Membranes can be classified into 4 categories depending on the size of the materials they remove from the carrier liquid. The 4 categories, listed from the largest to the smallest pore size, are MicroFiltration (MF), UltraFiltration (UF), NanoFiltration (NF), and Reverse Osmosis (RO)

a)  MicroFiltration (MF)
b)  UltraFiltration (UF)
c)  NanoFiltration (NF)
d)  Reverse Osmosis (RO)

a) MicroFiltration (MF):

MF is a low-pressure membrane process (0.3 to 3.3 bar) that removes particulate material ranging in size from 0.1 to 1.0 microns (1,000 to 10,000 angstroms) and larger.

MF is used for separating suspended or colloidal materials from a feed stream. Water, salts, and selected macromolecules pass through the semi-permeable membrane, and suspended solids are progressively concentrated.

In the case of water, bacteria (like Escherichia coli, Cryptosporidium parvum, or Giardia lamblia) can be separated from water with this technique.

b) UltraFiltration (UF):

UF is a low-pressure membrane process (0.5 to 10 bars) that separates materials in the 0.001 to 0.1 micron range (10 to 1,000 angstroms).

UF is used for the separation of high molecular weight dissolved materials (solute).

Water, salts, and low molecular weight species selectively pass through the semi-permeable membrane, and macromolecules and suspended solids are retained (progressively concentrated).

UF separates bacteria and viruses from the water stream that MF passes because the pore size is smaller.

c) NanoFiltration (NF):

NF is a high-pressure membrane process used for separating materials less than 0.001 microns (10 angstroms) in size.

NF is used for the separation of dissolved materials (solute).

Less expensive, NF is not as fine a filtration as RO, but it also requires less energy to perform the separation (i.e. less operation & maintenance cost). In water, NF is very effective in removing hardness (multiple-charged ions), total dissolved solids, and natural organic material (Disinfection-By-Product DBP precursor material) with MF pre-filtration. NF is also known, in the water markets, as the "Membrane softening".

The NF membranes fill the gap between UF and RO membranes.

d) Reverse Osmosis (RO):

In a tank of pure water and with the addition of a soluble salt, the "osmosis" is the natural process involving the fluid flow of across a semi-permeable membrane barrier. The chemical potential of the salt solution is reduced, and osmotic flow, from the pure water side across the membrane to the salt solution side, will occur until the equilibrium of chemical potential is restored. The osmotic pressure is a solution property proportional to the salt concentration and independent of the membrane.

In case of "Reverse Osmosis", application of an external pressure to the salt solution equal to the osmotic pressure will cause equilibrium. Additional pressure will raise the chemical potential of the water in the salt solution, and cause a solvent flow to the pure water side, because it has now a lower chemical potential.

The driving force of the RO process is applied pressure. The amount of energy required for osmotic separation is directly related to the salinity of the solution. Thus, more energy is required to produce the same amount of water from solutions with higher concentrations of salt.

Reverse Osmosis is a high-pressure membrane process (50 to 100 bars), used for separating low molecular weight materials, less than 0.001 microns (10 angstroms) in size.

RO is used for the separation of dissolved materials (solute).

Water selectively passes through the semi-permeable membrane, and salts and macromolecules are retained, and progressively concentrated