What is Septic Tank? : Working, Components, Design Criteria, Importances, Advantages & Disadvantages of Septic Tank

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A septic tank may be understood as an underground chamber through which the wastewater or the sewage is passed for preliminary treatment.

In other words, it may be defined as a primary sedimentation tank with large detention periods ranging from 6 to 12 hours.

✔ The basic principle of the septic tank for the primary treatment of sewage passing through it is the anaerobic decomposition of the organic matter present and settling of remaining waste particles.

✔ Septic tank is also commonly referred to as the on-site sewerage system as the sewage is disposed of at a particular instant on-site.

✔ In rural areas, it is used for the disposal of the night soil from the restroom, but in urban areas, it is used for the disposal of both wastewater and the night soil.

✔ The septic tank is designed to retain the wastewater in the tank for about 24 hours.

During the retention period, the biological decomposition of the night soil occurs by the aid of anaerobic bacteria.

Only a small part of the night soil thus remains, known as the sludge, which is then settled in the tank.

The treated wastewater that flows out through the tank is known as effluent.

Thus, the effluent from the septic tank is disposed of either by the municipal drainage system or simply through absorption by the soak pit.

✔ Septic tank may be made up of RCC, fiberglass, plastic, etc.



  1. Overview   

✔ Septic tanks are usually underground chambers commonly constructed using concrete, steel, fiberglass, etc.

Septic tanks can range in size from just five hundred gallons for small simple septic tank systems to thousands of gallons for large septic tank systems.

✔ For residential purposes, the size of the septic tank adopted is one thousand to fifteen hundred gallons.

✔ The septic tank may be designed as a single chamber or multiple chambers depending upon the requirement.

In general practice, the septic tank consists of two distinct chambers: the treatment chamber and the final chamber.

One inlet is provided in the treatment chamber, and one outlet is provided in the final chamber. The outlet is provided at a lower level than the inlet to ensure the rapid flow of the effluent without any obstruction in a particular direction only.

The treatment chamber is the working chamber of the septic tank where most of the biochemical process occurs.



  2. Objectives of Septic Tank  

The primary objectives of the septic tank can be listed as follows:

1. To create a storage or holding space for the collected sewage where the solid wastes can be separated from the liquid manure.

2. To break down the organic waste present in the sewage through biological decomposition with the aid of the bacteria.

3. Store the settled solids until they are removed or pumped out.



  3. Importance of Septic Tank  

1. It is necessary to collect the wastewater and night soil from the connected drains and toilets in the households that are generally not connected to the municipal sewer systems.

2. Septic tank efficiently treats the sewage before it is discharged to the environment thereby preventing the environmental degradation and pollution.

3. Septic tank allows the wastewater to be replenished by natural means and the treated water can also be re-used for purposes such as industrial works, irrigation, groundwater recharge etc.

4. Septic tanks are vital for safe disposal of the night soil, particularly from the restrooms.



  4. Working Mechanism  

✔ When the sewage enters the septic tank, it gets separated into three distinct layers: the sludge, clear zone, and scum.


septic tank

Fig: Single Chambered Septic Tank


✔ The solid particles present in the sewage that is heavier than the water settle at the bottom of the tank.

These settled solids are referred to as sludge.

✔ The septic tank is designed in such a way that no oxygen is present at the bottom of the tank thereby promoting the growth of only the anaerobic bacteria.

The anaerobic bacteria, in turn, digest the organic matter in the sludge.

Due to the decomposition, the particles become lighter and move upwards to the middle of the tank, commonly referred to as the clear zone.

The clear zone is the layer that usually consists of the brown greyish wastewater and anaerobic bacteria.

Sometimes, aerobic bacteria may also be present in this zone.


Design of Septic Tank

Fig: Double Chambered Septic Tank


The scum layer is the uppermost layer in the septic tank.

It mainly consists of suspended oil, grease, soap films, etc., lighter than water.

The anaerobic bacteria also digest the matters present in the scum layer. The digested particles become heavier than the water and thus settle at the bottom of the tank.

✔ For further treatment of the effluent water, an effluent filter may be placed outside the baffle walls. The effluent is then discharged into the drain field through the outlet and is thus absorbed into the soil.



  5. Components of Septic Tank  

Design of Septic Tank


1. Working Chamber:

The working chamber is the main working space where the anaerobic decomposition as well as the settling of the sewage particles takes place.


2. Inlet Pipe:

The inlet pipe is provided in the septic tank to pass the collected wastewater and the night soil inside the tank.


3. Baffle Wall:

The baffle wall is generally provided near the inlet pipe.

Baffle walls serve as the breakers for the incoming sewage. Such barriers also prevent the congestion of blockade of the outlet pipe by the overflow of the effluent.


4. Outlet Pipe:

The outlet pipe is provided in the septic tank to pass the collected effluent to the drain field for efficient disposal.

The outlet pipe is always provided at a level lower than the inlet pipe.


5. Roofing Slab:

The roofing slab is the top cover provided to the septic tank.

Generally, roofing slab comprises RCC slabs and may be circular or rectangular.


6. Ventilation Pipe:

The ventilation pipe of the septic tank is also commonly referred to as the Air Vent.

The main objective of providing the ventilation pipe is to facilitate air circulation inside the tank and prevent foul odor.

It is usually made up of cast iron or asbestos.

A wire mesh is provided at the top of the ventilation pipe to check the entry of flies, mosquitoes, and other insects.



  6. Design Criteria of Septic Tank  

✔ The septic tank design mainly depends upon the number of users expected to use it.

In this regard, the capacity of the sludge tank mainly depends upon the number of users and the sludge removal interval.

✔ It is a general practice to remove the sludge every two years.

The liquid capacity of the septic tank is usually taken as 130 litres to 170 litres per head.

✔ For small residential use, 130 liters per head is generally adopted as the tank’s liquid capacity.

The chamber of the septic tank is mainly made up of brick walls with cement mortar.

The thickness should not be less than 9 inches, and the foundation floor must be made of cement concrete of mix 1:2:4.

✔ It must be noted that both the inside as well as the outside faces of the wall and the top layer of the floor must be plastered with a minimum thickness of 12mm i.e. one and a half-inch thick.

The mix of the cement mortar for the plastering work should be 1:3.

Some water-resistant admixtures may be added to the mix.

✔ The floor of the septic tank must be designed to have a slope of 1:10 to 1:20 towards the inlet. This is done to ensure efficient collection and removal of the sludge.

The design criteria of the various components of the septic tank can be duly listed as follows:


1. Detention Period:
The detention period for the septic tank is taken as twenty-four hours.

In the design of the septic tank, the influent’s flow rate is taken equal to the effluent’s flow rate.


2. Dimensions of the Septic Tank :
The septic tank’s design must be done so that the tank’s width is not less than 750mm.

The length is usually taken as two to four times the width and the depth is taken as 1000 to 1300mm plus 300mm to 450mm is taken as the freeboard.

The minimum capacity of the tank is taken as 1 cubic meter.


3. Inlet and Outlet Pipes:
For the inlet wall, a T pipe or an elbow with a minimum diameter of 100mm must be used and submerged to a depth of 250 to 600mm below the liquid level.

For the outlet wall, a T pipe or an elbow with a minimum diameter of 100mm must be used and it must be submerged to a depth of 200 to 500mm below the liquid level.


4. Baffle Wall:
The inlet baffle wall is provided near the inlet region.

If the length of the wall is L, then the baffle wall is placed at a distance of L/5 from the wall.

The baffle wall’s thickness is kept between 50mm to 100mm.


5. Roofing Slab:
The thickness of the top slab must range from 75mm to 100mm based on the size of the tank.

For the circular cover, the minimum diameter must be kept as 500mm; for the rectangular cover, the minimum size must be kept 600*450mm.


6. Ventilation Pipe:
The diameter of the ventilation pipe should not be less than 50mm and not more than 100mm.

The ventilation pipe should extend at least 2m above the ground level.



  7. Advantages of Septic Tank  

1. Septic tank uses the natural method of waste decomposition and thus is good for the environment.

2. It has a long life span and lasts for several years.

3. Septic tanks are relatively affordable and economical.

4. Septic tanks are efficient in purifying the wastewater so that the effluent can be used for groundwater recharge.

5. The maintenance cost of the septic tank is less.

6. It is very desirable for rural areas.



  8. Disadvantages of Septic Tank  

1. It is not suitable for highly congested areas.

2. It must be cleaned at a regular interval of about two years.

3. In some cases, due to the lack of efficiency of the tank, it may lead to the contamination.

4. The septic tank may produce a foul odor due to the lack of proper maintenance. It may also aid the breeding of mosquitoes and flies.

5. During the rainy season, sewage overflow may lead to significant problems.

6. Regular maintenance is necessary for efficient sewage disposal.


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