Septic Tank Wikipedia – The Complete Guide

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 is a large underground tank that treats wastewater from homes and businesses that are not connected to a municipal sewer system. The tank is designed to separate solids from liquids and to allow bacteria to break down the organic matter in the wastewater. The treated wastewater is then discharged into the soil, where it is further purified.

Septic tanks are a common type of wastewater treatment system in rural areas and in areas where it is not practical to connect to a municipal sewer system. They are also used in some urban areas, such as areas with older homes that were built before sewer systems were installed.

Septic tanks work by using the natural processes of sedimentation and bacterial decomposition to treat wastewater. When wastewater enters the tank, it settles to the bottom, where the solids are separated from the liquids. The solids form a layer of sludge at the bottom of the tank, while the liquids rise to the top and form a layer of scum.

The bacteria in the tank break down the organic matter in the wastewater. This process produces methane gas, which is released into the atmosphere. The treated wastewater then flows out of the tank and into the soil, where it is further purified by the bacteria in the soil.

Septic tanks require regular maintenance to ensure that they function properly. The sludge and scum in the tank need to be pumped out periodically, and the bacteria in the tank need to be kept alive by adding septic tank additives. If a septic tank is not properly maintained, it can become overloaded and can pollute the surrounding environment.

Here are some of the benefits of using a septic tank:

• They are a relatively inexpensive way to treat wastewater.
• They are a reliable way to treat wastewater, even in areas with poor soil conditions.
• They are a low-maintenance system.

Here are some of the drawbacks of using a septic tank:

• They can be a nuisance to maintain.
• They can pollute the surrounding environment if they are not properly maintained.
• They are not suitable for all areas, such as areas with high water tables.

Overall, septic tanks are a reliable and effective way to treat wastewater in areas that are not connected to a municipal sewer system. However, they do require regular maintenance to ensure that they function properly.

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).

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.

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.

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.

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 = 18 Hours
• The capacity of tank required = 80000/24*18 = 60000 liter
• Capacity required for sludge accumulation = 30*5*100 = 15000 litre / year
• Total capacity required = 60000 + 15000= 75000 liter
• 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)
• Breadth = 3.25m. Length = 3.25* 4 = 13m.
• Total depth of Septic tank = 1.8+0.5 = 2.30 m

Soak Pit Design Calculation

1. Wastewater coming out from septic tank= 80000lit / day
2. Percolation rate = 1500 lit / m³ / day
3. Volume of filter media = 80000 / 1500 = 53 m³
4. Depth taken = 2.5 m
5. Area of soak pit = 53/2.5 = 21.2 sqm
6. Diameter of Soak well required = √21.2*4 / π = 5.2m

Septic Tank Construction Procedure

Septic Tank Construction : 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.

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

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 Of The Septic Tank

Half-yearly or yearly desludging of septic tanks 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.

Cleaning of the Septic Tank

To avoid sludge build-up and eventual effluent escape into the drainage field, septic tank treatment systems must be cleaned on a regular basis. If this happens, the leach field piping may become clogged, necessitating costly repairs.

The tank’s periodic time interval is determined by the tank’s volume in relation to the solids input, the number of indigestible solids, and the ambient temperature.

Septic tank systems are rarely cleaned more than once a year, and many users can reduce emptying to every 3 to 5 years with careful management.

When cleaning the septic tanks, it’s important to keep in mind that only a small amount of sludge should be left in the tank. When the tank is refilled, anaerobic decomposition is quickly restarted.

A well-designed and well-maintained septic tank system produces no odours. The tank’s periodic inspection should last for decades without requiring any maintenance.

A well-maintained concrete, fibreglass, or plastic septic tank should last for about 50 years.

Commissioning Of Septic Tank

For commissioning of the tank, it is suggested that sewerage system be complete and ready for operation, before connection is made to the building. Furthermore, the tank should be filled with water to its outlet before sewage is allowed in, and a small amount of well digested sludge should be present as a seed.

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

Septic tanks 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.

Indian Standard – Code of practice for installation of septic tanks – Attachment

1. IS: 2470 – Indian Standard – Code of practice for installation of septic tanks – Part 1 – Design criteria and construction – Bureau of Indian Standards – 1986
2. IS: 2470 – Indian Standard – Code of practice for installation of septic tanks – Part 2 – Secondary treatment and disposal of septic tank effluents – Bureau of Indian Standards – 1985