Which of the following is NOT a common filter media used in rapid filters?

Diatomaceous earth is not commonly used as a filter media in rapid filters, making it the correct choice in response to the question. Rapid filters are designed for the quick passing of water through filter media, allowing for efficient removal of particulates. The most prevalent media used in these systems are sand and crushed anthracite, which provide effective filtration and have qualities that enhance flow rates and sediment removal.

Granular activated carbon is also utilized in some water treatment processes for its ability to adsorb impurities and improve taste and odor. However, diatomaceous earth is more typical in a slower filtration process, like in pre-coat filters, where its fine particles form a layer that captures smaller particulates effectively but at much slower flow rates compared to the materials used in rapid filters. Thus, recognizing that diatomaceous earth is more suited for different filtration technologies distinguishes it from the other options listed, confirming it as the answer to the question regarding common media in rapid filtration systems.