Where filter is on it is not always filter in it! ... or what should and can filter my filter? During the filtering the water flows through the filter layers from top to bottom. It acts in addition to forces from the fluid flow and the gravitation on the grains of sand. systems, which would like to outwit these forces, have after my suggestion major disadvantages. After the end of the filter runtime, So when the pressure loss in the filter to its economic limit is, the filter must be regenerated by flushing. This is done in the reverse direction. Subtleties to that later. For the sake of completeness: The distances between the filter rinses are dependent not only on the contamination but also on the growth rates possibly in the filter layer domestic bacteria!