8.4.1 Disposal Pit

A disposal pit is an excavated or dug-out area designed for the intentional placement and containment of waste materials, such as sludge or other types of solid waste. Disposal pits are engineered structures that facilitate the controlled deposition of waste in a specific location. These pits are often used for long-term isolation of waste from the surrounding environment.

The disposal pit is a simple solution for variable volumes of sludge. It should be installed whilst maintaining ain safe distance from housing (>30 m) and in consideration of the groundwater table (>1.5 m above the highest level). The area in which one or several disposal pits are installed should be fenced to prevent potential accidents or unauthorized entry.

What are the requirements regarding sludge?

The sludge should be dewatered to have a dry matter content ranging between 20% and 35%. Aside from local regulations and guidelines, the recommended dry matter content depends on factors such as soil permeability, groundwater depth and potential risk of leachate migration. Generally, a higher dry matter content can reduce the volume of sludge, risk of leachate formation and potential contamination of surrounding soil and water sources.

What are essential design considerations?

The pit can have different shapes. A circular shape is recommended as they are usually more stable due to the natural arching effect of the ground around the hole, and there are no corners to concentrate the stresses. Rectangular or square pits are more likely to need support.

For circular pits, a diameter between 1 and 2 m is recommended. A depth of 3 m is recommended. However, if the water table is deep and the soil is suitable, a depth of 10 to 12 m is possible.

If planting is intended, the topsoil cover of about 0.5 m is essential. Trees will be planted into the topsoil, and the roots will spread and take in nutrients from the sludge as needed.

In what cases is a liner necessary?

A liner refers to a layer or barrier system with the primary purpose of preventing the migration of contaminants, particularly leachate (liquids formed as water percolates through sludge/waste), into the surrounding soil and groundwater.

A disposal pit requires a liner in the following scenarios:

• When the natural soil at the disposal site has high permeability, allowing water or contaminants to easily pass through.
• In areas where the disposal pit is located near or above the water table.
• If the disposal pit is situated in an area with potential for surface water runoff.
• When the sludge may contain substances that could harm the surrounding soil or compromise its quality.
• In case of intended long-term containment for hazardous materials.
• When communities are nearby and it is essential to protect public health and well-being.
• In case the sludge/organic waste contains still high amounts of organic prone to producing gases (e.g. methane).

A liner system can be made of geomembrane, clay (compacted bentonite or other clay materials), geotextile fabric or can be a composite of multiple materials.

What is to be considered for the operation of a disposal pit?

An essential aspect, specific to disposal pits, is the application of covering material. For this purpose, the soil removed during excavation can be added again little by little after each sludge filling. Adding a layer of soil (or ash – if available) helps…

• To control odours that may emanate from the deposited sludge, the soil has properties that can help absorb and neutralise odorous compounds.
• To create a cover or barrier over the deposited sludge to prevent the spread of disease vectors like flies or rodents.
• To facilitate further decomposition and stabilisation of the sludge, while the soil may have microorganisms that assist in this natural process.
• To minimize the potential of environmental contamination as it prevents rainwater from direct infiltration which may cause the carrying of contaminants into the surrounding soil or groundwater.