Between UF and RONanofiltration is a pressure-driven membrane purification process whose characteristics fall between those of ultrafiltration (UF) and reverse osmosis (RO, also called hyperfiltration). See sidebar: Figure 1. Particle Size Scale.

NF is also called low-pressure RO. Its membranes remove particles and small dissolved molecules and ions, usually hardness, iron, organics or color.

Typical NF applications include water softening, desalination of dyestuffs, acid and caustic recovery, and color removal.

The NF membraneAn NF membrane works just like an RO membrane. The separation process for NF requires that the water be forced through a semi-permeable membrane in the opposite direction of natural osmotic flow, leaving contaminants in the more highly concentration solution (reject water).

Nanofiltration membranes usually have three layers: a high-strength polyester base (approximately 150 microns thick), a microporous polysulfone interlayer (approximately 50 microns thick), and an ultra-thin polyamide barrier layer on the top surface (0.2 micron thick) See sidebar: Figure 2. NF Membrane Layers.

NF membranes are mostly packed in a spiral-wound configuration. This configuration offers lower replacement costs and a simpler plumbing system, as well as easier maintenance and greater design freedom compared with tubular and flat plate designs.

A cross-flow process is carried out by having a process solution or feedwater flow along a membrane surface under pressure. Cross-flow membrane filtration uses a high cross-flow rate to enhance permeate passage and to reduce membrane fouling. Retained solutes (such as dissolved salts) leave with the flowing process stream and do not accumulate on the membrane surface.

Within the systemBasic spiral-wound element construction makes use of flat sheet membrane cast onto a support fabric. The flat sheet membrane is enclosed between layers of feed spacer and permeate carrier. This process forms a leaf, which is sealed at each edge.

Many leaves are rolled into a cylinder around a perforated central tube. Finished element diameters can be in a range of 2.5” to 18” with a length up to 60” (see sidebar: Figure 3. NF Membrane Elements).

Standard test conditions for an NF element usually include feedwater, temperature, pressure, pH and recovery. The simplest NF system for POU/POE consists of a prefilter, an NF membrane filter, a booster pump, a storage tank for the treated water, and a flow regulator for the reject water, just like any RO system design.

Nanofiltration is a low- to moderately high-pressure (typically 50 to 450 pounds per square inch [psi]) process in which monovalent ions will pass freely through the membrane but highly charged, multi-valent salts and low molecular weight organic molecules will be rejected to a much greater degree (see sidebar: Table 1. NF and RO Rejection Rates, Parameters).

NF and RO: Similarities, differencesComparisons of NF with RO can be generally highlighted as followed:

Both are RO technology. The NF membrane is just a little “looser” — that is, RO will remove smaller-diameter particles than NF.

The key difference is the degree of removal of monovalent ions such as chlorides. RO removes monovalent ions at the 98 to 99 percent level at 200 psi. NF removal of monovalent ions varies between 10 and 90 percent, depending on the material and manufacture of the membrane.

In general, RO has less flow (produces less volume) than NF. NF and RO are generally in the spiral-wound configuration. RO membranes are characterized by high rejection of TDS (total dissolved

solids) in the range of 98 to 99.5 percent. NF membranes are characterized by higher water permeability than the RO

membranes and lower TDS rejection. NF membranes have sufficiently high rejection of selected constituents, i.e.,

hardness, metals (iron) and organic matter. Membrane fouling: A high concentration of organics can foul up membranes. NF

has low-fouling membranes and RO has high-fouling membranes. Since hard water has great potential to foul membranes, RO usually requires water to be softened first.

RO system permeate has very low hardness and alkalinity, and therefore, it has quite a high corrosion potential. This is why lower pH for RO permeate needs to be adjusted to neutral pH.

NF and RO membranes can be damaged by disinfectants like chlorine, unlike those for microfiltration and ultrafiltration.

This is why chlorine and chloramines must be removed in the system prior to the NF and RO membranes.

There are many benefits using nanofiltration technology as listed in sidebar: Table 1. Due to the limitations of RO and softeners, we expect to see more POU/ POE nanofiltration products in the marketplace in future.

Cang Li, Ph.D. (pictured, above) is director of research and development/quality control at Selecto Scientific, Inc., based in Suwanee, GA, a producer, and provider of service and installation for filtration technology for water and air treatment