Abstract: A method of producing active filters, particularly for use in treatment of water, is disclosed. A filter bed of buoyant plastic balls having a surface to which a powdered active material capable of adsorbing dissolved impurities will adhere when subjected to liquid flow rates no greater than a first value and from which the powdered active material will be removed when subjected to liquid flow rates having a second value greater than the first value is provided. A liquid containing the powdered active material is passed through the filter bed in the upward direction at a flow rate having the first value to cause the powdered active material to be distributed on and adhere to the surfaces of the plastic balls. A liquid to be purified is passed through the filter bed in the upward direction at a flow rate lower than the first value to cause impurities to be adsorbed by the powdered active material without removing the powdered active material from the surfaces of the plastic balls.