Microfiltration removes particles in the range of approximately 0.1 to 1 micron. In general, suspended particles and large colloids are rejected while macromolecules and dissolved solids pass through the MF membrane. Applications include removal of bacteria, flocculated materials, or TSS (total suspended solids). Transmembrane pressures are typically 10 psi (0.7 bar).
A cartridge filter with an absolute pore size of less than 10 μm is the suggested minimum pretreatment required for every RO system. It is a safety device to protect the membranes and the high pressure pump from suspended particles. Usually it is the last step of a pretreatment sequence. A pore size of 5 μm absolute is recommended. The better the prefiltration the less RO membrane cleaning required. If there is a risk of fouling with colloidal silica or with metal silicates, cartridge filtration with 1 to 3 μm absolute pore size is recommended. The filter should be sized on a flow rate according to the manufacturer’s recommendation and replaced before the pressure drop has increased to the permitted limit, but at least every 3 months.
Backflushable filters as final safety filters are generally not recommended because of their risk of breakthrough in case of a malfunction of their backflush mechanism, their lower efficiency and the higher biofouling risk. Backflushable fine filters may be used upstream of the cartridge filters to protect them. They are however, no substitute for disposable cartridges.
The cartridge filter should be made of a synthetic nondegradable material (e.g., nylon or polypropylene) and equipped with a pressure gauge to indicate the differential pressure, thereby indicating the extent of its fouling. Regular inspections of used cartridges provide useful information regarding fouling risks and cleaning requirements.
If the differential pressure across the filter increases rapidly, it is an indication of possible problems in the raw water supply or in the pretreatment process. The filter provides some degree of short-term protection for the membranes while corrective action is taking place.
Replacing cartridge filters more often than every 1 to 3 months usually indicates a problem with the pretreatment. The cartridge filter, however, is not meant to be a major component for the removal of high amounts of filterable solids. This would not only be an inefficient use of rather expensive filters, but would probably lead to premature failure of the membrane system due to the high probability that some of the unwanted material will break through. An alternative approach would be to use a second cartridge with larger pore size upstream.
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.
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.
In a cross-flow set-up (see figure below), the liquid is passed along the membrane surface at a particular speed. The permeate is able to pass through the membrane and the larger particles are left behind in a concentrated flow (the retentate). The hardened layer in this set-up is continuously removed by the cross-flowing supply flow.
• Specific advantages and disadvantages
The advantages of MF are:
• Low operating pressure required;
• Low energy consumption for semi dead-end set-up, compared to nano-filtration or reverse osmosis;
• Few manual actions required;
• Relatively cheap;
• No energy-consuming phase transfer needed, such as e.g. evaporation techniques;
• Quality of the produced permeate is not determined by the management.
• Possible disadvantages of MF are:
• Only suspended matter and bacteria removed ((~log 5 removal);
• Sensitive to oxidative chemicals (e.g. nitric acid, sulphuric acid, peroxide and persulphate in high concentrations);
• Damage can be caused by hard and sharp particles > 0.1 mm, whereby pre-filtration is necessary;
• Membrane damage if re-rinsed at pressure in excess of 1 bar.
Micro-filtration is primarily used as a pre-treatment step in the production of drinking and process water. It has excellent properties for removing suspended matter, bacteria and cysts. It is an alternative to classic sand filtration. Further, cross-flow MF is used in the:
• Dairy industry (cheese, milk,…);
• Food industry (clarification of fruit juice, wine, beer, etc.);
• Metal industry (oil/water emulsion separation);
• Textile industry (effluent treatment);
• Pharmaceutical industry (cold sterilisation).
MF is also implemented for the re-use of wastewater. In this case, it can be combined with a biological step in a membrane bioreactor.
• Boundary conditions
Membranes must be protected against hard particles larger than 0.1 mm. They can be removed by regular pre-filtration. Further, supply flows and pH conditions must be compatible with the membrane material.
MF can be implemented for removing the following parameters:
• Suspended matter (>99%);
• Harmful micro-organisms (e.g. bacteria, protozoa, algae, fungi) (>99%).
Further, MF can also be implemented to break down emulsions.
• Support aids
Support aids like bleach, peroxide, acid, alkali or detergent can be used to chemically clean the MF installation.
• Environmental issues
The concentrate from an MF has a high concentration of suspended matter and bacteria. This can be discharged together with wastewater if discharge norms are not breached. Rinse waters after chemical cleaning contain substances like bleach and formed AOX, peroxide, acid and alkali. These rinse waters can only be discharged to specific waste purification systems.