Even if you’re an expert in reverse osmosis (RO) membrane technology, you may be overlooking some applications using often neglected cousins of RO: nanofiltration (NF) and ultrafiltration (UF).
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 is not loose RO
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.
Inge Bisconer is Marketing Communications Manager for Osmonics Desal, Vista, CA.
Bjarne N. Nicolaisen, Nanofiltration: Where Does it Belong in the Larger Picture? Desalination Systems Inc., Product Technical Bulletin, December 1994.
Lee F. Comb, Advances in Membrane Technology for Beverage Water Treatment, Osmonics, Inc., 1993.
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