Design of Septic Tank

Septic Tank | Importance of Septic Tank | Components of Septic Tank | Advantages & Disadvantages of Septic Tank


  1. Introduction  

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 very large detention periods ranging from 6 hours to 12 hours.

It is an essential part of the sewage disposal system.

The basic principle of the septic tank for the basic treatment of sewage passing through it is the anaerobic decomposition of the organic matters present and the settling of other waste particles.

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

It is commonly adopted both in urban as well as rural areas.

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

The septic tank is designed is such a way that the wastewater is retained 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 which is known as the sludge which is then settled in the tank.

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

The effluent thus coming out from the septic tank is disposed either by the municipal drainage system or simply through absorption by the soak pit.

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

The main objective of the septic tank is for the basic treatment of sewage and its efficient disposal.

Mostly, in the septic tank, about 40% of the sewage is treated while the remaining 60% is treated in the drain field itself. In case of the municipal drainage system, the effluent is discharged to the drain pipes.


  2. Overview   

Septic tanks are usually underground chambers and are 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.

Commonly, 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 namely 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.


  3. 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 wastes.

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

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


  4. Importance of Septic Tank  

1. It is necessary for collecting 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 the safe disposal of the night soil particularly from the latrines in rural areas.


  5. Working Mechanism  

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

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

These settled solids thus comprise the scum layer.

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 matters present in the sludge.

Due to the decomposition, the particles become lighter and move upwards to the middle of the tank which is 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.

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

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

The anaerobic bacteria also digest the matters present in the scum layer due to which 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 on the outside of the baffle walls. The effluent is then discharged into the drain field through the outlet and is thus absorbed into the soil.


  6. Components of Septic Tank  

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 to the inlet pipe.

Baffle walls serve as the breakers for the incoming sewage. Such walls 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 that is provided to the septic tank.

In general practice, roofing slab comprises of 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 the circulation of air inside the tank and to prevent foul odour.

It is usually made up of cast iron or asbestos.

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


  7. Design Criteria of Septic Tank  

The design of the septic tank mainly depends upon the number of users expected to make use of 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 litres per head is generally adopted as the liquid capacity of the tank.

The chamber of the septic tank is mostly made up of brick wall with cement mortar.

The thickness should not be less than 9 inches and the foundation floor must be made up 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 such that it consists of 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 rate of flow of the influent is taken equal to the rate of flow of the effluent.


2. Dimensions of the Septic Tank :
The design of the septic tank must be done in such a way that the width of the tank 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 it must be 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 to 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 thickness of the baffle wall 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 and for 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.


  8. 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 to the extent 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.


  9. 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. Due to the lack of proper maintenance, the septic tank may produce a foul odour. It may also aid the breeding of mosquitoes and flies.

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

6. Regular maintenance is necessary for efficient sewage disposal.


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