The invention relates to methods and compositions for reducing the growth of microorganisms and other contaminants of water processing and holding systems. Rapid reduction in the growth of fungi, algae, spores, molds, bacteria, protozoa and/or other contaminating microorganisms found in water processing and holding equipment is achieved by placing a material comprising an antimicrobial composition, preferably one containing a metal oxide and a binder, in an area of such equipment where there is water movement or storage, thereby destroying contaminating water-borne or air borne microorganisms in the water and on the surfaces of the equipment in contact with the water.