What is a 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

Proper installation, regular maintenance, and pumping are essential for the health and longevity of your septic tank system. Choosing an experienced company like Byers Septic can ensure the job is done right and keep your system running smoothly.

Learn about the essential components of your septic tank system and how they work together to keep your home's wastewater flowing smoothly. 

Working and Components of a Septic Tank

In a conventional septic tank system, 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 is decomposed by the microorganisms forming Sludge. 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 is a biological filter that purifies the wastewater as it sinks into the ground.

Figure-2. Septic Tank; Image Credits: www.epa.gov

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 to the soak pit or drainage field through the outlet pipe. The water coming out of the main primary treatment chamber i.e. the septic tank system is not pure and is called as greywater that requires partial treatment before letting it into the ground soil. For all those septic tank systems that do not have a wastewater treatment unit thus requires a soak pit. 

Fig.3. Soak Pit Construction Layout

A soak pit does not allow the reuse of the water but helps it be safer to be discharged to the underground soil that would eventually join the groundwater. The figure above shows a detailed construction of a soak pit with different layers of materials involved in partial treatment. 

💡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.

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 table below.

   

No: of Bedrooms	Septic Tank Size in Gallons
1-3	900 to 1000
4	1250
5	1400
6	1550
7	1700
8	1850
9	2000

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 blackwater.
3. A 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 wastewater from 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. 

Also Read: What are the Various Wastewater Treatment Processes?