Abstract: In the biological purification of sewage water or drinking water, it is known to purify the water in a filter bed of granular filter medium, wherein the filter medium is transported from the bottom of the bed to its upper side with the aid of air, so as to obtain a continuously working filter. It is suggested in accordance with the invention that the amount of filter medium to be transported is divided into two or more parts which are transported in parallel, so as to prevent the flow conditions in the transport tube having a negative effect on the reactivity of the filter bed by decreasing the bacteria strain.

Abstract: A suspension is delivered to a filter bed comprised of granular filter medium in a continuously working filter at the lower part of the bed and is then permitted to flow upwards through the bed. With the intention of optimizing the capacity of the filter, the suspension is now guided into a central region of the bed. The suspension is divided from this region into at least two specific proportional quantities which are each led from the filter bed in a respective ring-shaped pattern at mutually different heights in relation to the extension of the bed, these ring-shaped patterns having different diameters and each being adapted to the extension of the filter medium in the direction of the diameter.