What size of sand and gravel do slow sand filters use?
The most important information to know is that coarse sand does not filter as well as fine sand; and, fine sand offers more resistance to the flow of water than coarse sand. Between .35 mm and .15 mm effective size with a uniformity coefficient of less than 2 is the desired (and most commonly used) range of sand sizes. 13 however all the sand in a layer should be the same effective size. The smaller effective size should be in the top 30 or 40 cm layer of sand. The gravel on the bottom should be large enough to not pass through the holes in the drain pipes and small enough to prevent sand from seeping into them. The drain pipes should be covered by at least 6 inches of gravel. Generally, the smaller the effective size of the sand used the better. If the effective size is too small however, water will not flow through the filter fast enough. If the effective size is too large the biolayer will not form effectively and the filter will not purify water. back to top 12.
