The following processes are grouped together under membrane filtration:

• Microfiltration (MF)
• Ultrafiltration (UF)
• Nanofiltration (NF)
• Reverse osmosis (RO)

Membrane filters are purely mechanical "fine sieves". These fine sieves consist of porous, artificially created films, so-called membranes, with precisely defined pore diameters. The separating layers are applied to so-called carrier material. Polymers or ceramics are used as carrier materials. Membrane filtration technologies are used for the widest variety of applications in the water treatment field, often in conjunction with classical processes.

The actual effective membrane layer is extremely thin and is located on the side facing the raw water.
The absolutely largest part of a membrane is the carrier material required for the physical design and strength. It is important that the pores of a membrane increase in size toward the pure water in order for the membrane to function properly. Otherwise, there is a risk of continuous blocking.

Membrane systems and any pre-processing require careful planning. The choice of the corresponding modules and the way in which the system operates must always be tailored to the raw water property.