What is the typical grain size of sand used in slow sand filters?

The typical grain size of sand used in slow sand filters is indeed in the range of 0.25 to 0.35 mm. This size is optimal as it strikes a balance between effective filtration and maintaining adequate water flow through the filter.

In slow sand filtration, the sand bed is crucial for trapping particles, debris, and pathogens, while allowing water to percolate through. Sand grains within this size range create enough surface area for biological activity, which aids in the breakdown of organic matter and the removal of microorganisms from water. If the grains are too small, the filter can become compacted, leading to reduced water flow and increased pressure head. Conversely, if the grains are too large, the filtering capacity diminishes, allowing larger contaminants to pass through.

Thus, the selection of a grain size between 0.25 to 0.35 mm ensures optimal performance of slow sand filters, thereby achieving the desired quality of potable water.