Ultrafiltration (UF)

The typical Ultrafiltration separation process is based on a sieving mechanism, with pores sizes between 0.01 and 0.1 μm, or 1-100 kD.

As fluids pass through, usually in a cross-flow configuration with low transmembrane pressures (TMPs) of 50-120 psi or 3.5-8 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.

Ultrafiltration 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 Ultrafiltration system maintenance include:

• Pre-treatment system – some membrane systems might include a pre-treatment step, such as cartridge filters or MF membranes. These will need to be exchanged periodically to maintain Ultrafiltration 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, Ultrafiltration systems will typically have an on-going 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.

Ultrafiltration concentrates particles that are greater than 0.01 µ or 1kD.  These particles include a number of materials, including, but not limited to:

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

Ultrafiltration membranes can be constructed of either ceramic or polymeric materials.  In the case of polymeric Ultrafiltration membranes, the specific polymers are typically offer polyethersulfone (PES) or polysulfone (PS).

Form factors for Ultrafiltration 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 Ultrafiltration membranes are polymeric, spiral-wound configurations.

Feed – The process stream that enters the membrane for clarification and/or fractionation

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

Diafiltration – The use of water (or another solvent) with the intent to increase the passage of soluble materials into the permeate (such as lactose or minerals) and increase the purity of components in the concentrate, such as protein