Abstract: The present invention relates to filtration of liquids, such as water, containing substances that desirably should be removed from the liquids. The invention relates in particular to a filtration cake, being produced in situ of a cleaning system from substances that desirably should be filtered away. Particular preferred embodiments of the present invention relate to treatment of waste water, where an activated sludge is provided, which sludge contains activated sludge flocs. These flocs are accumulated on a fluid penetrable support structure to form a layer of flocs, which subsequently is compressed to form a filter of desired properties. The invention also relates to adapting waste water—or other liquids containing biodegradable substances—to optimized biodegradation and filtration by deflocculating flocs in an activated sludge during or before a biodegradation step and subsequently promoting re-flocculation prior to filtering or other means of biomass separation.