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Microfiltration

Microfiltration membranes are created in several different designs. Spiral-wound membranes roll up many layers of flat membrane sheets around a central pipe that provides the water to be treated. Hollow fine-fiber configurations use a grouping of thousands of hollow tubes that are themselves constructed of membrane material. Microfilters, like ultrafilters, are normally found in a hollow fiber configuration.

Because microfiltration membranes are a physical treatment technology, they are subject to physical limitations. Water that is filled with particulates or organic materials can clog membranes. Some water, particularly surface water, may need pretreatment before passing through a membrane system. These systems also produce small volumes of highly concentrated solution which requires disposal.

Membranes are classified according to the size of the molecules that they are able to filter—Nominal Molecular Weight Cutoff or MWCO. Microfiltration has the highest MWCO, and thus, the largest pore size.

Microfiltration employs pore sizes from .03 to ten microns (usually 0.1 to two microns) and is effective to MWCO sizes of 100,000 Daltons or more. It is often used to remove sand, silt, clay, algae, bacteria and Giardia and Cryptosporidium.

Membranes are constructed of many different materials, which have their own pros and cons. Choosing the right membrane for conditions can be a challenge for water system managers.

Compared to other kinds of membrane technologies, microfiltration is less commonly used today.

Ultrafiltration

Ultrafiltration membranes are created in several different designs. Spiral-wound membranes roll up many layers of flat membrane sheets around a central pipe that provides the water to be treated. Hollow fine-fiber configurations use a grouping of thousands of hollow tubes that are themselves constructed of membrane material. Ultrafilters, like microfilters, are normally found in a hollow fiber configuration.

Because ultrafiltration membranes are a physical treatment technology, they are subject to physical limitations. Water that is filled with particulates or organic materials can clog membranes. Some water, particularly surface water, may need pretreatment before passing through a membrane system. These systems also produce small volumes of highly concentrated solution which requires disposal.

Membranes are classified according to the size of the molecules that they are able to filter—Nominal Molecular Weight Cutoff or MWCO. Ultrafiltration employs pore sizes from .01 to 0.03 microns and is effective to MWCO sizes of 10,000 Daltons or more. It is often used to remove sand, silt, clay, algae, bacteria, Giardia, Cryptosporidium, and viruses.

Membranes are constructed of many different materials, which have their own pros and cons. Choosing the right membrane for conditions can be a challenge for water system managers.

Nanofiltration

Nanofiltration membranes are created in several different designs. Spiral-wound membranes roll up many layers of flat membrane sheets around a central pipe that provides the water to be treated. Hollow fine-fiber configurations use a grouping of thousands of hollow tubes that are themselves constructed of membrane material. Nanofilters, like reverse osmosis, are normally found in a spiral-wound arrangement.

Because membranes are a physical treatment, technology they are subject to physical limitations. Water that is filled with particulates or organic materials can clog membranes. Some water, particularly surface water, may need pretreatment before passing through a membrane system. These systems also produce large volumes of concentrate, which requires disposal.

Membranes are classified according to the size of the molecules that they are able to filter—Nominal Molecular Weight Cutoff or MWCO. Nanofilters have an MWCO of approximately 1000 Daltons or less. The process requires very high water pressures to force source fluid through extremely small pores (as small as .001 micrometers or one nanometer, hence the name) in order to remove contaminants.

Nanofiltration is used to remove hardness, natural organic matter, and synthetic organic chemicals from water.

Source water must always be treated prior to nanofiltration, so that particulates do not foul the membrane and limit its efficiency. Waters high in iron, chlorine, and manganese may also require pretreatment. Even under ideal conditions, nanofiltration systems, like reverse osmosis systems, require regular membrane cleaning and periodic replacement.

Nanofiltration membranes are constructed of many different materials, which have their own pros and cons. Choosing the right membrane for conditions can be a challenge for water system managers.

Reverse Osmosis

Membrane water treatment systems were originally used only in desalination projects. But improvements in membrane technology have made them an increasingly popular choice for treating microorganisms, particulates, and natural organic materials that foul water’s taste and taint its clarity.

These systems consist of thin material sheets that technically do not have pores. Rather, the membrane allows water molecules to pass through it, but catches and retains other dissolved or suspended substances. The system pressurizes the solution to such an extent that water flows from a more concentrated solution, through the membrane, and into the more dilute solution—the opposite of natural flow by osmosis.

Reverse osmosis membranes are created in several different designs. Spiral-wound membranes roll up many layers of flat membrane sheets around a central pipe that provides the water to be treated. Hollow fine-fiber configurations use a grouping of thousands of hollow tubes that are themselves constructed of membrane material. Reverse osmosis, like nanofiltration, is normally found in a spiral-wound arrangement.

Source water must nearly always be treated prior to reverse osmosis, so that particulates do not foul the membrane and limit its efficiency. Waters high in iron, chlorine, and manganese may also require pretreatment. Even under ideal conditions, reverse osmosis systems, like nanofiltration systems, require regular membrane cleaning and periodic replacement.

The concentrated solution, minus its now-treated water, becomes a brackish waste product that retains contaminants. The volume of this concentrate may be up to half of the total source water volume—a larger amount than that produced by conventional membrane filtration. Disposal of this water is a major management concern, and many current methods—including release to sewers or deep wells—carry environmental consequences.

Reverse osmosis units are easily scaled and can be a good option for small—even portable—system requirements in areas where electrical current is available and reliable, and staff can be trained in the use of additives to prevent scale formation.

Reverse osmosis membranes are constructed of many different materials, which have their own pros and cons. Choosing the right membrane for conditions can be a challenge for water system managers.

Electrodialysis/Electrodialysis Reversal

Electrodialysis and electrodialysis reversal treatment systems use electricity and a series of membranes to separate salts from source water and to concentrate them into a solution for disposal.

When electric current is applied to source water, chloride ions gravitate to the one end and sodium ions are drawn to the other. Moving in either direction, these minerals pass through stacks of membranes, which trap them in channels dedicated to containing the highly concentrated solution. This waste product, which must be disposed of properly, may amount to some 30 percent of the total source water treated; 15 to 20 percent is more typical.

The water produced by these treatments must also be treated for organic compounds (if they are a concern) and microbes—either before or after the electrodialysis process. Because source water does not physically pass through membranes in these systems, most organic contaminants are not removed.

Source water for these systems must also be prefiltered, to reduce turbidity, though membranes are less prone to fouling than in other systems because source water does not pass through them. Membranes are also kept clean by the periodic reversal of the system’s polarity, which causes ion flow to occur in the opposite direction and reduces buildup.

Electrodialysis and electrodialysis reversal systems require large amounts of energy to produce the constant current that drives purification and pumps water through the system. For this and other reasons, they are not used as much in large water treatment facilities as some of the other technologies described here. Rather, they are more commonly used for medical and laboratory applications that need ultrapure water.

However, they are easily scaled to small system use and typically operate automatically with few maintenance and operation requirements. Electrodialysis is less well suited to point of use or point of entry systems than are reverse osmosis or nanofiltration.

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