Microfiltration (MF)

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:

• Bacteria
• Algae
• Fungi
• Yeast cells
• Microorganisms
• Fats

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.

Flux

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)

Fouling

The deposition of solids on the surface of a membrane

Permeate

The liquid stream that passes through the membrane (aka filtrate)

Retentate

The liquid stream that is rejected by the membrane (aka concentrate)

Concentration Factor

The ratio of initial feed volume to retentate or concentrate, which is an indication of target volume reduction achieved by membrane filtration