Septic Tank: Principles of Design of Septic Tanks and Size Calculations -lceted LCETED INSTITUTE FOR CIVIL ENGINEERS

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May 7, 2022

Septic Tank: Principles of Design of Septic Tanks and Size Calculations


Use of Septic Tanks

Where there is no public sewerage facility, the wastewater has to be taken into a septic tank and treated. The septic tank is an integrated sediment and digestion tank. It works on the principle that aerobic bacteria thrive in the presence of oxygen in the air and anaerobic bacteria thrive in water or saturated water.

In a septic tank, sewage is retained for a period of time so that there is maximum digestion of organic solids in the sewage and heavy solids settle. It works on the anaerobic digestion of wastewater. In a properly constructed septic tank, millions of anaerobic microorganisms break down human waste into gases and liquids.

The solid settles to the bottom and is called a sledge. Usually, in the tropics, it is desirable to break the storage period from 24 to 36 hours. If all the water used is pumped into the septic tank, we estimate the required capacity by providing storage of 180 to 200 litres per person using the tank. If wastewater is excluded, it will be much less. A tank capacity of fewer than five persons is not recommended. 300mm (12 inches) free board is usually provided. Length is usually made three times the width. Details of the layout of small and large septic tanks are shown in the figure below. The details and dimensions of the inlet and outlet of the septic tank are very important and should be set carefully. Septic tanks are also provided with inspection covers and ventilation ducts with mosquito nets.

Use of Septic Tanks

(a) Simple type for a residence with two chambers


Use of Septic Tanks

(b) Ideal septic tank with three chambers for large installations

Septic tanks


Also use: Water-Sump Tank Calculator | Tank Volume Calculator | Sump Volume Calculator

Principles of Design of Septic Tanks

The following are the general principles to be followed in the construction of large sized septic tanks:

1. The capacity to be provided is to be a minimum of 100 litres per user and the minimum number for design should be 5 persons. The minimum retention period should be 24 to 36 hours.


2. The minimum inner width should be 75 cm (2.5 ft). The length should be three times the width. It is customary to divide a medium to large tank into 3 equal chambers. For small tanks, as in ordinary individual residences with 4 to 5 persons, two divisions may be adequate.


3. The liquid depth should be 1 to 2 m only (normally 1.5 m or 5 ft) and the top level of the tank should be 30 cm above the liquid level, thus forming an air space.


4. There will be two cross walls inside the tank with three chambers. The first wall is provided near the entry point and should be constructed on an R.C.C. (1:11 / 2:3) lintel placed 30 cm above the bottom of the tank. It should extend 15 cm above the liquid level. The first chamber is sometimes referred to as the grit chamber.


The second cross wall should be built from the floor level to 15 cm above the liquid level. Holes with honeycombed brickwork are to be provided at a depth of 40 cm from the liquid level in this wall. The water will pass through the holes to the third chamber and the scum will be floating on the top in die second chamber. [In a small two-chamber tank, the second cross wall is omitted as shown in Fig. above


5. The base slab is to be R.C.C. (1: 2: 4) and the walls can be one-brick walls for tanks up to 3.6 m (12 feet) in length. (For large tanks, it can be made as twin tanks side by side with a middle common wall.) For easiness in the cleaning, the top cover slab can be precast slabs which can be easily lifted up and covered with earth to make it airtight: If the top is a continuous slab, manholes should be provided for cleaning.


6. The inside and outside should be plastered with 1 : 3 cement mortar (12 mm thick). The bottom concrete floor is given a 1-in-20 slope towards the inlet side so that the sludge will get collected there. The sludge collected should not be spread on the ground. It should be removed from the site or should be buried in a pit.


7. The inlet level should be higher by 50 nuns than the outlet level.


8. A vent pipe of 100 mm diameter with a wire mesh cowl is to be provided above the first chamber. Its height should be 2 m when the septic tank is 15 m away from the building. If the tank is nearer the vent pipe should extend to 2 m above the top of the building.


9. The outlet should have a dispersion trench so that the effluent can be led to a dispersion trench provided at least 15 cm below the ground level. The stoneware pipes laid in the dispersion trench should be laid with loose joints with sockets in the downstream direction (different from the normal) to allow easy dispersion of the outflowing water (Fig. below).


(As a thumb rule, a tank with an inside dimension 1.5 m x 0.75 m in plan and 1.8 m deep with two divisions can serve 5 persons and a tank with an inside dimension of 2.3 m x 1.1 m in plan and 1.8 m deep with three divisions can serve 20 persons.)

Design of Septic Tanks

Details of dispersion trench: 1. Effluent from the septic tank, 2. Distribution chamber (1 x 1 m) with baffle at the centre to distribute the flow, 3. 75 to 100 mm stoneware pipes laid with tight joints, 4. Unglazed stoneware pipes laid with open joints and large end downstream, 5. Stoneware pipe, 6. 15 to 25 cm depth of gravel or crushed stones, 7. 15 cm graded coarse aggregate topped with sand, 8. Earth filled to 15 cm above the ground level and turfed.

Appropriate sizes of septic tanks based on the number of persons

Standard size of septic tank in feet

Sizes of Septic Tank Based on Number of Persons



What is the purpose of a septic tank?

Septic tanks are often used instead of sewage systems in rural areas, campgrounds and tourist areas to treat human waste and separate solids and liquids from sewage. The liquid portion of the waste is removed through a drainage field where natural filtration takes place in the soil.



What is a septic tank and how does it work?

A septic tank is an airtight chamber made of brick, concrete, fibreglass, PVC or plastic, through which the black water from the cistern or automatic toilets flows and the grey water flows through a pipe from the septic tank. inside a building or an outside bathroom for primary treatment.


Which type of septic tank is best?

The best choice is a precast concrete septic tank. Precast septic tanks have many advantages over plastic, steel or fibreglass tanks. That is why many cities and towns actually need to use concrete septic tanks.



IS Code for septic tank design?

IS: 2470

Indian standard code of practice for installation of septic tanks (IS: 2470) - Bureau of Indian Standards (1986)


How to calculate the size of the septic tank you need?

The size of the septic tank should be calculated using this formula, C = A + P (rq + ns)


C is the capacity of the septic tank,

A is the constant,

P = number of users,

r = minimum retention time,

q = average litre of sewage flow per person per day,

n = number of years of sludge storage and

S = amount of sludge accumulation per person per year.


In the British code, A = 2000litres, rq + ns = 180 litres, so the rounding formula, we get


C = 2000 litres + 180P.



we have to find a capacity of septic tank size for 6 users, but this value


C = 2000 litres + 180 × 6 = 3080 litres,


the minimum size of the septic tank For 6 users, could be 2.5m × 1.2m × 1.2m with respect to their length, breadth and depth. 

The inside of the tank needs to be spaced 1 to 1.5 feet above the water level, so the total height should be 6 feet and the tank capacity should be 3600 litres of liquid waste including depth.

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