What material is specified for the top layer over sand media in a sand filter?

Coarse gravel or stone is the specified material for the top layer over sand media in a sand filter because it plays a crucial role in the function and performance of the filter system. The coarse gravel allows for effective drainage and prevents the sand layer from clogging by supporting the weight of the overlying materials while maintaining adequate permeability.

The design of a sand filter typically involves a layered approach, where each layer serves a specific purpose. The coarse gravel serves as a physical barrier that restricts finer particles from migrating upward into the sand layer, thus maintaining the filtration efficiency. It also ensures that water can freely flow through the filter, preventing waterlogging and promoting the proper functioning of the septic system.

Using materials like fine dirt, sand, or clay for the top layer would hinder the proper functioning of the sand filter. Fine dirt could easily wash into the sand layer, leading to clogging, while clay could seal the surface, preventing water from passing through. Sand might seem similar to the underlying layer, but it would not provide the necessary structure and drainage capabilities needed at the top of the media. This layered structure is key to maintaining good hydraulic and treatment performance in a sand filter system.