Membrane Filtration Tutorial
The cross flow membrane filtration process has emerged since the early 1960’s as one of the newer industrial separation processes.
Today there are significant amounts of membrane filtration equipment operating in the water purification, food processing and paint industries to name a few.
The processes of most interest to industry are Reverse Osmosis, Nano-Filtration, Ultra-filtration and Microfiltration. Other membrane process such as Dialysis, Hemo-dialysis, Liquid Membranes, Catalytic Membranes, Electro-dialysis and Pervaporation will not be considered here.
Membrane processes are distinguished by the size of the "pores" within the membranes. Note that the question of porosity is a mute one. Some will say that Reverse Osmosis and possibly nano-filtration membranes are non-porous but they will both pass water and hence we will consider them porous membranes.
Reverse Osmosis
Utilises a membrane that selectively restricts flow of solutes while permitting the flow of the solvent. The solvent is almost always water.
Major processes using reverse osmosis include:
- Making potable drinking water from seawater
- Manufacture of high purity waters for the electronics industry
- Process water purification – e.g. boiler water
- Process de-watering
- De-watering and concentrating food and beverages.
- Typically these membranes have a molecular weight cut-off of less than 500.
The membranes are normally rated on the retention of NaCl in water. Good membranes rating in excess of 99% rejection.
Nano-Filtration
Often referred to as "Loose R/O". Typically have molecular weight cut-offs in the range 200-5000. Used where desalination of the solute (retentate) is the objective, particularly where equivalent ionic material is preferred to be retained.
As with reverse osmosis membranes, pre-treatment of the feed is essential. A reducing environment is preferred. Heavy metals, particularly Iron, can cause disastrous membrane degradation – as can Chlorine. It is normal to adjust pH and add anti-scalants to prevent deposition of carbonates and sulphates as the concentration of the feed increases.
Uses of Nano-Filtration include:
- Concentration of sugar solutions while reducing salt concentration – Lactose Concentration.
- Beverage concentration.
These membranes are often rated for MgSO4 rejection. Good membranes reject 97% or more.
Ultra-Filtration
Typically, these membranes have molecular weight cut-off in the 2000 to 200,000 range.
Ultra-filtration is used to concentrate oil in water emulsions, the by-product of many metal working industries. Concentration of dilute latex and the removal of virus from solutions to achieve "vaccine manufacturing conditions" are also practiced. In this country a major use is in the separation and purification of whey protein, produced as a by-product of the cheese and casein making industries, since the protein is retained while the lactose, lactic acid, minerals, non-protein nitrogen and water pass through.
Micro-Filtration
A broad definition of micro-filtration is the separation of particles from true solutions.
The usual particles retained on a micro-filtration membrane range in size from several µm down to about 0.2µm. Obviously, at the 0.2µm end of this range, very large macro molecules are retained putting paid to our "definition" above.
Micro-organisms such as bacterial are a particularly important class of particles retained on micro-filtration membranes, hence the importance of the process in potable water treatment.
Often, microfiltration is used as a pre-treatment for other membrane processing.
Many applications are in the medical, pharmaceutical and Bio-Processing industries. Examples are:
- Clarification of fermentation broths
- Sterile filtration
- Cell recycling in continuous fermentation
- Cell harvesting and washing
- Enzyme purification