The selection of membranes offered by the various suppliers in the business may appear to be confusing since many materials may be used to make membranes, and they are provided under an array of trade names. In reality, relatively few materials are actually used in quantity, and only a few basic membrane types form the bulk of the membranes being sold and used.
Cellulose acetate (CA) is the “original” membrane and is used for RO, NF and UF applications. The material has a number of limitations, mostly with respect to pH and temperature.
Polyvinylidene difluoride (PVDF) is a traditional membrane material, but it is not used much because it is difficult to make membranes with good and consistent separation characteristics. Its main advantage is its high resistance to hydrocarbons and oxidizing environments.
Also called thin-film composite membranes, they appear under various acronyms such as TFC and TFM, and were made to replace cellulose acetate
RO membranes. The main advantage is the combination of relatively high flux and very high salt rejection, 99.5% NaCl rejection commonly achieved with composite RO membranes. They also have good temperature and pH resistance, but do not tolerate oxidizing environments.
Composite membranes are made of two-layer and three-layer designs, the precise composition of which is proprietary.
MEMBRANES – MATERIALS, STRUCTURE, LIMITS
Around 1980, FilmTec marketed the two-layer design which immediately became the industry standard for water desalination, and this type of membrane has dominated the water desalination market ever since. The membrane has been improved over the years but the basic design remains unchanged, and today there are several companies making this type of membrane.
In the mid-1980s Desalination Systems, Inc. (DSI) began making composite membranes with a three-layer design. These membranes had difficulties competing with the two-layer membranes in water desalination but proved to work better on industrial process streams where it is more stable and less prone to fouling. The three-layer design is available for RO and NF, and it is still the best choice for treating a vast array of difficult process streams. DSI is the only producer of three-layer composite membranes.
Total worldwide consumption of membranes, based on membrane surface area, is approximated as follows.
Composite RO membranes: 85%
Composite NF membranes: 3 – 5%
Polysulfone UF and MF membranes: 5 – 7%
Other membranes: 3 – 5%
Materials like polyacrylonitrile (PAN), ceramic materials (SiO2) and cellulose (hydrolyzed cellulose acetate) are included in the group of “other membranes.”
SELECTION OF MEMBRANE MATERIAL
It can be difficult to select the right membrane and membrane material for a given process, and some general information about the process environment must be available to make a proper selection. The first step is to determine the preferred process (RO, NF, UF or MF) and look at the membrane materials available. Based on the process environment the best-suited membrane material can then be selected.
As a general rule, only well planned and well-performed pilot tests will provide good answers to membrane selection questions for given processes.