Septic Tank – The Complete Guide 2020 Edition

From Septic Tank guide to septic tank construction procedure, and Septic Tank Capacity Calculation to Septic Tank Cost. We Covered almost Everything in this Complete Guide.

The design criteria of the septic tank, based on certain assumptions, have been given in detail. The design of inlets and outlets have been modified for the smooth working of the tanks. Arrangements for installing tanks in parallel for the treatment of sewage for the population of more than 300 persons have been included. The procedure for the removal of the sludge has been elaborated. This standard covers the design and construction of a septic tank for small installations ( up to 20 persons ) as well as large installations.

When its Come to Hide or Decorate the septic tank then you can pave interlock tiles over the septic tank.

What is a Septic tank?

A septic tank can be defined as a primary sedimentation tank with large detention time (12 to 36hrs against a period of 2hrs in an ordinary sedimentation tank).

An Illustration With Text Descriptions Of A Septic Tank Using A Section View To Detail The Inner Process And Components.
An Illustration With Text Descriptions Of A Septic Tank Using A Section View To Detail The Inner Process And Components.

Terminology Used in SEPTIC TANK

For the purpose of this standard, the following definitions shall apply.

    • Effluent – The supernatant liquid discharge from a septic tank.
    • Invert – The lowest point of the interior of a sewer or drain at any cross-section.
    • Scum – The greasy and other substances floating on the surface of sewage.
    • Septic Tank – A water-tight single-storeyed tank in which sewage is retained sufficiently long to permit sedimentation.
    • Sewage – The liquid waste of a household or community including human excreta.
    • Sludge – Sludge is the settled solid matter in semi-solid condition.
    • Surface Water – The run-off from precipitation and other water that flows over the surface of the ground.
    • Supernatant Liquor – The layer of liquid overlying the settled solids which have separated from it.
    • Suspended Solids – The solids which are suspended in a sewage or effluent.
    • T. W. L. – The top water level in a tank.
    • Waste Water ( Sullage ) — The discharge from washbasins, sinks and similar appliances, which does not contain human excreta.
Structural Details Of Septic Tank
Structural Details Of Septic Tank

Dimensions of Septic Tank

Septic tank shall have a minimum width of 750 mm, the minimum depth of one metre below water level and minimum liquid capacity of 1000 litres.

For rectangular septic tanks, the length of the tank shall be 2 to 4 times the width. Suitable sizes of septic tanks are given below.

Suitable sizes of septic tanks for use of 5, 10,15,20,50 persons are given in the following table.

No. Of Users Length in Mtr. Breadth in Mtr. LIQUID DEPTH if Cleaning Interval is 1 Year LIQUID DEPTH if Cleaning Interval is 2 Year
5 1.5 0.75 1.0 1.05
10 2.0 0.90 1.0 1.40
15 2.0 0.90 1.3 2.00
20 2.3 1.10 1.3 1.80
Suitable sizes of septic tanks for use of 5, 10,15,20,50 persons are given in the following table (Image).
From the above table, you can easily calculate the size of the septic tank for 25 users.

Using the above method you can calculate the design of a septic tank for 200 users.

For circular tanks, the minimum diameter shall not be less than l-35 m and operating depth shall not be less than 1-O m.

Inlet – The design of septic tank inlets shall be such as to introduce the crude sewage with the least possible disturbance of the settled sludge or the surface scum.

For tanks, not more than 1200 mm wide, T-shaped dip-pipe not less than the nominal bore of the incoming drain may be provided. The pipe shall be fixed inside the tank, with top limb rising above scum level and the bottom limb extending about 300 mm below the top water level. Typical sketch of the septic tank is given in Fig.

For tanks in excess of 1200 mm in width, two or more submerged inlets are preferable. The typical sketch of the septic tank is shown in Fig.


One method of providing such inlets is by the use of submerged bends of the same nominal bore ( not less than 100 mm ) set as closely together as practical in a shallow sump formed within a small benched chamber.

It is important that the invert of the benched channel be 58 mm above the top water level and the inlet ends of the submerged bends should be set flush with the floor of the sump which should be not less than 75 mm below top water level.

The sump may be the full width of the chamber, but should not exceed 1.5 times the nominal bore of the inlet bends in other direction.

The inverts of the outlet ends of these bends should are between 300 and 525 mm below the top water level in the tank.

A septic tank baffle should be provided 150 mm from the inlet end of the tank, extending 150 mm below the invert of the inlet pipes and 150 mm above the topwater level.

Outlet – The final outlet for tanks which are less than1 200 mm wide should be by 100 mm nominal bore dip-pipe fixed inside the tank with a top limb rising above scum level and the bottom limb extending to about l/3 of the liquid depth below top water level.

The invert of the outlet pipe shall be 50 mm below the invert of the inlet pipe.

For wider tanks, it is necessary to use a weir outlet extending the full width of the tank and protected by a scum board fixed 15~ mn from the weir and extending 150 mm above and l/3 of liquid depth below the top water level.

A deflector should be formed either in the structure ‘of the end ( outlet ) wall or by a purpose-made deflector to prevent rising particles from reaching the outlet weir.

This deflector should be located150 mm below the base of the scum board and protrude 150 mm into the tank.

Concrete Septic Holding Tanks In The Ground
Concrete Septic Holding Tanks In The Ground

Here is an example is taken from https://www.indiawaterportal.org/ to Understand Easily,

Design a Septic tank for a colony with 100 Quarters. Assume all other data required.

Given:- No Of Quarters = 100 Nos


• Assumed for calculation:-

  1. Water supply=200 liter/per/day
  2. No of persons per Qrs = 5Nos
  3. Sewage generation = 80% of water supply
  4. Detention period = 18 hours
  5. Cleaning period = once in a year
  6. Length : Breadth (L:B) = 4:1 & Depth of Storage of water = 1.8m
  7. Sludge deposit = 30 lit/person/year
  8. Min Free Board required = 30cm (Rec= 50cm)

Septic Tank Calculation

• Total Waste water coming to septic tank = 100*5*200*80/100=80000 lit/day

Detention period = 18hours

the capacity of tank required =80000/24*18=60000lit

Capacity req for sludge accumulation = 30*5*100 = 15000lit / year

Total capacity req = 60000 + 15000= 75000lit

Plan area of the Septic tank = 75/1.8=41.67m²

L:B taken as 4:1, 4B*B= 41.67,B = √ ( 41.67/4)

B = 3.25m. L = 3.25* 4 = 13m.

Total depth of Septic tank = 1.8+0.5= 2.30m

Soak Pit Design Calculation

Wastewater coming out from septic tank= 80000lit / day
Percolation rate = 1500 lit / m³ / day

Volume of filter media = 80000 / 1500 = 53m³

Depth taken = 2.5m

Area of soak pit = 53/2.5 = 21.2 sqm

Dia of Soak well req= √21.2*4 / π = 5.2m

Septic Tank Construction Procedure

Building a septic tank from start to finish is not a job for an amateur. Indeed some states require septic tanks to be installed by a certified installer [source: Hygnstrom, Skipton, Woldt]. We will now learn something about a septic tank and what you can do in its construction.

  1. Step #1. Calculate the Area and Size of Septic Tank. Determine the capacity The size of a septic tank depends on the number of people in the house. You will have to consult with your installer to determine what size tank you need.
  2. Step #2. Type or Material selection for Septic Tank. Decide what type of tank to buy Septic tanks can be made of steel, fiberglass, or reinforced concrete. Discuss these options with your installer and decide what’s best for you.
  3. Step #3. Choose the Place of Septic Tank (as per Vastu – optional). Determine the location Your septic tank should be located somewhat downhill from the house. This will allow you to take advantage of gravity between the house and the tank, by having the waste flow down the pipe into the tank.
  4. Step #4. Excavate You can dig the hole to put your tank in yourself.
  5. Step #5. Calculate the dimensions and the exact position of the hole you will need to dig by consulting with your installer or with a structural engineer. You will also need to excavate an area for a pipe of at least 4 inches (10 centimeters) in diameter, running from the house to the tank so that for every 12 inches (30.5 centimeters) of pipe length there is a drop of ¼ inch (.64 centimeters). You will also need to run a pipe from the tank to the leach area, where the liquid waste seeps into the soil.

What is the procedure for the construction of the septic tank?

SEPTIC TANK CONSTRUCTION

Floor – It is essential that the floor of the tank be watertight and of adequate strength to resist earth movement and to support the weight of the tank, walls and contents.

The floor may be of cement concrete of minimum M 15 grade and a minimum slope of 1: 10 may be provided towards the sludge outlet to facilitate desludging.

Walls – The walls should be of such thickness as to provide adequate strength and water tightness.

Walls built out of brick should not be less than 200 mm thick and should be plastered to a minimum thickness of 12 mm inside and outside with cement mortar not weaker than 1 : 3; where they are built out of the stone masonry. They should have a minimum thickness of 370 mm.

SLUDGE WITHDRAWAL

Half-yearly or yearly desludging of septic tank is desirable. Small domestic tanks, for economic reasons, maybe cleaned at least once in 2 years provided the tank is not overloaded due to use by more than the number for which it is designed.

NOTE – Frequent desludging inhibits the anaerobic action in the tank. Normally, the tanks are cleaned when tha sum of the depth of the scum and the sludge is observed to exceed half the depth of the tank.

A portion of sludge not less than 25 mm in depth should be left behind in the tank bottom which acts as the seeding material for the fresh deposits.

The digested sludge should be withdrawn through a dip pipe of not less than 150 mm dia under a hydrostatic pressure of at least 450 mm. The sludge pipe shall deliver the sludge to the sump and be provided with a delivery valve to draw the sludge as required.

Portable pumps may also be used for desludging in which case there will be no need for sludge pipe or sludge pump. Manual handling of sludge should be avoided.

When removal of the sludge is carried out the scum in the first tank should not be disturbed more than necessary, thir scum is needed to ensure efficient operation.

Sludge from septic tanks may be delivered into covered pits or into a suitable vehicle for removal from the site. Spreading of sludge on the ground in the vicinity should not be allowed.

COMMISSIONING OF SEPTIC TANK

The sewerage system should be complete and ready for operation before the connection is made to the building. The tank shou!d be filled with water to its outlet level before the sewerage is let into the tank.

It should, preferably, be seeded with small quantities of well-digested sludge obtained from septic tanks or sludge digestion tanks.

In the absence of digested sludge a small quantity of decaying organic matter, such as digested cow dung may be introduced.

Question. What should height Buffel wall in the septic tank?

Answer – A baffle should be provided 150 mm from the inlet end of the tank, extending 150 mm below the invert of the inlet pipes and 150 mm above the top water level.

Question – How to close permanently septic tank?

Answer – Simply Fill the Mud in to Tank.

Why bed slope is to be provided in the septic tank?

Answer – The floor may be of cement concrete of minimum M 15 grade and a minimum slope of 1: 10 may be provided towards the sludge outlet to facilitate desludging.

Question – Maximum height of outlet gas pipe of the septic tank for better gas discharge ??

Answer – Generally the ventilating pipe may extend to a height of about 2 m when the septic tank is at least 20 m away from the nearest building and to a height of 2 m above the top of the building when it is located closer than 20 metres. The ventilating pipe may also be connected to the normal soil ventilating system of the building where so desired. 

 

Septic Tank - The Complete Guide 2020 Edition 1
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