design of septic tank

Septic Tank Design & Construction Details

Septic Tank Design & Construction Details

 Septic Tanks:

  A septic tank is also known as a sewage collection system is 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 system 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 limited to rural areas only.

The term “septic” is expressing the anaerobic bacterial condition that immerge 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 the capacity of sewage is 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 such a designed which allows the heavy solid waste to settle down from wastewater.

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

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


Construction and Working of Septic Tanks:

  • 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 roof of the tank.
  • 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, solid 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 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 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.

Cleaning of the septic tank:

  • 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.
  • The periodic time interval of 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 to 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.


Septic Tanks Potential problems: 

  • Sometimes with 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 material 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.

Septic Tank Design based Per User Consumption:

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

Septic Tank Design & Construction Details

Let 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 person  – 450 liters/Day
  • Washing clothes & Utensils – 35 Liters
  • Cleaning House – 15 Liters
  • Other – 10 Litres

Total – 520 Litres/Day Approximately

We consider detention time as 3 days. So the designed tank should have the capacity to retain the household wastewater 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 Septic tank design 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 Length and Width ratio of Septic Tank is 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 (free board should be provided at least 300mm)

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

= 4.725x 1000

= 4725 Liters

Septic Tank Design & Construction Details


British Standard Calculation:

British standard given following formula calculate 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 liter per day

            N – Number of years

S – Sludge accumulation in litres per person/year

Simplification of (rq + ns) = 180 Litres

We can Rewrite the formula C=A + 180 P

C = 2000 + (180 × 5)

   = 2900 Litres

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