Soak Pit Design| Soak Pit Construction

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Written By Sourabh Negi

I cover sustainable building construction and green technology.

A soak pit is generally a circular, porous, covered pit, through which the effluent or wastewater is allowed to be absorbed or discharged into the surrounding soil if there is no need to reuse it. It is also called sewage pit, soakaway, or leach pit. As it is a mechanism of water treatment thus soak pit design should be property done keeping in mind the standards laid.

It is a common technique for the discharge of waster water into the surrounding soil which helps in recharge of the ground.

What is the need for Soak Pit?

The wastewater coming from the septic tank is not pure enough to directly discharge it into the adjacent ground. It contains a large amount of organic matter ( 200 to 250 mg/l). Also, the Biochemical oxygen demand (BOD) of such effluent is around 100 to 200 mg/l which is quite high.

Such impure water if disposed directly into the soil can create severe health problems thus needs to be treated to such an extent where it can be directly discharged into the ground.

Basic Information Required for Soak Pit Design

  • Information related to the nature of soil and subsoil conditions should be obtained for soak pit design.
  • Approximate depth of water table and seasonal changes if any.
  • An approximate flood level is also required to be obtained.
  • Soil exploration should be done to a sufficient depth to determine the underlying soil condition.
  • Sometimes it may be possible that there is an impervious granite layer under sand or an impervious clay layer under the pervious topsoil. To know such conditions soil exploration is required to be done.
  • The site plan of the proposed or existing building, as well as the reduced level of the site, is required to be known.
  • The quantity of discharge from the septic tank is required to be known.
  • Wells, small streams, and ditches in the vicinity if any.
  • Land area available for the absorption and permeability of the surrounding is required to be known.
  • The risk of infecting any surrounding well, underground water supplies, swimming pool, etc should be known.

Soak Pit Design for Wastewater as per IS code

The soak pit design shall be as per the standards laid. Here the design of the soak pit as per Indian Standards has been shown.

Soak Pit Dimensions

The soak pit may be of any suitable shape preferably circular. It should have at least a cross-section dimension of 0.90 m and a minimum depth of 1.0 below the invert level of the inlet pipe. The soak pit may be filled with stone aggregates or may be kept empty.

Soak pit design of empty soak pit with brick or stone lining
Soak pit design drawing of empty soak pit with lining

When the soak pit is kept empty it should be lined with brick, stone, or concrete blocks. And lining should be done with dry open joints to let the effluent or wastewater flow outward. This lining should be supported by at least a 7.5 cm thick backing of coarse aggregate.

The lining above the inlet level of the inlet pipe should be plastered with cement mortar. In the case of a soak pit filled with stone or brick aggregates, no lining is required because the filling material will support the surrounding soil.

Soak pit design of filled soak pit without lining.
Soak pit design drawing of filled soak pit without lining

In this case, only a top masonry ring is required to be constructed. This is done so to prevent the damage done by the flooding of the pit or by surface runoff. Inlet pipe should not be provided at the surface level but it should be provided at least 90 cm below the top surface to prevent mosquito nuisance.

If the pit is large in size the top portion may be narrowed to reduce the size of RCC cover slabs.

Allowable rate of effluent applications to the soil absorption system

Percolation rate (Minutes)The maximum rate of effluent application (per/m2/day)
Upto 1204
2143
3118
4102
590
1065
1552
3037
4533
6026

Functions of Soak Pit

  • It collects the wastewater or effluent from the primary treatment unit like a septic tank.
  • Water gets treated to such an extent that it may be possible to discharge it into the ground.
  • Treated water is allowed to discharge into the ground.
  • It helps in groundwater recharge.

Advantages of Soak Pit

  • No technical know-how is required. It is a very easy and effective method.
  • Construction of soak pit is very simple It requires cheap quality or easily available material like stone, bricks, etc.
  • The initial cost is very low thus can also be used in rural areas.
  • Due to its low operation cost, it has wide usage.
  • Small land area is required
  • Helps in groundwater recharge

Disadvantages of soak pit

Soak pit can be constructed only at that place where the ground is porous. In places where the ground is relatively impermeable soak pit cant be constructed. It cant be used without the primary treatment units like a septic tank because of the risk of clogging.

They cant be used in areas prone to flooding or in areas having high water tables because the effluent may get mixed with the water which can create health problems. Although It is suitable for all temperatures. But it cannot function properly in places having very low temperatures because of the freezing of the ground.

If the percolation rate is more than 30 minutes, the soil is unsuitable for soak pits. If the percolation rate is more than 60 minutes, the soil is unsuitable for any soil absorption system.

Maintenance of Soak Pit

Normally soak pit lasts for 3 to 5 years. When the soak pit becomes full, it should be excavated, cleaned, and refilled. To increase the lifespan of the soak pit it should be regularly cleaned. To facilitate easy cleaning a removable lid should be placed preferably of concrete over the soak pit.

Health aspects

IT should be sufficiently distant from the drinking water source preferably 30 m away from wells and natural water sources so that such sources do not get contaminated. Sever health concerns is not present in soak pits as long as primary treatment units like the septic tank are used.

As the treatment process is happening below the ground there is less possibility of contact with the living life.

Summary

A soak pit is generally a circular porous covered pit, through which the effluent or wastewater is allowed to be absorbed into the surrounding soil if there is no need to reuse it. It is a cheap and effective method of secondary treatment of effluent.

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