Soak Pit: Meaning & Design

Soak Pit

The Soak Pit which is also called the soakaway or soakage pit is a closed rectangular or circular, covered-up construction with porous or perforated walls, which is connected to the primary treatment unit or directly connected to the washroom and even some specific types of toilet.

It allows water to slowly penetrate into the ground. A soak pit, which is lined with porous materials that provide foundational support to prevent the collapse of the underground chamber, may also be used for separate treatment of greywater (water from domestic sanitation, like showering and from kitchen areas).

It is one of the most commonly used techniques to discharge industrial and domestic wastewater (after primary treatment) safely into the ground and even helps with the recharge of groundwater.

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Soak Pit Meaning

The Soak Pit which is also called the soakaway or soakage pit is a closed rectangular or circular, covered-up construction with porous or perforated walls and a chamber that allows water to slowly soak into the ground

How Does Soak Pit Work

The soak pit receives effluent from the primary treatment unit or washroom/latrine for the safe disposal of wastewater (greywater and black water) and gradually allows the effluent to percolate into the surrounding soil.

As the effluent contains organic materials in it, (as it is grey and black water), the water in it percolates through the neighboring soil from the soak pit, small particles in the effluent are filtered out by the soil matrix and organic matter is digested by microorganisms.

Thus, these are best suited in porous soils with good infiltration and absorptive properties. Hard-packed clay and rocky soils are not suitable for its construction. The workings of the soak pit are similar to a leach field but it occupies considerably less space than a leach field as well as less operation and maintenance work.

But they can also generally receive less influence, and groundwater pollution can be greater than it is in leach fields.

Need for Soak Pit

The effluent from a primary treatment unit (e.g. septic tank, twin-pits for pour-flush toilets, biogas settler, anaerobic baffled reactor, etc.) and wastewater or greywater from industry or domestic use are impure, thus cannot be allowed to directly release in the environment.

If it is discharged in the open it creates not only unhygienic conditions and foul odor but may also invite epidemics and diseases. So a certain degree of treatment must be provided to it before it is safely disposed of on the ground. For such partial and light treatment, a soak pit is provided. Where it also recharges the groundwater below.

A soak pit is also a great way of collecting and using stormwater to recharge and raise the groundwater table. Which makes it a viable option for rainwater harvesting.

Soak Pit Function

A sock pit serves follows functions,

• It collects wastewater from the primary treatment unit or domestic use.
• It purifies the collected wastewater to some extent and serves as a partial treatment unit.
• It allows the discharge of relatively clean and non-harmful water to the surrounding ground.
• It permits the reuse of wastewater to recharge the groundwater.
• It can also serve the sole function of collecting and infiltrating stormwater from the paved areas to the ground and allow relinking of the earth below to the hydrological cycle.

Soak Pit Design

A pit is dug into the ground and either lined with porous material or unlined and filled with gravel and coarse rocks.

A stratum of sand and fine gravel is spread across the lowermost to aid in the dispersion of the flow. The pit is then covered on top.

Design Considerations of Soak Pit

The following conditions should be considered for the design of the soak pit.

1. Generally, the soak pit should consist of a chamber of approximately 1 m³ and the depth of it should be kept between 1.5 m and 4 m deep.
2. Its bottom should never be less than 2 m above the groundwater table.
3. It can either be lined with a porous material to provide additional support and prevent the collapse of walls if it is to be left empty. Or it can be left unlined and filled with gravel and sand to prevent the walls from collapsing, which will still provide adequate space for wastewater.
4. Though in both the above-mentioned cases, a layer of sand and fine gravel is to be spread across the bottom of the soak pit to help disperse the flow.
5. A removable lid (preferably concrete) should be provided to seal the pit until it needs to be maintained to allow for easy future access.
6. It should be constructed at a safe distance of at least 30 m away from local drinking water sources as it can possess the threat of contamination of groundwater.
7. It should also be kept well away from high-traffic areas so that the soil above and around it is not compacted.
8. It should be made sure that the site does not gather water from surface runoff, as too much mud filtered in the pit will clog the gravel and sand layer which will block the seepage and water will stagnant.

Operation and Maintenance of Soak Pit

A well-sized soak pit can last for 3 to 5 years without any type of maintenance. Though it requires periodic care to extend its life.

Particles and biomass filtered will eventually clog the soak pit and it will need to be cleaned or have the material replaced. Maintenance of the soak pit involves excavation, cleaning, and either washing and reclaiming the filter material or replacing new gravel, rock, and sand.

The lifespan of a soak pit can be stretched longer by filtering the influent to prevent excessive accumulation of solids. Clogging of soak pits is expected after a period of time owing to the removal of particles and the growth of biomass within the chamber.

This clogging will ultimately lead to the failure of the system unless the chamber contents are emptied and cleaned or replaced. For maintenance, a removable lid should be provided so as to provide easy access.

Applicability of Soak Pit

The soak pit does not provide adequate treatment to the raw wastewater and can get clogged really easily. It should be made sure that only pre-settled black or grey wastewater post-primary treatment is admitted in the soak pit.

Soak pits are appropriate technology for rural and suburban areas. It relies on the sufficient absorptive capacity of the soil. So they are not appropriate for areas prone to flooding or have high ground.

Advantages of Soak Pit

Soak Pit has the following advantages,

1. It can be built with locally available materials and is easy to repair.
2. It is a very simple technology to construct, operate and maintain for all kinds of users.
3. Its installation requires quite a small land area compared to other technology such as say a leech field.
4. It also helps to recharge the groundwater table below.
5. Very minimal initial cost and operation costs are required for the soak pit. So it is very affordable.
6. It is power-conservative technology and is sustainable.
7. It is a suitable technology for rural or suburban areas without a proper drainage system.

Disadvantages of Soak Pit

The following are soak pit disadvantages,

1. The primary treatment of wastewater is necessary to prevent the high risk of clogging.
2. Its installation may negatively affect groundwater and soil properties, in case it is too near the groundwater table or if effluent is highly toxic.
3. It is not a suitable technology for countries/areas with colder climates.
4. These are not very effective and efficient technology where the daily volume of discharged effluents is high.
5. They are only suitable for areas where the soil is porous and allows the percolation of water.
6. If the groundwater table is at least 2 m below the soak pit, then it is suitable.
7. It is not an appropriate technology for the highly congested area.

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