What type of filter media is most commonly used in drain fields?

Gravel is the most commonly used filter media in drain fields due to its excellent drainage properties and ability to support the filtration of effluent. It provides a large surface area that allows for the effective movement of water and the aeration of microorganisms that help break down contaminants in the wastewater. Gravel also facilitates the passage of water while preventing the clogging of the system, which is essential for maintaining its functionality and longevity.

In addition to its drainage capabilities, gravel is less susceptible to compaction compared to other materials, which can impede water flow. The size and shape of gravel particles are ideal since they create voids for easy drainage while still allowing for effective filtration.

Other materials like clay and silt tend to retain water and can create an impermeable barrier, which is undesirable in drain fields. Concrete does not serve as filter media; rather, it may be used in constructing structures within the drainage system but does not provide the necessary filtering action desired in a drain field.