Replacing about 6" of sand with 6" of anthracite coal is called ________ a filter.

Replacing about 6 inches of sand with 6 inches of anthracite coal is referred to as capping a filter. In filtration systems, particularly in water treatment processes, capping involves placing a material that has a different filtration characteristic on top of an existing medium. Anthracite coal has a higher density and a larger surface area compared to sand, which enhances the filter's ability to remove finer particles and improve overall filtration effectiveness.

This method is implemented to maximize the filtration capacity without completely replacing the existing filter media. By using anthracite coal, the filtration process benefits from its specific properties, such as improved water quality and enhanced removal of contaminants. The change in the top layer helps in creating a more efficient filter, capitalizing on the advantages of different media.