber. In an anaerobic chamber, organic solids settle at the bottom of the tank where anaerobic bacteria act on it and convert complex unstable compounds to simpler stable compounds.

Household or domestic septic tank-The simplest form of a septic tank for a household may be either a single-compartment tank cylindrical or rectangular in shape. A household tank should not be less than 2300 liters capacity. In a two-compartment Septic tank, the first compartment should hold not less than 2300 liters. The capacity of the first compartment is 2/3 to 1/2 of the entire tank. Two-compartment tanks are more successful than one compartment septic tank. Some of the desirable features in the design of household septic tanks include:

1. The Building materials used in the construction of septic tanks should be watertight and corrosion-resistant.

2. The natural ventilation provided should be adequate. Ordinarily, the open end of the inlet and outlet will be adequate, or an open shaft leading to the ground surface will be sufficient. The upper end of the shaft should be shielded with a cowl, to prevent access of the small birds, vermin, animals, etc.

3. Manholes should be provided for inspection and clearing of the septic tanks. 

4. Baffles wall should be provided near inlet and outlet to prevent higher surface velocities and escape of scum respectively. The inlet baffle wall may extend 30 cm below the liquid surface

5. Household tanks are cleared manually after an interval of two to three years.

4. The capacity required for sludge digestion at 0.033 Cu mt per capita at 25°C.

5. Volume of digested sludge assumed as 0.00021 Cu mt per capita per day.


Design of Septic Tank for a Building
How to Design a Septic tank?

The size of a building septic tank is governed by the following factors:

1. The floor area of the grit chamber has to be adequate to reduce the velocity of flow and to permit sedimentation.

2. Capacity should be adequate to provide a detention period varying from 12 hours to 3 days; 24 hours period is usually considered satisfactory.

3. The depth below the partition wall opening should be sufficient to permit sludge accumulation for a predetermined period. A minimum area 0:07 sq mt per user in grit chamber and minimum volumetric content of the grit chamber of 0:02 cu mt per user is required.

4. Septic tank should have a minimum width of 0-75 mt and a minimum depth of one meter below the water level. It should be of minimum 1 cu mt capacity. The length of the tank should be 2 to 4 times the width.

5. Every septic tank should be provided with a ventilation pipe of at least 5 cm diameter.


Commissioning of Building Septic Tank
How to Commissioning of Septic tank?

The tank should be filled with water to its outlet level before sewage is led into the septic tank. It should be seeded with small quantities of well-digested sludge obtained from the sewerage line.

Desludging of septic tanks -A Building Septic tanks have to be dislodged periodically. The interval of the desludging depends upon the design of the septic tank and the capacity in relation to the users. The period may vary from 6 months to 2 years. While desludging, the Septic tank should not be made completely sludge free as some sludge is essentially required for seeding the sewage is to which is to be accepted afterward.

Disposal of sludge -Removed sludge may be delivered into cess-pools or into vehicles for removal from the site. Spreading of the sludge on the ground should be discouraged

Disposal of Effluent of the septic tank -The effluent of septic tanks may be discharged into perennial streams or it may be used for gardening or irrigation. The latter method should as far as possible be discouraged because it may cause nuisance and health hazards to nearby residents. If neither of the methods is practicable, the effluent can be disposed of by discharging it into soak pits or seepage pits, leaching cess-pools, and dispersion trenches. Effluent from all these is absorbed by the soil.

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