What Is Septic Tank | How Does A Septic Tank Work | Septic Tank Design based Per User Consumption

What Is Septic Tank | How Does A Septic Tank Work | Septic Tank Design based Per User Consumption

What Is Septic Tank?

A septic tank collects and treats wastewater in a property that isn’t linked to the mains sewer system and is, thus, most commonly used in rural areas.

Septic tank

The term ‘septic’ means’infected with microorganisms’; the tanks contain bacteria that break down organic waste.

The inclusion of a bacterial treatment mechanism would be the difference between a septic tank and a cesspit, and such structures have both industrial and residential appliances.

Installed underground, a septic tank makes use of natural processes to treat the sewage it stores. Usually made up of two compartments or chambers, the tank receives wastewater from an inlet pipe.

If you’re in need of maintenance servicing or work on a Septic Tank, follow this link, otherwise read on to learn more about Septic Tanks, and how they may be emptied.

Also, read: IS 516:1959 Most Important Point (Method of Tests For Strength of Concrete)

How Does A Septic Tank Work?

A septic tank May digest organic matter and separate float-able matter(e.g., oils and grease) and solids in the wastewater  A septic tank will be connected with two pipes (for inlet and outlet).

The inlet pipe can be used to transport the water waste from the house and collect it in the septic tank. It’s kept here long enough so that the liquid and solid waste are separated from each other.

Septic tank

The second pipe is the outlet pipe. It can also call the drain field. This pipe moves out of the pre-processed wastewater from the septic tank and spreads it evenly from the soil and watercourses.

When waste-water was collected after a while, it will begin to separate into 3 layers. (as shown in the image above) The top layer is oils and grease and floats above all the waste.

This commonly referred to as”scum.” The middle layer contains wastewater along with waste particles. The third and bottom layer consists of particles that are heavier than water and form a layer of sludge.

Bacteria within the tank does it is best to break down the solid waste, which then allows liquids to drain and separate away easily.

What’s left at the bottom of the tank is what needs to be periodically removed as part of general maintenance. This is one of these reasons why a septic tank is only a basic form of sewage treatment.

Also, read: What Is Inverted Beam | Advantages of Inverted Beam | Purpose of Inverted Beam

Septic Tank Construction

It’s best to construct a single compartment for small capacity septic tanks. Larger capacities are the best fit for a both-compartment tank.

The wall separating the both is about two-thirds the length away from the inlet. It, therefore, perform better than the single compartment tank.

Septic tank

Both compartments interconnect above the sludge storage using pipes or square openings. A plastic or concrete tank would be the best to use.

The floor and side walls are made with plastered and concrete to render a smooth surface. The floor has a slight slope of 10 percent towards the sludge outlet.

As a result, slung moves out more easily. The top cover of a septic tank is watertight and very thick. But a manhole of relative size is created. This allows inspection and emptying of the tanks.

Also, read: 23 Different Types Cements Available in India and As Per IS Standards

Septic Tank Design based Per User Consumption

The proper capacity of a septic tank is essential otherwise wastewater backflow toward the house. The septic tank system much designed considering a future increase in wastewater generation design proper capacity septic tank that may last long for years.

Let take one example of House – 4 Members.

Daily Water Usage for a House – 4 Person

Cooking  – 8 Liters

Bathing & Toilet – 90 Liters/Person, So for 4 person  – 360 liters/Day

Washing clothes & Utensils – 28 Liters

Cleaning House – 12 Liters

Other – 8 Litres

Total – 416 Litres/Day Approximately

We consider detention time as 4 days. So the designed tank should have the capacity to retain the household wastewater at least 4 days.

Total wastewater in 4 days – 416 × 4 = 1664 Liters

So, we take more than that about 1800 Liters capacity minimum for a home. For Septic tank design depth of the tank should not be less than 1.5m.

Take sludge settled down per person – 30 liters/year. So here we take sludge removal 1 year once.

Total Accumulated Sludge = 30 litres x 4 persons x 1 years = 120 Litres

Total Septic Tank Capacity = 1800+120 = 2120 Liters

We know that 1 cubic meter = 1000 Liters = 2120 /1000 = 1.92 Cum

Area required @ 1.8m depth = 1.92 / 1.5 = 1.28 Sqm

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

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

So, 4 B2 = 1.28 Sq. m where B=0.565 m

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

So that L = 4 × 0.75 = 3m

L – 3 m; B – 0.75 m ;

Depth = 1.5 + 0.3 = 1.8 m (free board should be provided at least  300mm)

Septic Tank capacity = 3 x 1.8 x 0.75 = 4.05 Cum

Septic Tank capacity = 4.05x 1000

Septic Tank capacity = 4050 Liters


British Standard Calculation:

British standard given following formula calculate the wastewater flow for a septic tank.

C=A + P (r q + n s)


C = Capacity in liters

P = Number of People

A = 2000 Litres as constant

R = Detention period of Sewage in Days

Q = Sewage Flow in liter per day

N = Number of years

S = Sludge accumulation in liters per person/year

Simplification of (r q + n s) = 180 Litres

We can Rewrite this formula C=A + 180 P

C = 1800 + (180 × 4)

C = 2520 Litres

3 thoughts on “What Is Septic Tank | How Does A Septic Tank Work | Septic Tank Design based Per User Consumption”

  1. Dear Sir/madam
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  2. what i have seen here is a good design of septic tanks of which the capacity is fit for households. i am actually very much interested for a capacity that fits an Institution. Am happy of the breakdown even if one is not an engineer one can understand it immediately.


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