What size should a soakaway be?

What size should a soakaway be?

Stormwater Sinkhole Design

A soakaway, also known as a seepage pit, is a covered, porous-walled chamber that allows water to slowly percolate into the ground. The settled effluent from a Collection and Storage/Treatment or (Semi)Centralized Treatment technology is discharged into the subway chamber from where it infiltrates into the surrounding ground.

The Soak Pit can be left empty and lined with a porous material (to provide support and prevent collapse), or left unlined and filled with large stones and gravel. The stones and gravel will prevent the walls from collapsing, but will leave adequate space for wastewater. In both cases, a layer of sand and fine gravel should be spread on the bottom to help disperse the flow. The soak pit should be between 1.5 and 4 m deep, but never less than 1.5 m above the water table.

A Soak Pit does not provide adequate treatment for primary wastewater, the pit will quickly become clogged. A Soak Pit should be used to discharge settled sewage or graywater. Soak pits are appropriate for rural and peri-urban settlements. They depend on the type of land with sufficient absorption capacity. They are not suitable for areas that are prone to flooding or with a high water table.

Formula for calculating a soakaway

The soakaway is a hole dug in the ground, filled with stones, which facilitates the infiltration of water into the ground. It is used to evacuate grey water (water from sinks and showers and from domestic activities such as cooking, washing up, washing clothes) or rainwater when there are no gutters, channels or networks to drain it. It should not be used for wastewater (gray water + black water from latrines or toilets) because it would directly contaminate the water table.

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A concrete cylinder, called a conduit, usually about 1 m in diameter, is buried in the ground. The conduit is watertight throughout the impermeable layer of soil (at least 50 cm). Further on, it is equipped with large holes that disperse the water in the permeable soil. The conduit is filled with stones of 40 to 80 mm diameter. The pipe that carries the water to the absorption well pours it at its center into a plate that allows it to be dispersed evenly. All this ensures that the water does not fall down the wall and does not exit through a single hole in the conduit, which would locally saturate the soil and prevent proper water infiltration.

Types of absorption wells

They can replace or be complementary to the oxidation field. They consist of excavations of more or less variable diameter and depth. In these the water infiltrates through walls and floor that should be taken permeable, it is recommended to fill with gravel to the approximate height of 1m to achieve a good distribution of water to the bottom”.

The absorption field allows the final treatment and distribution of sewage. A conventional system consists of perforated pipes surrounded by materials, such as gravel and pieces of tire covered with geotextile fabric and clay soil. To treat sewage, this system relies heavily on the soil where microorganisms help remove organic matter, solids and nutrients that remain in the water. As the effluent flows continuously into the soil, microbes that digest the sewage components form a biological layer. The layer reduces the movement of water through the soil and helps prevent the area below the layer from becoming saturated. Water must flow through the unsaturated soil so that the microbes there and in the layer can ingest the waste and nutrients in the effluent. The grass covering the absorption field system also uses the nutrients and water to grow.

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Absorption wells with water

A soakaway well may or may not have an ademe or contrademe; the filter is always necessary to avoid silt (mud or garbage that obstructs a water conduit) from the well; the complementary works depend on the requirements demanded by the authority in each demarcation, normally three filters must be contemplated:

MSI is an engineering company that specializes in the construction of absorption wells with state-of-the-art technology, highly qualified personnel and, in short, experience that translates into savings and quality execution with results that meet the needs of each client.

If you need help to build an absorption well, contact us, our proven methodology will allow you to be safe in an infrastructure work that will result in important benefits for the environment and for your economy.

What size should a soakaway be?
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