Sand is a natural, loose material used in filter cartridges as a filter medium for mechanical filtration. As water passes through the sand layer, solid insoluble substances such as turbidity, rust, mud, or other sediments are captured. The efficiency of filtration depends on the size of the sand grains, which is professionally referred to as "grain size". Finer sand captures even very small particles but can clog faster, while coarser sand allows more water to pass through, but its ability to capture fine impurities is lower.