No, in slow sand filters, while sand provides much of the filtration process, it is not the main filtering medium.
Slow sand filters utilize a biological layer called the "Schmutzdecke layer" consisting of bacteria, protozoa, and other microorganisms. This biological layer is responsible for most of the filtration that occurs in a slow sand filter. The sand acts as a support matrix for this biological layer, providing a surface area for the microorganisms to colonize and function. Therefore, the sand supports the main filtering medium, but it is not the filtering medium itself.
