This section is not about the “microporous polymeric membranes” used to make pleated membrane micro-filters and also to collect and concentrate bacteria in microbiology labs. Those are discussed in, Mechanical Filtration. Membrane systems are entirely different: they have no “pores” and they do not “filter” anything. Instead, they are formulated to be so extremely hydrophilic (“water-loving”) that water dissolves into the membrane, and other materials (chemicals, particles, microbes) are simply excluded by their chemical nature. Exceptions-things membrane systems don’t remove well-include small, water-like molecules such as alcohols, and other very small molecules, even if they aren’t much like water, such as THMs and other VOCs. Once water molecules are in the membrane, they find their way to the other side by diffusion and leave everything else behind. The three major technologies using membrane systems are reverse osmosis (RO), nanofiltration, and ultrafiltration. Several membrane types are in use, and the most important ones are the cellulose acetate types, the polyamide types, and the thin-film composite (TFC) types. The polyamide membranes are readily damaged by disinfectant chlorine, but the cellulose acetate membranes are not. TFCs are intermediate in chlorine sensitivity, and both are often protected by activated carbon or granular brass prefilters. Chlorine-resistant cellulose acetate membranes are used where the absence of chlorine would allow degradation of the membrane by bacteria.