Soak Pit ( Leach Pit) : 2 Types, Function, Design, Advantages & Disadvantages of Soak Pit

A soak pit may be defined as a closed porous chamber that is connected to the primary treatment unit of a residential or commercial building that permits the passage of wastewater through it.

✔ It allows the wastewater collected from the septic tank of the building to soak into the underlying ground.

It is also referred to as a soak way or leach pit or seepage pit.

✔ A soak pit is a relatively efficient construction that allows the infiltration of wastewater collected in a septic tank or even a certain type of latrine deep into the ground when the wastewater is not intended for reuse.

It is a commonly used technique for discharging wastewater and also helps in the recharge of groundwater.

✔ As the wastewater penetrates the ground; smaller particles get filtered by the matrix of the soil itself while the organic waste present in the wastewater is decomposed and digested by the micro-organisms present in the soil.

It is most suitable in places where the soil is highly porous.

A well-constructed soak pit can last for a minimum period of 3 to 5 years. 

✔ If it is constructed with proper design and over highly porous soil; it may even last up to 10 years or more. This value also depends upon the number of users.

( For example: If the soak pit is designed for 10 users but there are only 5 people living in the home then life will be more.)

  1. Needs of Soak Pit  

✔ The wastewater is also known as greywater that is obtained from the primary treatment unit of the building such as a septic tank, settler, twin-pit latrine, etc which is impure and thus cannot be allowed to percolate directly into the ground.

✔ A certain degree of treatment must be provided to the collected wastewater before it is allowed to settle.

For such partial treatment, a soak pit is generally provided.

The wastewater infiltrated from the soak pit can be used for the recharge of groundwater.

  2. Design  Considerations of Soak Pit  

1. The design of a typical soak pit generally consists of a pit of approximately 1 cubic meter with a depth of 1.5m to 4m.

2. It must not be constructed nearby a drinking water source as it can pose contamination of the drinking water source.

4. It must be noted that the soak pit is not constructed in an area subjected to a high volume of traffic as it may cause compaction of the pit.

5. A thin layer of sand and gravel may be provided at the bottom of the pit to disperse the flow of wastewater.

7. A lid preferably a concrete lid must be constructed to cover the soak pit.

  3. Functions  

1. It serves the function of a receiver and collects the wastewater from the primary treatment unit.

2. It allows the collected wastewater to get purified to some extent.

3. It thus allows non-harmful water to infiltrate into the ground.

4. It thereby helps in the recharge of groundwater.

  4. Types of Soak Pit  

i. Lined Soak Pit

A lined soak pit is used when the inner volume is kept empty.

✔ The lining may be of bricks, stone, or concrete blocks, with dry open joints, and with at least 7.5 cm backing of coarse aggregate.

This pit is also known as an empty soak pit. 

ii. Unlined Soak Pit

✔ If the inner volume is filled with stone and brick aggregates, no lining is required, except for the masonry ring provided at the top. Such a soak pit is named an unlined soak pit.

Fig: Unlined Soak Pit

  5. Operation / Maintenance  

∼ Without any type of maintenance, a well-sized soak pit should last for a minimum of 3 to 5 years.

∼ Soil should not be compacted and the soak pit must be kept far away from the high-traffic area. Otherwise, the pit may clog due to particles and biomass and require to be cleaned or moving.

∼ A removable lid should be provided so that it could be easily maintained.

  6. Health Aspects  

∼ Since the soak pit is mostly constructed underground, there is less chance to come in contact with humans and animals.

∼ It should be at a safe distance from drinking water sources. So, it should be located at least 30 m (100′ ) distance away from the water tank. ( if not possible; a minimum of 10 m at any condition).

∼ It can be constructed in the most sensitive communities because it is usually odorless and not visible.

  7. Factors Affecting The Filling Period  

a. Nature of soil

If soil is highly porous; the leach pit lasts longer and vice versa.

b. Number of users and designed users

If actual users are less than designed users then the leach pit lasts longer.

  8. Advantages  

1. It is a relatively simple and easy technique.

2. The construction is simple and locally available materials can be used for its
construction.

3. It requires only a small area.

4. The initial cost, as well as the operation cost, is affordable.

5. It helps in the recharge of groundwater bodies.

  9. Disadvantages  

1. It is applicable only in areas where the soil is porous and allows the percolation
of wastewater.

2. It is not suitable for highly congested areas.

3. There is a high risk of clogging in the absence of primary treatment.

4. It is not suitable for cold climatic conditions.