Septic Tank Design & Construction Details

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.

These systems are designed to address the need for on-site wastewater treatment, particularly in areas where centralized sewerage systems may be impractical or unavailable. The thoughtful design of septic tanks involves a comprehensive understanding of hydraulic principles, biological processes, and environmental considerations to create a reliable and environmentally sustainable solution.

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.

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 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

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)  

Where,

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.

Summary:

Septic Tank Design Components	Description
Inlet Pipe	Connects the household wastewater from toilets, sinks, and drains to the septic tank, allowing for the entry of sewage.
Septic Tank	A watertight underground chamber designed to separate solids from liquids, allowing bacterial digestion of organic matter.
Baffles	Internal dividers within the septic tank that help slow the flow of wastewater, promoting the settling of solids and preventing them from entering the outlet.
Outlet Pipe	Connects to the septic tank and allows treated effluent (liquid) to flow out and into the drain field for further processing.
Effluent	Treated liquid wastewater that exits the septic tank and enters the drain field for final purification and absorption.
Drain Field	An area of soil where effluent is dispersed and undergoes further treatment through microbial action and natural filtration.
Soil Absorption System	Consists of trenches or beds filled with gravel and perforated pipes, facilitating the absorption of effluent into the surrounding soil.
Vent Pipe	Allows the release of gases and odors from the septic tank, preventing pressure buildup and ensuring proper functioning.
Access Ports	Openings or covers that provide access to the septic tank for inspection, maintenance, and pumping activities.
Tank Inlet and Outlet Filters	Optional filters installed at the tank’s inlet and outlet to trap floating solids and prevent them from entering the drain field.

Watch Video: Septic Tank Design Calculations

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