Membrane filtration is a technique used to separate particles from a liquid in order to purify it.
In membrane filtration, a solvent is passed through a semi-permeable membrane. The membrane's permeability is determined by the size of the pores in the membrane, and acts as a barrier to particles which are larger than the pores, while the rest of the solvent can pass freely through the membrane. The result is a cleaned and filtered fluid on one side of the membrane, with the removed solute on the other.
There are three membrane filtration processes:
Microfiltration [MF] - MF is a low pressure [up to 100 psi (7 bar)] process for separating larger size solutes from aqueous solutions by means of a semi-permeable membrane. This process is carried out byhaving a process solution flow along a membrane surface under pressure. Retained solutes (such as particulate matter) leave with the flowing process stream and do not accumulate on the membrane surface.
Pore ranges from 0.1 - 3 µm (micron meter).
Clarification of dark juices for example, in the clarification of wine and dark juices, MF is to separate the suspended solids from the juice to produce a low turbidity juice while allowing the passage of color and flavor.
Ultrafiltration [UF] - UF is a low pressure [up to 150 psi (10 bar)] process for separating solutes from aqueous solutions by means of a semi-permeable membrane. UF provides an essentially complete barrier against particles larger than the pore size, bacteria and the much smaller viruses usually found in the feed water.
UF operates by a surface removal mechanism resembling a fine sieve with a highly uniform pore size. Any particles greater than the pore size are rejected. This characteristic makes UF membranes ideal for meeting absolute filtration quality requirements.
In addition to high removal efficiency and an absolute removal rating, UF membranes tend to be more compact, allow higher automation with unattended operation and have lower chemical usage.
The pore size is approx. 0.02 µm (micron meter).
Microbiological contaminants rejection [Rejection of Cryptosporidium]
Reverse osmosis pre treatment [Reduction of SDI and TOC (Total Organic Carbon)]
Reduction of Turbidity [colloids, proteins, large organic molecule]
Nanofiltration [NF] – NF is a low to moderately high pressure [up to 450 psi (31 bar)] process. Pore sizes range between UF and RO. Pores have not been observed in NF membranes under any microscope, however, water can still pass through the membrane and multivalent salts and low molecular weight organics are rejected. It is difficult to predict the performance of NF membranes since membrane rejection is influenced by the size, structure and charge of the components in solution. As a result, piloting is highly recommended for NF applications, even if a detailed feed water analysis is available.
Overall reduction of TDS (Total Dissolved Solids)
Color and TOC ( Total Organic Carbon).
Separation of organic from inorganic matter (in special food and wastewater applications)