MICROFILTRATION (MF)

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.
most important use of microfiltration is the filtration of aqueous solutions, namely in the treatment of drinking water.

Method and installation description
Micro-filtration (or MF for short) is one of the pressure-driven membrane processes in the series micro-filtration, ultra-filtration (UF), nano-filtration (NF) and reverse osmosis (RO). The micro-filtration process uses a membrane – a simple permeable material – which, in the case of micro-filtration, only allows particles smaller than 0.1 microns to pass through it. The micro-filtration membrane can consist of various materials like, for example, polysulfone, polyvinyldifluoride (PVDF), polyethersulfone (PES), ZrO2 and carbon. The pore size varies between 0.1 and 5 microns. Because the pores are large compared to other mentioned filtration techniques, pressure – needed to send the liquid through a micro-filter membrane – is limited to 0.1 to 3 bar.
Micro-filter membranes are offered in various configurations by suppliers, with each configuration having a specific use and accompanying advantages and disadvantages. Possible membrane configurations include:
Pipe-shaped membranes: capillary, hollow fibre or tubular;
Plate-shaped membranes: flat plate or spiral
In addition to the specific membrane configurations, one can also identify a few set-ups. The 2 most commonly used methods are dead-end and cross-flow set-ups. The names refer to the way in which the supply is sent to the membrane. In dead-end MF, the supply is sent directly to the membrane. The pollution layer will thus form on the supply side of the membrane surface. This layer contains all particles that have been separated on the basis of their size (sieve effect). This layer is periodically rinsed away by briefly re-sending the produced liquid (permeate) through the membrane in an opposite direction to the production flow. This helps to loosen the hardened layer, and makes it ready for disposal. This is referred to as a semi dead-end set-up.
This re-rinsing may not be enough to remove the layer from the surface if the hardened layer is too strongly compressed or if the bond with the membrane is very strong. In this case, chemical cleaning must be implemented with, for example, bleach, peroxide, acid and alkali or detergent.
The figure below shows a dead-end set-up, where M = the membrane.