Abstract: A method for the hydroponic cultivation of plants which includes passing a nutrient liquid through a filter which has been impregnated with one or more cultures of microorganisms capable of controlling the growth or germination of pathogenic fungi. The impregnated filter releases a portion of the microorganisms into the nutrient liquid as the liquid passes through the filter. The nutrient liquid containing the microorganisms is then contacted with at least the roots of the plants, or parts thereof. The invention is also directed to the filter unit which includes a pre-filter for removing particulate matter, an assembly containing fiberous material impregnated with the biomaterial, a flow means for transporting the nutrient liquid through the fiberous material so that the biomaterial is released in a continuous, controlled manner into the nutrient liquid, and a discharge means to discharge the nutrient liquid containing a portion of the biomaterial from the filter unit.

Abstract: A filter device for use in the treatment of water to render it potable, which is of particular value in emergency situations, comprises exo-polysaccharide producing, gram-negative bacteria supported upon a water-permeable material which is non-toxic to microorganisms and to human beings, is resistant to temperatures within the range from -15.degree. C. to +65.degree. C., and is not readily biodegradable. The device may be freeze-dried to enable it to be stored until it is required for use and may then be reactivated by the addition of water. It may be used as the surface layer in a slow-sand filter.