semi - permeable membrane and membrane / support assemblies and systems for microfiltration , ultrafiltration , nanofiltration , reverse osmosis , electrodialysis , electrodeionization and piezodialysis filter systems are formed of polymer membranes , including fine particle moieties or threads of substantially water insoluble , oligodynamic bioactive dispersed material . when these membranes systems are used in aqueous liquids the dispersed material dissolves slowly and provides a continuous biostatic and / or biocidal action over a long duration to counteract membrane fouling propensities of the system environment .