Reverse osmosis, also known as hyperfiltration, is the finest filtration known. This process will allow the removal of particles as small as ions from a solution. Reverse osmosis 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 reverse osmosis membrane, while rejecting other ions and contaminants from passing. The most common use for reverse osmosis is in purifying water. It is used to produce water that meets the most demanding specifications that are currently in place.

Reverse osmosis 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 reverse osmosis 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. As an example, say for every gallon of water that is pumped into the membrane housing, 2/3 will pass through the membrane to become usable water, and the remaining 1/3 will continue along the exterior of the membrane carrying the contaminants with it.  The reason that all of the water does not flow through the membrane is it is moving very fast at a high pressure (about 160 psi), and the pores in the filter are very tiny.  If the water were slowed down, then more of the water would pass through the membrane, but without enough water flowing along the outside of the membrane, they would foul (the pores would get plugged).  The process of reverse osmosis 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.

Reverse osmosis 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 reverse osmosis 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.

While the Clifton Water District originally considered four solutions—ion exchange, lime/soda ash softeners, developing an alternative water supply and membrane technology—only the latter two proved economical. However, securing an alternative water source was to be both geographically and financially challenging.  Meanwhile, membrane technology proved to be the most cost effective solution. Effective in lowering TDS and hardness concentrations while also producing highly filtered water as an added benefit, membrane technology clearly became Clifton Water District’s most desirable choice.

The Charles A. Strain Water Treatment Plant opened in September 1957. Featuring a 12 MGD-capacity blended water treatment system which relies on a 2.4 MGD NF system to meet water quality requirements. The plant consists of four skids in parallel, each having two multimedia filters in parallel followed by cartridge filters. The purification process begins by pumping water from a clearwell to a multimedia filter consisting of six layers of four different materials—anthracite, manganese, greensand, gravel and garnet. It then flows to the nanofiltration unit, where the water is injected with sodium bisulfate, sulfuric acid and antiscalant before passing through the cartridge filters to provide added filtration of suspended solids for the nanofiltration membranes. The product water is then blended with conventionally treated water.

Clifton Installation RO Skid

Tests showed that the Clifton Water District’s nanofiltration plant now generates cleaner, healthier, aesthetically pleasing water. By significantly lowering total hardness by 80%, sulfate by 98%, TDS by 50%, TTHMP by 95%, HAAP by 99%, and total particle concentration by 97%, the new nanofiltration system produces a water supply that is now compliant with both the NSDWR and Primary Drinking Water Standards.

The Clifton Water District initially estimated that the new nanofiltration treatment plant would cost approximately $4.5 million. Close cooperation with Osmonics lowered expenses by approximately $1 million. Ironically, during the winter of 1997 – 1998, the Clifton Water District did not purchase water from Grand Junction but instead sold water to it.

A comparative analysis of water quality before and after NF treatment shows remarkable improvement in purity.

Although virtually identical in looks to RO membranes, NF and UF membranes serve distinctly different separation functions. Dramatic levels of resource recovery, efficiency improvement and pollution prevention are compelling incentives for the industry to continue to use NF and UF.

Membrane separation technology removes substances ranging in size from ionic to molecular. These substances are so small they typically are measured in Angstroms (1 Angstrom = one 10 billionth of a meter) or molecular weight (MW). Membranes have been developed with mass transfer properties and pore sizes such that ionic, molecular and organic substances measuring between 1 and 1000 Angstroms (MW between 100 and 500,000) are removed or rejected.

A key difference between each membrane type is in the size of the pores. RO membrane pores are the smallest, measuring between 1 to 15 Angstroms.

While each of the four membrane types have similarities, they each perform very different functions in varying applications. In general, RO and NF membranes are capable of separating substances as small as ions from feed streams while UF and microfiltration (MF) membranes typically separate larger molecules. All four membrane types allow water to pass.

For example, RO membranes typically reject most of the ionic and organic species from the feed stream, allowing only water to pass. NF membranes are usually used to reject high percentages of multivalent ions and divalent cations while allowing monovalent ions to pass.

UF and MF membranes reject molecules on the basis of size. UF membranes retain particles larger than about 15 to 200 Angstroms and MF membranes retain particles from about 200 to 1000 Angstroms. UF and MF membranes are typically rated in terms of pore size, or porosity, while RO and NF membranes are rated by terms of percent salt rejection and flow.

Nanofiltration often has been wrongly categorized as a "loose RO" membrane. The differences are subtle, but distinct. Most notable is NF’s ability to reject only ions with more than one negative charge, such as sulfate or phosphate, while passing single charged ions.

Another distinctive feature is its ability to reject uncharged, dissolved materials and positively charged ions according to the size and shape of the molecule in question.

Finally, the rejection of sodium chloride with NF varies from 0 to 50 percent, according to the feed concentration. Although these differences may appear insignificant, they have far reaching implications in many applications.

In contrast, "loose RO" is an RO membrane with reduced salt rejection. This effect has proven desirable for a number of applications where moderate salt removal is acceptable since operating pressures and power consumption are significantly lowered. So, in exchange for less than complete salt removal, costs are reduced.

The table lists some comparative rejection values for four types of membranes. RO rejects almost every contaminant listed; "loose RO" rejects salts to a lesser degree, but other contaminants well; NF passes salts with only one negative charge, but rejects more than one negative charge; and UF rejects only those molecules with a large enough molecular weight (humic acid or larger) while passing others.

The following are a few examples of applications where NF and UF have been used successfully:

• Seawater desalination. One of the major obstacles to efficiently desalting seawater is the tendency for RO membranes to become fouled with silt and organics. UF can remove these fouling constituents before the water reaches the RO membrane, reducing fouling and increasing efficiency.

• Sugar industry. NF and UF membranes are routinely used to concentrate sugar and clarify sugar streams in the sugar industry. NF typically is used where traditional heat concentration processes are undesirable or inefficient. NF membranes consistently separate sugars of a specific molecular weight and remove 60 percent of the water, concentrating raw sugar juice from 12 to 30 Brix, a scale that measures the weight of sugar in solution.

UF membrane’s sharp molecular weight cut-off capabilities are used to clarify sugar streams. Color, tannins and other undesirable organic components are preferentially rejected while sugar molecules are allowed to pass.

• Dairy industry. Some of the most successful membrane applications are in the dairy industry where the production of whey, a protein by-product of cheese making, creates a pollution and disposal problem. Although whey consists of high-quality protein and lactose, the high ratio of lactose to protein and the low solids content make it unusable as is. In modern cheese-making facilities, UF, NF and RO are used to render liquid cheese whey into whey protein powder, concentrated lactose and reusable water.   Typically, whey is first treated with UF to reject and concentrate the protein fraction, from which protein power is then produced. The UF permeate containing the lactose and salts is then nanofiltered to concentrate the lactose and pass most of the salts. Finally, the NF permeate (salty water) is then desalinized by RO for reuse in the dairy operation.

• Textile industry. The textile industry uses valuable dyes, which are clearly visible if discharged into public waterways. In addition, these dyes have been shown to be trihalomethane (THM) precursors possessing carcinogenic properties. Thus, their disposal creates both an aesthetic and environmental wastewater problem. At the same time, the textile industry continually seeks to conserve water and would economically benefit from dye recovery. NF membranes address all these issues. First, textile dyes are rejected, recovered and reused. Second, waterway pollution is avoided. And third, reusable water is produced.

The textile industry also uses synthetic sizing agents, which are expensive and non-biodegradable and pose significant waste treatment problems. Ultrafiltration membranes are used to recover and reuse these agents, avoiding expensive chemical and waste treatment costs.  NF and UF membrane technologies continue to meet customer demands on a daily basis. Be aware of their subtle differences and take advantage of their separation capabilities in the components you sell, the equipment you build and the systems you design.

References:

1. Bjarne N. Nicolaisen, Nanofiltration: Where Does it Belong in the Larger Picture? Desalination Systems Inc., Product Technical Bulletin, December 1994.
2. Lee F. Comb, Advances in Membrane Technology for Beverage Water Treatment, Osmonics, Inc., 1993.
3. Munir Cheryan, Ultrafiltration Handbook, Technomic Publishing Co., 1986.

Reprinted with permission from Water Technology.  For more information, contact Editor, Water Technology, 13 Century Hill Drive, Latham. NY 12110; (512)783-1281