Microfiltration and Ultrafiltration Membranes

Microfiltration (MF) is a pressure-driven separation process, which is widely used in concentrating, purifying or separating macromolecules, colloids and suspended particles from solution. MF membranes typically have nominal pore sizes on the order of 0.1–1.0 μm. MF processing is widely used in the food industry for applications such as wine, juice and beer clarification, for wastewater treatment, and plasma separation from blood for therapeutic and commercial uses. In biotechnology industries, MF concerns applications such as cell recycle and harvesting, separation of recombinant proteins from cell debris, and purification of process streams.

Membranes with a pore size of 0.1 – 10 µm perform micro filtration. Microfiltration membranes remove all bacteria. Only part of the viral contamination is caught up in the process, even though viruses are smaller than the pores of a micro filtration membrane. This is because viruses can attach themselves to bacterial biofilm. Micro filtration can be implemented in many different water treatment processes when particles with a diameter greater than 0.1 mm need to be removed from a liquid.

UF, a kind of membrane separation technique, is able to remove the particle materials in solutions. This separation process is usually used in industry and research areas for purifying and concentrating macromolecular (103–106 Da) solutions, especially protein solutions. Like microfiltration, ultrafiltration is based on size exclusion or particle capture. Ultrafiltration membranes are defined by the molecular weight cut-off (MWCO) of the membrane used, and cross-flow or dead-end mode is applied in different areas. The ultrafiltration membranes are only accessible for small molecules, such as leachate molecules, inorganic salts, and micromolecular organics, rather than some macromolecules like SS, colloid, protein, and bacteria. UF is always used as a way to pretreat leachate with lower organic concentration before nanofiltration (NF).

Ultrafiltration is a popular separation process used for product recovery and pollution control in industrial wastes, wastewater treatment, oil emulsions, medical therapeutics, biological macromolecules, and colloidal paint suspensions. he main problem related to ultrafiltration process in industries is membrane fouling which limits the potential of this technique. Fouling affects the membrane performance and causes an increase in operating costs due to increased labor for maintenance and a shorter membrane life. Membrane cleaning is very essential to significantly influence membrane performance. Membrane cleaning can be categorized into physical and chemical cleaning.

For complete removal of viruses, ultra filtration is required. The pores of ultra filtration membranes can remove particles of 0.001 – 0.1 µm from fluids. UF membranes typically operate between 50 – 120 PSI (3.4 – 8.3 bar) and are dependent on transmembrane pressure to drive the separation process. Other polymeric ultrafiltration membrane characteristics include robust chemical and temperature resistance, and low fouling tendencies if proper pretreatment is employed.

Ultrafiltration is the filtration method of choice for people who prefer minerals left in their water but still want microscopic contaminants taken out. A UF system may be selected over an RO system because it wastes less water to the drain. Someone may choose UF in California where water use is regulated. Someone in South Carolina, where the water has few dissolved minerals to begin with, may choose UF since RO wouldn't be necessary. Sometimes, ultrafiltration is used to recycle effluent water after filtration, so the water can be reused for irrigation.

Anyone who has studied at the very least high school level biology (and paid attention) is familiar with the concept of a membrane, in particular a semi-permeable membrane. Biological living cells are wrapped in semi-permeable membranes that keep their functions separate from the surrounding environment. The semi-permeable aspect allows only certain ions and small organic and other molecules to pass into or out of the cell. The membrane can be selective in either a passive or active capacity. Ultrafiltration (UF) and microfiltration (MF) processes utilize a semi-permeable membrane to separate microcontaminants from a water stream.

Microfiltration and UF purification are more alike than they are different. As mentioned in the introduction, they are both non-biological (non-living), membrane-based separation technologies. These system processes work by applying differential pressure across a semi-permeable membrane and that pressure forces water, solutes, and small particulate matter through the membrane while larger solids are retained on the other side.

These processes both also make for beneficial pretreatment steps for reverse osmosis and nanofiltration (RO/NF). Membranes need to be deployed in properly designed systems and sometimes need periodic cleaning so they can last as long as possible without replacement. Filtration and precipitation pretreatment reduces concentrations of larger solid particles and reduce severity of membrane fouling.

The membranes for these microfiltration and UF purification systems are available in the same configurations. Plate and frame, tubular, hollow fiber, and spiral wound are possible options. These different configurations offer their own pros and cons. There are also different materials the membranes can be composed of, namely polymers, ceramic, and a few metal versions.

It all boils down to pore size. On the membrane separation scale, micro- and ultrafiltration are coarser than nanofiltration (NF) and reverse osmosis (RO), but are still finer than media filtration. Although there is not universal agreement, commonly used definitons have microfilter pores within the range of 0.1 to 10 microns and ultrafiltration membrane pores within 0.005 to 0.1 microns. Ultrafiltration membranes are typically rated by the molecular weight of the solids they remove and are typically rated from 5000 to 300,000 molecular weight cut-off. The membrane selected for a treatment system is based on the size of the smallest particles to be retained in the feedwater. The difference in their pore size determines the applications for which ultrafiltration purification or microfiltration treatment process would be the most applicable to be applied for the specific application.