What is microfiltration?

What is microfiltration?

Microfiltration is a filtration process using a microporous media that retains the suspended solids of a fluid. The pore size of the membrane ranges from 0.1 to 1 micron or microns.

Microfiltration is different from reverse osmosis and nanofiltration in that it does not require pressure. It is often used as a pretreatment for reverse osmosis or as a stand-alone filtration process.

How does it work?

Microfiltration is a filtration process using a micrometer-sized (μm) filter. Filters can be at atmospheric pressure or with a vessel at a certain pressure (maximum 25 psi) ⁽²⁾, but they usually work at low pressures.

These filters are porous and allow water to pass through them, removing organic matter, suspended solids, small colloids, bacteria and turbidity.

What are the applications of microfiltration?

Microfiltration is mainly used as a pretreatment step in the production of drinking water and industrial water. It has excellent properties to eliminate suspended solids, bacteria and cysts. It is an alternative to the classic sand filter. In addition, Crossflow MF is used for:

Microfiltration has many fields of application, such as:

• Cold sterilization of beverages (juices, wines, beer…) and pharmaceutical products (cold pasteurization)
• Dairy industry (cheese, milk…)
• Juice, wine and beer clarification
• Separation of bacteria from water
• As a pretreatment for reverse osmosis
• Oil and water separation
• Effluent treatment
• Metallurgical industry (separation of water/oil emulsion);
• Textile industry (wastewater treatment);

MF is also used for wastewater reuse. In this case, it can be combined with a biostage in a membrane bioreactor.

Classification of separation membranes, according to their pore openings

• Microfiltration: 0.1 to 1 micron (μm)
• Ultrafiltration: 0.01 to 0.1 μm
• Nanofiltration: 0.001 to 0.01 μm
• Reverse osmosis (hyperfiltration): 0.0001 to 0.001 μm

Remember that:

1 mm = 1000 μm

1 μm = 1000 nanometers (nm)

1 nm = 10 Angstroms (Å)

Advantages and disadvantages of microfiltration

The advantages of MF are:

Low operating pressure required.
Low energy consumption in semi-inactive units compared to nanofiltration or reverse osmosis.
Virtually no manual operations are required.
It is relatively inexpensive.
It does not require energy-intensive phase transitions such as the evaporation technique.

Disadvantages of microfiltration (MF)

Sensitivity to oxidizing chemicals (such as high concentrations of peroxides and nitric, sulfuric persulfates);
Hard and sharp particles > 0.1 mm can cause damage and therefore require more open pore pre-filtration;
The diaphragm will fail if the diaphragm is flushed again with a pressure higher than 1 bar.

In conclusion

Microfiltration generally operates in cross-flow mode and termination mode. In cross-flow filtration, the stock solution flows along the membrane surface and only a small portion of the liquid passes through the membrane as permeant. The concentrate is bound to continuously depolarize the concentration used to clean the membrane.

For this reason, cross-flow filtration is preferred for the filtration of liquids with a high concentration of solids. Typical cross-flow velocities are up to 6 m/s in the tubular module geometry. During cross-flow filtration, the liquid flows perpendicular to the membrane surface, which causes the trapped particles to accumulate on the membrane surface and form a filter cake.

Increasing the filter cake height during the filtration period reduces the filtration flow rate. Therefore, during dead-end operations, the membranes must be cleaned regularly by backwashing or possibly by mechanical or chemical cleaning methods.

The most important application of microfiltration is the filtration of aqueous solutions, especially in the treatment of drinking water and beverages. In beverage production, industrial applications include beer and wine filtration and milk and whey processing. In biotechnology, microfiltration is used to retain biomass in the fermentation fluid. The most important microfiltration operation in the metallurgical industry is the separation of water and oil.

References

1. Huisman, I.H. MEMBRANE SEPARATIONS | Microfiltration. Encyclopedia of Separation Science. s.l. : Academic Press, 2003.
2. Wes, Byrne. Reverse osmosis. A practical guide for industrial users. Littleton : Tall Oaks Publishing Inc., 2002. 0-927188-03-1.

Last update: March 26, 2021.