Membrane water treatment systems were originally used only in desalination projects. But improvements in membrane technology have made them an increasingly popular choice for removing microorganisms, particulates, and natural organic materials that foul water’s taste and taint its clarity.
Water treatment membranes are thin sheets of material that are able to separate contaminants based on properties such as size or charge. Water passes through a membrane; but depending on their size, larger particles, microorganisms, and other contaminants are separated out.
Some of these systems are pressure driven, depending on water pressure to separate the particles based on size. Microfiltration employs the largest pore size, and can remove sand, silt, clay, algae, bacteria, Giardia, and Cryptosporidium. Ultrafiltration can also remove viruses. Nanofiltration systems provide nearly complete protection against viruses, remove most organic contaminants, and can reduce hardness in water. Reverse osmosis systems are dense membranes that remove almost all inorganic contaminants and all but the smallest organic molecules.
Electrodialysis combines membrane technology with the application of electrical current, to separate contaminants based on charge. Unlike other membrane processes, the source water never passes through the membranes during electrodialysis. It is not used as much in large water treatment facilities as some of the other technologies described here. Rather, it is mostly used for medical and laboratory applications that need ultrapure water.
Membranes, especially reverse osmosis and nanofiltration, can be a good option for small-scale water treatment systems faced with a wide variety of contaminants. However, they often produce larger quantities of wastewater (or “concentrate”) than most other treatment systems—as much as 15 percent of the total treated water volume—and can become clogged with clay or organic materials if particle-rich source water is not filtered first.
Maintenance is generally not difficult, but it can be expensive since the main action required is to replace the membrane as necessary. Maintenance problems tend to involve leaking and fouling of membranes.