Microfiltration (MF)A Polymeric Membrane Solution
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What is Microfiltration?
In the spectrum of membrane technology, MF has the largest, or most “open,” pore sizes – which in turn equates to rejection of particles in the micrometer size range. Examples of solids that will not pass through the membrane into permeate include fats, bacteria, yeast and suspended solids greater than 0.1-10 µ. Other high value materials such as proteins, sugars, dissolved salts, and lower molecular weight molecules will pass through into permeate, making this format attractive for removal of contaminants from recirculating or higher-value feed streams.
While there are many form factors (e.g. tubular, spiral wound, plate and frame) and materials of construction (e.g. polymeric, ceramic) for microfiltration, the most commonly used is polymeric spiral-wound technology. Solecta is proud to offer polyvinylidene fluoride (“PVDF”) spiral-wound membranes in a variety of pore sizes and feed spacers to accommodate the needs of numerous process applications.
Industrial Applications of Microfiltration
MF is used broadly across process industries, most namely dairy, food ingredients, biotechnology/life sciences, beverages, and automotive manufacturing operations. Some of the key applications across these industries include the following:
- WPI production (whey defatting)
- Cheese production (brine clarification)
- Skim milk production (casein/whey fractionation)
- Sugar/sweetener processing (dextrose clarification prior to refining)
- Gelatin processing (downstream clarification)
- Other fermentation processes (clarification)
- Enzyme manufacturing (downstream processing of bulk fermentation)
- Beer, wine, and juice production (color removal and clarification)
- Paint recovery (clarification)
- Oil-water separation (for water/waste treatment and/or recovery of oils)
In these applications, microfiltration can replace more costly filtration options, such as rotary-drum vacuum filters, which also use filtration aids such as diatomaceous earth. Microfiltration can greatly simplify the separation process and avoid the on-going operational cost and burden of burden of disposal of these aids.
Otherwise, MF technology is broadly used in industrial water and wastewater treatment processes. In water treatment applications, the technology can be used for clarification of plant process water and/or as pre-treatment for advanced membranes, such as nanofiltration or reverse osmosis. Additionally, microfiltration can also be used in wastewater treatment, to help remove suspended solids either as stand-alone technology or embedded in a more complex process, such as membrane bioreactors (MBRs).
FAQs on Microfiltration
The typical microfiltration separation process is based on a sieving mechanism, with the option of both isotropic and anisotropic membranes with pores sizes between 0.1 and 10μm.
As fluids pass through, usually in a cross-flow configuration with low transmembrane pressures (TMPs) of 30 psi or <2 bar, the membranes retain those particles in a process stream called retentate. The fluid that passes through the membrane is referred to as permeate.
As the membranes separate solids, particles can agglomerate at the membrane surface, causing what is referred to as fouling. This phenomenon can slow down the flow rate and/or increase pressure across the membrane, if the process is not optimized. Maintaining proper operational best practices, such as backwashing in some membrane formats and/or cleaning procedures in others, can prevent fouling. Automation can also help detect this by monitoring flow and/or pressure drop across the membrane.
Microfiltration systems are very robust and typically require little routine maintenance. When they do, it is recommended that qualified technicians work on these systems to maintain optimal performance. Some of the typical maintenance tasks around microfiltration system maintenance include:
- Pre-treatment system – some membrane systems might include a pre-treatment step, such as cartridge filters. These will need to be exchanged periodically to maintain microfiltration system performance.
- Gauges and other instrumentation – as these systems typically include both manual and automated instrumentation, it is important to check and ensure these are operating correctly. In the case of automation, it is important to calibrate these instruments per the manufacturers’ recommended protocol.
- Valves, solenoids, and other wear parts – it is not uncommon for valves to become stuck and/or freeze, particularly if they are not exercised during normal operations. It is important to turn valves off and on periodically, as well as check to ensure solenoids are operating correctly, to maintain system reliability.
- Element replacement – when a membrane has gone past its useful life, it will need to be replaced. Loss of performance will typically manifest itself in reduced rejection or clarification results and/or through changes in flow rate. Ensure that technicians are qualified to replace elements, so they don’t damage elements upon installation, and also ensure proper seals to prevent system leakage.
Otherwise, microfiltration systems will typically have a periodic cleaning protocol, which maintains the health and reliability of the membrane system. These cleaning chemicals will ensure proper flow and rejection, as well as prevent unwanted microbiological contamination – ensuring optimal performance and sanitary conditions.
Microfiltration removes particles that are greater than 0.1 µ. These particles include a number of materials, including, but not limited to:
- Yeast cells
Microfiltration membranes can be constructed of either ceramic or polymeric materials. In the case of polymeric microfiltration membranes, the specific polymer is typically polyvinylidene fluoride or PVDF.
Form factors for microfiltration include tubular (shell and tube design), plate and frame (membrane flat sheet sandwiched in between support plates), hollow fiber (comparable to a straw), and spiral-wound (alternating layers of flat sheet, feed spacer, and permeate carrier).
The most common microfiltration membranes are polymeric, spiral-wound configurations.
The rate of extraction of permeate, which is typically measured in LMH (liters per square meter of membrane surface per hour – l/m2/h) or GFD (gallons per square foot of membrane surface per day – gal/ft2/day)
The deposition of solids on the surface of a membrane
The liquid stream that passes through the membrane (aka filtrate)
The liquid stream that is rejected by the membrane (aka concentrate)
The ratio of initial feed volume to retentate or concentrate, which is an indication of target volume reduction achieved by membrane filtration