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Septic Tank: Working, Components and Features

A Septic tank is a watertight treatment chamber that is used to treat black water and grey water from cisterns, pour-flush toilets, buildings, etc. Septic tanks are made out of masonry, concrete, fiberglass, plastic, or PVC.

Fig.1. Septic Tank and Soak Pit System
Image Credits: WHO

Working and Parts of a Septic Tank

In septic tanks, the primary treatment of black and gray water is performed by two stages: Settling and Anaerobic reactions. The reaction finally provides a harmless effluent that can be either infiltrated into the ground or transported to a semi-centralized treatment plant.

The wastewater entering the septic tank is allowed to separate into three forms; They are solids, effluents, and scum.

Solid in the water settles to the bottom and it is decomposed by the microorganisms. Scum (oil & grease) floats on the top. Effluent exits the tank through underground perforated pipes to a soak pit or drainage field. Here, the arrangement act as a biological filter that purifies the wastewater as it sinks into the ground.

Figure-2. Septic Tank; Image Credits:

The main parts of a septic tank system are:

1. Inlet and Inlet Baffle: The wastewater enters the tank through the inlet. Inlet baffles are installed on the inlet pipe inside the tank to prevent scum from collecting and backing up into the inlet pipe. It also plays an important role in helping solids to settle in the tank. The water passes through the baffle, and its speed of flow reduces, helping to get more detention periods to settle down.

2. Sedimentation Tank: Inside the tank, the heavy particles sink at the bottom. The solids (proteins, fatty matter, carbohydrates, cellulose) are decomposed by microorganisms in the absence of oxygen, called an anaerobic reaction, into simpler compounds. Mainly, nitrogen is converted into ammonia and the colloidal matter is flocculated, liquefied and finally digested.

3. Baffle walls: These are walls provided in septic tanks to achieve longer detention time for the sewage to allow settlement of solids, and to keep the floating scum layer from plugging the inlet pipe.

4. Outlet and Outlet Baffle: The treated effluent before going out through the outlet, goes through a baffle. These outlet baffles help prevent the scum and any solids to enter and pass through the outlet pipe.

5. Soak Pit/Drainage Field: The final effluent is discharged out to the soak pit or drainage field through the outlet.

Size and Capacity of Septic Tanks

The capacity of septic tanks is planned based on the number of users. Usually, septic tanks are 8 x 4.5 x 6 feet ( l x b x h) in dimension. Usually, septic tanks are designed to take up to 1000 to 1500 gallons.

1000-gallon septic tank: 8 feet 2 inches long, 5 feet 8 inches wide, and 5 feet 1 inch deep. A 1,000-gallon tank is sufficient for a three-bedroom house. The usual minimum tank size required for the different number of bedrooms is mentioned in the table-1 below.

No: of Bedrooms

Septic Tank Size in Gallons


900 to 1000













Features of Septic Tank

1.    As per the United States Environmental Protection Agency (EPA), more than one in five households in the United States, go for septic tanks to treat their wastewater.

  1. The Septic tank hence is basically a sedimentation tank that can be either rectangle or cylindrical in shape. 
  2. It is a type of small-scale decentralized treatment unit for greywater and black water.
  3. Septic tank is one of the simplest forms of septic systems used for treating wastewater from buildings.
  4. Septic tanks can be used to treat the wastewater from the household plumbing fixtures like toilets, showers, laundry, etc.
  5. The septic tanks are buried deep underground from 4 inches to 4 feet which is dependent on the site conditions, the shape of the tank, and the slope.

Note: The outlet is placed lower than the inlet. Else, in case of any overflow in the septic tank, the soak pit gets filled and clogged.

Hence, Proper design, construction, and maintenance are required for the proper working of the septic tank system.

Also Read: What are the Various Wastewater Treatment Processes?


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