What is often added to replace some of the sand in filtration systems?

The correct choice is crushed anthracite, which is commonly utilized in filtration systems to replace a portion of the sand. Crushed anthracite is a type of coal that has undergone a specific grading process to create granules suitable for filtration. Its properties, including a high density and a large surface area, allow it to effectively capture particles and contaminants from water as it passes through the filtration media.

Using crushed anthracite alongside sand enhances the overall filtration process by improving the removal of impurities. It is especially effective due to its ability to support both physical and chemical adsorption, which aids in the purification of water.

The other materials listed, while potentially useful in different contexts, do not typically serve the same purpose in filtration systems. Granular activated carbon is primarily used for adsorbing organic compounds and odors. Sandstone may not provide the same level of filtration efficiency due to its inconsistent grain size and texture. Coal dust, although derived from coal, lacks the structure and density characteristics that make crushed anthracite an effective filtration medium. Thus, crushed anthracite is preferred for its specific filtration benefits.