Microfiltration (MF)

MF is a form of filtration that has two common forms. One form is crossflow separation. In crossflow separation, a fluid stream runs parallel to a membrane. There is a pressure differential across the membrane. This causes some of the fluid to pass through the membrane, while the remainder continues across the membrane, cleaning it. The other form of filtration is called dead-end filtration or perpendicular filtration. In dead-end filtration, all of the fluid passes through the membrane, and all of the particles that cannot fit through the pores of the membrane are stopped.

Pore sizes of MF membranes are in the range of 0.05 to 3 mm. Thus, MF typically operates at low transmembrane pressures to minimize build-up of the suspended solids at the membrane surface. Pressures of 0.3 to 3.5 bar and cross-flow velocities of up to 3-6 m/s in tubular modules are common. On an industrial scale, MF is usually carried as a multistage (stages-in-series) operation in a feed-and-bleed mode of operation. MF is the most open membrane and separates macro-materials and suspended solids. Crossflow MF is used in a number of applications, as either a prefiltration step or as a process to separate a fluid from a process stream. Dead-end MF is used commonly in stopping particles in either prefiltration or final filtration before a fluid is to be used. Cartridge filters are typically composed of MF media.

Ultrafiltration (UF)

UF is a form of filtration that uses membranes to preferentially separate different fluids or ions. UF is a low-pressure fractionation of selected components by size. UF is not as fine a filtration process as nanofiltration, but it also does not require the same energy to perform the separation. UF also uses a membrane that is partially permeable to perform the separation, but the membrane's pores are typically much larger than the membrane pores that are used in nanofiltration. UF is most commonly used to separate a solution that has a mixture of some desirable components and some that are not desirable. One of the uses that demonstrates the usefulness of UF is electrodeposition paint recovery. In this instance the paint, composed of a resin, a pigment and water are separated into two streams that can be reused. The first stream includes the water and a small amount of the paint resin, which can be used to rinse the parts later in the process. The paint pigment is separated from that stream and can be reused in the paint bath, allowing the bath to be concentrated to a usable level.

UF is capable of concentrating bacteria, some proteins, some dyes, and constituents that have a larger molecular weight of greater than 10,000 daltons. Depending on the molecular weight cutoff selected, the membrane will concentrate high molecular weight species while allowing dissolved salts and lower molecular weight materials to pass through the membrane. UF is typically not effective at separating organic streams. UF membranes are used in numerous industries for concentration and clarification of large process streams.

Nanofiltration (NF)

NF is a form of filtration that uses membranes to preferentially separate different fluids or ions. NF is not as fine a filtration process as reverse osmosis (RO), but it also does not require the same energy to perform the separation. NF also uses a membrane that is partially permeable to perform the separation, but the membrane's pores are typically much larger than the membrane pores that are used in RO. NF is a membrane technology on the rise because it is more cost effective than RO membranes in certain applications.

NF is most commonly used to separate a solution that has a mixture of some desirable components and some that are not desirable. An example of this is the concentration of corn syrup. The NF membrane will allow the water to pass through the membrane while holding the sugar back, concentrating the solution. As the concentration of the fluid being rejected increases, the driving force required to continue concentrating the fluid increases. NF is capable of concentrating sugars, divalent salts, bacteria, proteins, particles, dyes, and other constituents that have a molecular weight greater than 1000 daltons. NF, like RO, is affected by the charge of the particles being rejected. Thus, particles with larger charges are more likely to be rejected than others.

NF is not effective on small molecular weight organics, such as methanol. The largest users of the NF technology are municipal drinking water plants. A future trend is for NF to replace lime softening to achieve an industry standard of 50 parts per million (ppm) of alkalinity plus meet the federal THM limits. NF is well established in the dairy industry for cheese-whey desalting. Other growing markets are in RO pretreatment; pharmaceutical concentration; kidney dialysis units; and maple sugar concentration.

Reverse Osmosis (RO)

RO, also known as hyperfiltration, is the finest filtration known. RO is the most complex technique in membrane separation. This process will allow the removal of particles as small as ions from a solution. High pressures of about 15.0 to 70.0 bar are required in order to overcome the high osmotic pressures across the membrane. This permits water to flow from the concentrated feed stream to the dilute permeate - a direction that is just the reverse of what would occur naturally during osmosis.

RO is used to purify water and remove salts and other impurities in order to improve the color, taste or properties of the fluid. It can be used to purify fluids such as ethanol and glycol, which will pass through the RO membrane, while rejecting other ions and contaminants from passing. The most common use for RO is in purifying water. It is used to produce water that meets the most demanding specifications that are currently in place.

RO uses a membrane that is semi-permeable, allowing the fluid that is being purified to pass through it, while rejecting the contaminants that remain. Most RO technology uses a process known as crossflow to allow the membrane to continually clean itself. As some of the fluid passes through the membrane the rest continues downstream, sweeping the rejected species away from the membrane.

The process of RO requires a driving force to push the fluid through the membrane, and the most common force is pressure from a pump. The higher the pressure, the larger the driving force. As the concentration of the fluid being rejected increases, the driving force required to continue concentrating the fluid increases. RO is capable of rejecting bacteria, salts, sugars, proteins, particles, dyes, and other constituents that have a molecular weight of greater than 150-250 daltons. The separation of ions with RO is aided by charged particles. This means that dissolved ions that carry a charge, such as salts, are more likely to be rejected by the membrane than those that are not charged, such as organics. The larger the charge and the larger the particle, the more likely it will be rejected.