Membranes can be divided into four categories: microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO). Each of these processes relies on pressure and size exclusion to filter the water. Separation is based on the pore size with microfiltration having the "loosest" pores and reverse osmosis having the "tightest" pores.

As the pore size becomes smaller, the membrane becomes tighter. As a result, higher pressure is needed to force the water through it. Both RO and NF change the chemical-or ionic-nature of the raw water to produce the product water, whereas UF and MF are classic particle removal processes. The filtration spectrum of RO, NF, UF and MF relative to sizes of common materials is shown below.
The development of membranes started about 40 years ago with the introduction of the first reverse osmosis membranes for desalination purposes. Since then, membrane technology has seen a tremendous growth in development effort, creating a whole new and extensive market of membrane filtration applications.  Membrane filtration is now applied in nearly all water areas: from the production of potable water, to the treatment surface water or the recycling of industrial and municipal effluent. Capillary hollow-fiber membranes have become the standard in providing an absolutely fail-safe barrier to waterborne pathogens. Although membrane filtration is widely applied in drinking water production plants all over the world, it is new to small-scale point-of-use applications. The reason behind this is that producing and applying membranes is a relatively labor-intensive, complicated and therefore expensive process. However, thanks to several breakthrough new technologies, Filtrix has succeeded in introducing membrane technology to smaller-sized applications.