design of septic tank

Septic Tank Design & Construction Details

What Is Septic Tank?

A septic tank is an underground wastewater treatment system commonly used in rural and suburban areas where a centralized sewage treatment plant is not available.

It typically consists of a large, watertight concrete or plastic container that separates solid waste from liquid waste.

The solid waste settles at the bottom of the tank, while the liquid waste flows out of the tank and into a drain field for further treatment and disposal.

Septic tanks are designed to break down organic matter and prevent the release of harmful substances into the environment.

The solids that settle at the bottom of the tank undergo anaerobic digestion, which reduces the volume and treats the waste to some extent.

The liquid waste that flows out of the tank is further treated as it infiltrates into the soil, where bacteria and other microorganisms continue the treatment process.

Regular maintenance is crucial to the proper functioning of septic tanks. This includes pumping out the tank to remove the accumulated solids, checking for leaks and damage, and monitoring the drain field for signs of clogging or saturation.

Septic tanks are a cost-effective and environmentally friendly option for wastewater treatment in areas without access to a centralized sewage system. However, it is important to be mindful of the amount and type of waste that enters the tank, as overloading the system can lead to failure and environmental damage.

A septic tank also known as a sewage collection system also treats sewage on a small scale which is common in areas with no connection to main sewerage pipes.

Septic tank for Small House
Septic Tank Design & Construction Details 6

Septic tank system design is one type of onsite sewage collation system. It is estimated that about 25 % of the population in North America depends on septic tanks.

This system can include suburbs and small towns as well as rural areas. In European countries, dependency is limited to rural areas only.

The term “septic” is expressing the anaerobic bacterial condition that immerges in the tank and which decomposes or mineralizes the waste discharged into the tank.

Sometimes additional decompose bacterial agents added to the tank will accelerate the digestion of solids in the tank.

A septic tank generally has a capacity of sewage between 1,000 and 1,500 gallons which is connected to an inlet wastewater pipe at one end and to a septic drain field at the other.

Wastewater from the house is directed into the septic tank. This tank is designed which allow the heavy solid waste to settle down from wastewater.

The bacteria available in sludge partially decomposed this solid waste. Light-weighted waste like grease and oil particles floats and forms a layer of scum on top of the wastewater.

A Baffles wall system is provided at the inlet and outlet of the tank to help prevent scum and solids from escaping.

Design of Septic Tank

The Septic tank design example for home is as follows,

The proper capacity of a septic tank is essential otherwise wastewater backflow toward the house. The septic tank design considers a future increase in wastewater generation designs proper capacity septic tanks which can last long for years.

Septic Tank Design & Construction Details

Septic tank design example calculation for residential buildings is given below:

Let’s take one example of House – 5 Members

Daily Water Usage for a House – 5 Person

  • Cooking  – 10 Liters
  • Bathing & Toilet – 90 Liters/Person, So for 5 people – 450 liters/Day
  • Washing clothes & Utensils – 35 Liters
  • Cleaning House – 15 Liters
  • Other – 10 Litres

Total – 520 Litres/Day Approximately

In septic tank design, we consider detention time as 3 days. So the designed tank should have the capacity to retain household wastewater for at least 3 days.

Total wastewater in 3 days – 520×3 = 1560 Liters

So, we take more than that about 2000 Liters capacity minimum for a home. For the Septic tank design, the depth of the tank should not be less than 1.8m.

Take sludge settled down per person – 30 liters/year. So here we take sludge removal 2 years once.

Total Accumulated Sludge = 30 litres x 5 persons x 2 years = 300 Litres

Total Septic Tank Capacity = 2000+300 = 2300 Liters

We know that 1 cubic meter = 1000 Liters = 2300/1000 = 2.3 Cum

Area required @ 1.8m depth = 2.3/1.8 = 1.2 Sqm

We take the Length and Width ratio of the Septic Tank as 4:1 or 2:1

Length(L): Breath(B) ratio taken as 4 : 1

So, 4 B x B = 1.2 Sq. m where B=0.54m

(Note: Minimum width of tank should not be less than 750mm)

So that L = 4×0.75 = 3m

L – 3m; B – 0.75m;

Depth = 1.8+0.3 = 2.1m (freeboard should be provided at least 300mm)

Septic Tank capacity = 3 x 2.1 x 0.75 = 4.725 Cum

= 4.725x 1000

Septic tank design capacity = 4725 Liters

Read More: Estimation Of A Building With Plan

Septic Tank Design 3 Chambers | 2 Tank Septic System Design

Septic Tank Design & Construction Details
Septic Tank Design & Construction Details 7

British Standard Design of Septic Tank

The British standard is given following the formula septic tank design and calculates the wastewater flow for a septic tank.

C=A + P (rq + ns)  


C – Capacity in liters

P – Number of People

A – 2000 Litres as constant

R – Detention period of Sewage in Days

Q – Sewage Flow in liters per day

N – Number of years

S – Sludge accumulation in liters per person/year

Simplification of (rq + ns) = 180 Litres

We can Rewrite the formula C=A + 180 P

C = 2000 + (180 × 5)

   = 2900 Litres (Septic tank design capacity)

Septic Tank Construction Details

Nowadays, the design of the septic tank main has two chambers each of which is equipped with a manhole cover.

These chambers are separated by means of a dividing wall which has openings located about midway between the floor and the roof of the tank.

Septic Tank is installed where there is no proper drainage system. In order to store, the Wastage or sewage for 10-30 Days Septic tank is constructed and the same is designed. This tank is usually installed below 1.5m-1.8m from ground level. 

The details of the construction of the septic tank are as follows,

Firstly, the wastewater from the house is allowed to enter the tank through an inlet pipe, allowing solids to settle and scum to float.

During the period of detention, solids settle down is get digested by an anaerobic process reducing the volume of solids.

The remaining water is then diverted into the second chamber where further settlement takes place with the excess liquid then draining in a relatively clear condition from the outlet into the drain field or seepage field.

This comparatively clean water is now taken to a distribution chamber where the wastewater is channeled into one or more perforated pipes set in trenches of gravel.

Under this system water slowly infiltrates (seeps) into the underlying soil.

The bacterial components and waste of this water are trapped or adsorbed by soil particles or decomposed by microorganisms.

The septic tank treatment almost removes disease-causing organisms, organic matter, and most nutrients (except nitrogen and some salts).

The partially cleaned wastewater then either moves to the groundwater or evaporates from the soil.

Septic Tank Cleaning

The Septic tank treatment system requires regular and periodic cleaning to avoid the build-up of sludge and eventual escape with the effluent into the drainage field. If this happens, it may clog the leach field piping requiring expensive repairs.

In the septic tank cleaning process, the periodic time interval of the tank emptied depends on the volume of the tank relative to the input of solids, the number of indigestible solids, and the ambient temperature.

Generally, septic tank systems require cleaning rarely more than once a year, and by careful management, many users can reduce emptying to every 3 to 5 years.

While cleaning the septic care should be taken that, only a small residue of sludge should be left in the tank. Anaerobic decomposition is rapidly restarted when the tank refills.

A well-designed and properly maintained septic tank system is odor-free. The periodic inspection of the tank should last for decades with no maintenance.

If a septic tank is a well-maintained concrete, fiberglass, or plastic tank should last about 50 years.

Read More: Design Of Staircase Calculation – Riser And Tread

Septic Tanks Potential Problems

Sometimes excess wasting of cooking oils and grease can fill up the upper portion of the septic tank and can cause the inlet drains to block.

It is a fact that grease and oil are often difficult to degrade and can cause odor problems and difficulties with periodic emptying.

Flushing materials such as sanitary towels, cotton buds, etc. which are non-biodegradable hygiene nature will rapidly fill or clog a septic tank and these materials should not be disposed of in this way.

Some people use a waste grinder to dispose of waste food will cause a rapid overload of the system and early failure.

Septic tank system damage due to some chemical contact such as pesticides, herbicides, materials with high concentrations of bleach, or any other inorganic materials such as paints, solvents, etc. Such materials inhibit bacterial function.

Roots of a tree growing above the tank and shrubbery or the drain field may clog and or rupture them.

Excessive water in the septic tank due to some plumbing leakage may lead to an overload of the septic tank system.

How a septic tank separates solid and liquid waste

A septic tank separates solid and liquid waste through a process of gravity separation. The tank is designed with a large, watertight container that allows waste to enter and settle.

As the waste enters the tank, it begins to separate into two layers: a layer of solid waste that settles at the bottom of the tank and a layer of liquid waste that floats on top.

The liquid waste is lighter and has a lower specific gravity than the solid waste, so it rises to the top of the tank and flows out of the tank through a series of pipes into a drain field.

Solid waste, on the other hand, settles at the bottom of the tank and undergoes anaerobic digestion, which is a process of breaking down organic matter without the presence of oxygen.

This separation of solid and liquid waste allows for effective treatment of the waste in a septic tank. The liquid waste is further treated as it infiltrates into the soil, where bacteria and other microorganisms continue the treatment process.

Solid waste is broken down by anaerobic digestion, reducing the volume and treating the waste to some extent.

By separating solid and liquid waste, septic tanks allow for efficient and effective treatment of wastewater in areas where a centralized sewage treatment system is not available.

However, it is important to regularly pump out the solid waste from the tank to prevent clogging and ensure the proper functioning of the system.

Cost-effectiveness of septic tanks compared to centralized sewage systems

Septic tanks can be cost-effective compared to centralized sewage systems in rural and suburban areas, where the cost of connecting to a centralized system can be high.

Installing a septic tank requires less infrastructure and can be more cost-effective for property owners in these areas.

In addition, the ongoing operating costs of a septic tank can be lower than those of a centralized sewage system.

Septic tanks do not require the same level of maintenance and upkeep as centralized systems, and they do not require the same level of skilled labor to operate.

However, it is important to note that the cost-effectiveness of septic tanks can vary greatly depending on the location, size of the property, and type of soil.

In some cases, the cost of installing a septic tank may be higher due to the need for additional components such as pump systems, alarm systems, or drain fields.

In areas where a centralized sewage system is available, the cost of connecting to the system may be lower in the long run, as the ongoing costs of operating a septic tank can add up over time.

It is important to weigh the costs and benefits of each option when deciding on the best wastewater treatment solution for a property.


In conclusion, septic tanks play an important role in the treatment of wastewater in rural and suburban areas where a centralized sewage treatment system is not available.

They are designed to separate solid and liquid waste, treating the waste through a combination of anaerobic digestion and further treatment in the soil.

Regular maintenance is crucial to ensure the proper functioning and to prevent environmental damage. Despite its advantages, it is important to be mindful of the amount and type of waste entering the septic system to avoid overloading and potential failure.

Overall, septic tanks provide a cost-effective and environmentally friendly solution for wastewater treatment in areas without access to centralized systems.

Watch Video: Septic Tank Design Calculations


What is a septic tank?

A septic tank is an underground container used for the treatment and disposal of wastewater from homes or buildings that are not connected to a centralized sewer system. It allows for the separation and decomposition of solids from the wastewater.

How does a septic tank work?

Wastewater from toilets, sinks, and drains enters the septic tank, where solid waste settles to the bottom as sludge, and lighter particles float as scum on the top. The liquid effluent in the middle layer flows out of the tank into a drain field or leach field for further treatment and absorption into the soil.

What are the basic design considerations for a septic tank?

The design of a septic tank involves considerations such as the size of the tank, the number of occupants or fixtures it will serve, the soil conditions, local regulations, and the water usage patterns of the household or building.

How is the size of a septic tank determined?

The size of a septic tank is determined based on factors such as the number of bedrooms or occupants in the building, water usage, and local regulations. It is typically calculated using standards and guidelines provided by local health departments or relevant authorities.

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