The filtration comes from sand particles that are about 0.35 mm in diameter and fairly uniform. The solids in the water collect mostly on top of the filter and are scraped away at intervals of 1 to 6 months. At this time the sand is cleaned by vigorous flushing with water flowing upward. This type of filter is popular in Europe because a previous step for coagulation can be omitted. Few such filters have been used in the U.S. because the water sources tend to have fine clay that penetrates and clogs them.
Sand adsorbs organic compounds on which microorganisms can feed.
The highest nutrient concentration is right on the sand granules, so that is where the organisms grow. Microbial growth clogs filters and shortens the interval until cleaning. Too much head loss is the signal that cleaning is required. Chlorination kills the microorganisms and keeps the filter going longer. Time until cleaning is a matter of hours, not days.