Microfiltration

Microfiltration is a pressure-driven process in which a membrane is applied to separate particles from an aqueous solution. Microfiltration is defined as the filtration of a suspension with colloidal or other fine particles having a linear dimension of roughly 0.02 µm to 10 µm. Typical operating pressure for microfiltration is relatively low, lying between 0.02 MPa and 0.5 MPa.

Screening or typical surface filtration is the term used to describe an operation with a membrane whose pores are smaller than the particles to be separated. If the membrane pores are larger then the particles can penetrate into the membrane phase. Nevertheless they can still be separated from the liquid phase if they interact with the inner membrane surface and can finally be adsorbed. In this case the term used is “deep-bed filtration” because the filtration effect takes place over the entire membrane phase.

Dynamic microfiltration separates micrometre-sized particles from liquid and gaseous media. Typical applications include the separation of bacteria, E. coli, yeasts, emulsified oils and fats as well as the separation of particles and fine dust from production processes.

Microfiltration is generally operated in the cross-flow as well as the dead-end mode. In cross-flow filtration, the raw solution flows along the membrane surface with only a small portion of the liquid passing through the membrane as a permeate. The concentrate is circulated in a loop to reduce concentration polarisation continuously and is thus used to clean the membrane. For this reason, cross-flow filtration is preferably applied for the filtration of liquids with a high solids concentration. Typical cross-flow rates range up to 6 m/s in tubular module geometries. In dead-end filtration, the liquid flows perpendicular to the membrane surface so that the retained particles accumulate at the membrane surface and form a filter cake. The filter cake increases in height throughout the filtration period resulting in a decrease in permeate flux. Therefore the membranes in dead-end operations have to be cleaned at regular intervals either by backflushing or possibly by using chemical or mechanical cleaning methods.

The most important use of microfiltration is the filtration of aqueous solutions, namely in the treatment of drinking and beverage water. In beverage production, industrial applications include the filtration of beer and wine as well as in the processing of milk and whey. In biotechnology, microfiltration is used for the retention of biomass from fermentation broths. The most important microfiltration operation in the metal-working industry is oil/water separation.

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