Introduction of Soak Pit Design
If there is no intention or need to reuse wastewater, collected rainwater or gray water, immersion pits can offer an economic opportunity for partial treatment of waste-water or ash from a primary treatment (eg septic tank, double pits for flush toilets, biogas collector, deflector anaerobic reactor, etc.) and a relatively safe way to discharge it into the environment and thus recharge groundwater bodies.
As the waste-water (gray water or black water after primary treatment) penetrates the soil from the immersion well, small particles are filtered through the soil matrix and organics are digested by microorganisms.
The waste-water effluent is absorbed by the soil particles and moves horizontally and vertically through the soil pores. The subsoil layers must, therefore, be permeable to water in order to avoid rapid saturation. High daily volumes of discharged effluents must be avoided (HEEB et al. 2008).
Thus, immersion wells are more suitable for soil with good absorption properties; clay, hard or rocky soil is not appropriate. Immersion wells are used in the same way as leach fields but require less space, as well as less operation and maintenance.
But they can also generally receive less influence, and groundwater pollution can be greater than in leach fields.
Design Considerations of Soak Pit Design
The immersion well, basically consisting of a single well (usually approximately 1 m3), should be between 1.5 and 4 m deep, but, as a general rule, never less than 2 m above the water table.
It must be located at a safe distance from a source of drinking water (preferably more than 30 m). The immersion well must be kept away from areas of heavy traffic, so that the soil above and around it is not compacted.
It can be left empty and lined with a porous material to provide support and prevent collapse, or left unfinished and filled with thick stones and gravel.
The rocks and gravel will prevent the walls from collapsing, but will still provide adequate space for wastewater. In both cases, a layer of sand and fine gravel should be spread over the bottom to help disperse the flow.
To allow future access, a removable cover (preferably concrete) must be used to seal the well until it needs to be maintained.
Health Aspects / Acceptance of Soak Pit
As long as the immersion well is not used for raw sewage, and as long as the previous collection and storage/treatment technology is working well, health concerns will be minimal.
The technology is located underground and, therefore, humans and animals should not have contact with the effluent.
It is important, however, that the immersion well is located at a safe distance from a source of drinking water (preferably at least 30 m). As the immersion well is odorless and is not visible, it must be accepted by even the most sensitive communities.
Operation and Maintenance of Soak Pit
A good-sized immersion well should last between 3 and 5 years without maintenance. To prolong the life of immersion well, care must be taken to ensure that the effluent has been clarified and/or filtered to avoid excessive accumulation of solids.
The immersion well must be kept away from areas of heavy traffic, so that the soil above and around it is not compacted.
Particles and biomass will eventually clog the well and will need to be cleaned or moved. When the performance of the immersion well deteriorates, the material inside the immersion well can be excavated and replenished.
To allow for future access, a removable (preferably concrete) lid should be used to seal the pit until it needs to be maintained.
Applicability of Soak Pit
A soak pit does not provide adequate treatment for raw wastewater and the pit will quickly clog. It should be used for discharging pre-settled blackwater or greywater.
Soak pits are appropriate for rural and peri-urban settlements. They depend on soil with sufficient absorptive capacity, whereas clay soils, as well as hard-packed or rocky soils, are not appropriate.
They can be used in almost every temperature, although there may be problems with pooling effluent in areas where the ground freezes.
They are not appropriate for areas prone to flooding or that have high groundwater tables and should be constructed at a distance of at least 30 m from drinking water wells in order to prevent cross-contamination.
Soak Pit: Need and Functions
A soak pit or a soakaway is a closed porous chamber that is directly connected to a primary treatment unit of a residential or commercial building.
It serves the function of letting the wastewater coming from the septic tank to slowly soak into the underlying ground.
Why Need for Soak Pit of Soak Pit Design?
The water coming out from a primary treatment chamber is not pure. The primary treatment unit can be a septic tank, a biogas settler, anaerobic baffled reactor, twin-pits, etc.
The effluent water coming out of these units are called greywater, which must be subjected to a partial treatment before letting it into the ground soil.
For this partial treatment, most of the wastewater management system needs a soak pit. There is no intention to reuse the wastewater that is coming from the primary treatment unit.
But the partial treatment of effluent water through a soak pit is a safer way of discharging the water into the environment.
Function of Soak Pit
The greywater passing through the soak pit is subjected to filtration. This results in the settlement of smaller particles of effluents at the bottom of the soak pit.
These smaller particles are digested by the microorganisms which are a sustainable process of degradation. The filtered water is then discharged out through the porous wall of the soak pit.
In General, the Important Functions of a Soak Pit Are Enlisted Below:
- Soak pit serves the purpose of receiving the effluent water from the primary treatment
- Soak pit undergoes the partial treatment of the effluent water coming out of the primary treatment unit.
- The soak pit discharges clear and non-harmful water to the ground.
- The soak pit is designed in such a way that the treated water comes out of the porous walls of the soak pit.
- The soak pit helps to recharge the groundwater bodies.
Advantages of Soak Pit
Heres, pros of Soak pit are as follows.
- It can be built and repaired with locally available materials.
- Technique simple to apply for all users.
- A small land area is required.
- Low capital costs; low operating costs.
- Recharging groundwater bodies.
Disadvantages of Soak Pit
Heres, cons of Soak pit are as follows.
- Primary treatment is required to prevent clogging.
- May negatively affect soil and groundwater properties.
- Applicable only were soil conditions allow infiltration, the groundwater table is at least 5 m below the soak pit, there is no risk for flooding and any water well is in a distance of at least 30m.
- Difficult to realize in cold climate.
- Should be avoided for high daily volumes of discharged effluents.
Summary of Soak Pit
A soak pit, also known as a soakaway or leach pit, is a covered, porous-walled chamber that allows water to slowly soak into the ground.
Pre-settled effluent from a collection and storage/treatment or (semi-) centralized treatment technology is discharged to the underground chamber from which it infiltrates into the surrounding soil.
Frequently Asked Questions (FAQ) about Soak Pit Design
What is a soak pit?
A soak pit, also known as a soakaway or leach pit, is a covered, porous-walled chamber that allows water to slowly soak into the ground. It receives pre-settled effluent from a primary treatment unit and facilitates its infiltration into the surrounding soil.
Why is a soak pit necessary?
A soak pit is necessary for the partial treatment of effluent water from primary treatment units like septic tanks, biogas settlers, or anaerobic baffled reactors. It ensures that the wastewater undergoes further filtration and safe discharge into the environment, helping to recharge groundwater bodies.
What type of wastewater can be discharged into a soak pit?
Soak pits are designed to handle greywater and pre-settled blackwater. They are not suitable for raw sewage as it can quickly clog the pit.
How deep should a soak pit be?
A soak pit should be between 1.5 and 4 meters deep, but it should always be at least 2 meters above the water table to prevent contamination of groundwater.
What are the soil requirements for a soak pit?
The soil must have good absorptive capacity to prevent rapid saturation. Soak pits are not suitable for clay, hard-packed, or rocky soils.
How far should a soak pit be from a drinking water source?
A soak pit should be located at least 30 meters away from any drinking water source to avoid the risk of contamination.
How long does a soak pit last?
A well-constructed and maintained soak pit can last between 3 to 5 years without needing significant maintenance.
What maintenance does a soak pit require?
To prolong its life, ensure the effluent is clarified and filtered to prevent excessive solid accumulation. Over time, particles and biomass will clog the pit, necessitating excavation and replenishment of the material inside the pit.
Can a soak pit be used in all climates?
Soak pits can be used in almost all climates, but there may be issues with effluent pooling in areas where the ground freezes.
Are there any disadvantages to using a soak pit?
Yes, primary treatment is required to prevent clogging. Soak pits may negatively affect soil and groundwater properties and are only suitable where soil conditions allow infiltration.
They should be avoided in areas with a high groundwater table, flood-prone regions, and where there is a risk of freezing or high daily volumes of effluents.
What are the advantages of using a soak pit?
Soak pits have several advantages: they can be built and repaired with locally available materials, require a small land area, have low capital and operating costs, and help recharge groundwater bodies.
Is a soak pit visible or odorous?
No, the soak pit is located underground and is designed to be odorless, making it an acceptable solution even for sensitive communities.
