Demystifying Membranes - Part 1

• Jul 01, 2003

This is the first in a two-part series on membrane elements and treatment systems. Part One compares the advantages and disadvantages of four types of membrane separation technologies. Part Two, which will appear in an upcoming issue, will clear up some common misunderstandings about the properties of membrane technologies.

The pressure-driven membrane separation technologies of reverse osmosis (RO), nanofiltration (NF), ultrafiltration (UF) and microfiltration (MF) have been around, in some form or another, for more than 40 years.

In spite of their generally long history of use, these technologies are shrouded by a degree of confusion and misunderstanding. For example, there is a lack of complete agreement among the experts on the exact salts rejection mechanism of reverse osmosis and nanofiltration. Whereas the principle of "size-exclusion" or "sieving" dictates the specific rejection of suspended solids and non-ionic contaminants by microfiltration and ultrafiltration respectively, the ionic rejection of reverse osmosis and nanofiltration membranes is not as easily explained. On the other hand, there is absolutely no doubt that they work, and work well in many different applications.

The best way to define these four technologies is by their function or performance characteristics.

Microfiltration
This technology involves the removal of particulate or suspended materials ranging in size from approximately 0.01 to 1 micron (100 to 10,000 angstroms).

Ultrafiltration
This membrane technology is used to separate materials typically smaller than 0.01 micron (100 angstroms). The removal characteristics of UF membranes can be described in terms of "molecular weight cutoff" (MWCO), the maximum molecular weight of compounds that will pass through the membrane pores into the permeate stream. MWCO terminology is expressed in Daltons. Basically, ultrafiltration is used to remove dissolved non-ionic contaminants while suspended solids are removed by microfiltration.

Nanofiltration
This technology is an intermediate process between ultrafiltration and reverse osmosis. The MWCO properties of nanofiltration membranes range 300 to 1,000 Daltons ( 25 angstroms). Ionic rejections vary widely depending on the valence of the salts and characteristics of the specific NF polymer; multivalent salts such as magnesium sulfate (MgSO₄) are rejected as much as 99+ percent, while monovalent salts such as sodium chloride (NaCI) may have rejections as low as 20 percent.

Reverse Osmosis
This commonly used process removes all dissolved organic (non-ionic) solids with molecular weights above approximately 100 Daltons, as well as a high percentage of ionic materials. Because reverse osmosis membranes are not perfect (they will typically remove 95 to 99 percent of the ionic contaminants), they are usually used as pretreatment to a final "polishing" deionization unit for high purity water production.

Background
Compared to other solids/liquid separation technologies, membranes offer the following advantages:

• Continuous process, resulting in automatic and uninterrupted operation
• Low energy utilization involving neither phase nor temperature changes
• Modular design -- no significant size limitations
• Minimum of moving parts with low maintenance requirements
• No effect on form or chemistry of contaminants
• Discrete membrane barrier to ensure physical separation of contaminants
• No chemical addition requirements

Table I compares the four membrane separation technologies in a number of important areas.

Table I. Membrane Technology Comparison Chart

Feature	Microfiltration	Ultrafiltration	Nanofiltration	Reverse Osmosis
Polymers	ceramics sintered metals polypropylene polysulfone polyvinylidene fluoride polytetrafluoro- ethylene polyacrylonitrile	ceramics sintered metals cellulosics polysulfone polyvinylidene fluoride	thin film composites cellulosics	thin film composites cellulosics polysulfonated polysulfone
Pore Size Range (micrometers)	0.01 - 1.0	0.001 - 0.01	0.0001 - 0.001	<0.0001
Molecular Weight Cutoff Range (Daltons)	>100,000	2,000 - 100,000	300 - 1,000	100 - 200
Operating Pressure Range (psi)	<30	20 - 100	50 - 300	225 - 1,000
Suspended Solids Removal	yes	yes	yes	yes
Dissolved Organics Removal	none	yes	yes	yes
Dissolved Inorganics Removal	none	none	20-85 percent	95-99 percent
Microorganism Removal	protozoan cysts, algae, bacteria*	protozoan cysts, algae, bacteria*, viruses	all*	all*
Osmotic Pressure Effects	none	slight	moderate	high
Concentration Capabilities	high	high	moderate	moderate
Permeate Purity	high	high	moderate-high	high
Energy Usage	low	low	low-moderate	moderate
Membrane Stability	high	high	moderate	moderate

*Under certain conditions, bacteria will grow through the membrane

The Fight against Fouling

Fouling is the single greatest cause of membrane system failure and the source of most of the negative impressions surrounding these technologies. It is usually the result of the accumulation of suspended particulate material, precipitated salts, oxides or hydroxides or insoluble organic contaminants on the surface of the membrane. This accumulation inhibits the flow of permeate through the membrane, and in the case of RO and NF, can result in an increase of salts passage into the permeate stream.

In an effort to minimize fouling, a number of membrane device configurations have been developed over the years. The devices (elements or modules) in common use today are as follows:

• Spiral wound
• Plate and frame
• Capillary fiber
• Tubular

There are examples of these device configurations used in most of the technologies (MF, UF, NF, RO); however, in terms of cost and fouling resistance, each configuration has its strengths and weaknesses.

Spiral Wound
The spiral wound device is far and away the most common device configuration used in reverse osmosis and nanofiltration applications today. This is largely due to the fact that it is the least expensive configuration, resulting from its relatively high packing density (membrane surface area per unit volume), as well as the fact that it is a very competitive design (five of the six leading manufacturers are based in the United States). Because of the close spacing of the membrane leaves, this design has the greatest susceptibility to fouling.

Figure 1 illustrates the spiral wound membrane element.

Plate and Frame
The plate and frame membrane device has taken many shapes and sizes over the years, and has been used primarily for the technologies of microfiltration and ultrafiltration. The challenge of packing as much membrane area into as small a volume as possible, as well as problems associated with sealing and gasketing to prevent leakage and cross-contamination, have limited the utilization of this device. On the other hand, plate and frame elements are basically quite fouling resistant compared to the spiral-wound configuration.

Figure 2 illustrates the plate and frame configuration.

Capillary Fiber
This device consists of unsupported hollow fibers that have been extruded or spun from such polymers as polysulfone, polyethersulfone, polyacrylonitrile, polyvinylidene fluoride or similar thermoplastic materials, which can be constructed with pore sizes usually in the microfiltration or ultrafiltration categories. The capillary fibers (also known as hollow fibers) are constructed with inside diameters of several millimeters and wall thicknesses of fractions of millimeters. In most cases, the permeate flow is from the inside of the fiber to the outside (lumen-feed), although the elements with the feed flow on the outside (and the permeate exiting from the lumen side) are considered to be more fouling resistant. This construction is widely used for treating surface water sources to meet the new U.S. Environmental Protection Agency (EPA) drinking water regulations. Although used only in special applications, this configuration is available in ceramic and carbon-based materials.

Figure 3 illustrates the capillary fiber membrane element.

Tubular
These devices are differentiated from capillary fiber devices because they are constructed of a membrane-coated backing material (as with the spiral wound) that is wound into a tubular shape. Because this construction is more pressure tolerant than capillary fibers, the diameters of tubular devices can be made larger, thereby improving their tolerance to suspended solids. Most of the ceramic and other inorganic membrane devices today are in the tubular configuration. The inside diameters of tubular devices typically range from about 7 mm to 25 mm, and the flow is virtually always lumen-feed.

Figure 4 illustrates the tubular configuration.

Table 2 compares all of these devices with regard to packing density and fouling resistance. In general, the higher the fouling resistance, the higher the element cost per unit area of membrane.

Table 2. Membrane Element Configuration Comparison

Element Configuration	Packing Density*	Fouling Resistance**
Spiral Wound	high	low
Plate and Frame	low	high
Capillary Fiber	medium	low
Tubular	moderate	high

*Membrane area per unit volume of space required.

**Tolerance to suspended solids.

Whereas the construction of spiral wound elements make them very difficult, if not impossible to backwash; the other three configurations can be designed for easy backwashing, thereby improving their fouling tolerance even more.

This article originally appeared in the July/August 2003 issue of Environmental Protection, Vol. 14, No. 6.

This article originally appeared in the 07/01/2003 issue of Environmental Protection.