Among membranes' attractions as substitutes for conventional water-treatment technologies, such as filtration, distillation and ion-exchange are:
- energy savings;
- operating ease; and
- system design flexibility.
Also, membranes enable water providers to remove the threat of agents and contaminants from water with greater confidence than with conventional treatments, such as sand filtration.
Water suppliers today are thus examining membranes for -- among other goals -- achieving removal of
- microbes (bacteria, pyrogens, giardia and cryptosporidium cysts) and trihalomethane (THM) precursors and pesticides,
in addition to extracting
- color from surface water and
- potable water from brackish, alkaline sources or seawater.
Potentially, membrane filtration can provide for one-step removal of turbidity and THM precursors as well as for disinfection.
The Nature of Membranes
A membrane essentially is a barrier that separates two fluids and selectively restricts transport of the fluid's constituents.
The job of the membrane as a water-treatment element is to separate an influent stream into two effluent streams -- the permeate (having passed through the semi-permeable membrane) and the concentrate (solute), or reject component.
The four commonly accepted categories or 'classes' of membrane are defined by the size of the material being removed from the carrier fluid. From smallest to largest pore size they are: reverse osmosis (RO), nanofiltration (NF), ultrafiltration (UF) and microfiltration (MF).
Membranes reject three categories of solute: organic, inorganic and particulate. RO, NF and some UF membranes show high rejection of naturally occurring organics and some specific inorganics. However, inorganic solutes have been successfully removed by electrodialysis (ED), RO and NF.
Transport through a membrane comes about by convection or by diffusion of individual molecules induced by a gradient provided by an electric field, concentration, pressure or temperature. Specific mechanisms of mass transport or solute rejection by membranes include sieving, diffusion, adsorption, and electrostatic interaction.
Membrane Separation Processes
Reverse Osmosis (RO). RO essentially is a pressure-driven membrane-diffusion process for separating dissolved solutes. It eliminates dissolved solids, bacteria, viruses and other organisms from water and goes beyond filtration, which succeeds only at removing some suspended materials larger than one micron. Still, RO mostly finds use for producing potable water from salt-bearing (primarily brackish) water. It involves no phase change, and uses -- relative to distillation -- little energy.
The pore diameter (0.5 nanometer (nm) to 1.5 nm) of RO membranes -- being the smallest of all membrane types -- allows only the most minute organic molecules and uncharged solutes, along with the water, to pass through. Ninety-five to 99 percent of inorganic salts and charged organics are rejected.
The bulk of RO membranes are polymeric -- cellulosic acetate (CA) and aromatic polyamides. Two types of RO membranes predominate: asymmetric, or skinned, and thin-film composite (TFC). The support material commonly constitutes polysulfones, with the film constituting polyamines, polyureas and other polymers.
Membranes now can be formulated to remove virtually all dissolved solids and impurities or specific impurities from the water.
Nanofiltration (NF). NF has been termed 'loose RO' since its membrane pores are larger than RO's. It concentrates divalent salts, bacteria, particles and other constituents that have a molecular weight greater than 1000 and works in situations requiring high organic and moderate inorganic removals.
In separating different fluids or ions, NF typically affords higher recoveries and requires considerably lower operating pressure than does RO, but at the expense of allowing more salt passage.
Membranes for NF are of CA and aromatic polyamide offering salt rejections from 95 percent (for divalent salts) to 40 percent (for monovalent salts). They have an approximate molecular-weight cut-off (MWCO, the smallest molecular-weight species for which the membranes have more than 90 percent rejection) for organics of 300.
Ultrafiltration (UF). Successful use of UF (i.e., concentration of rejected solutes) depends primarily on particle size and to some extent, on particle charge. Typically, water-plant rejected species include bio-molecules, polymers and colloidal particles. The driving force for transport across the membrane is pressure differential -- characteristically two to 10 bars (atmospheres) but as high as 25 to 30 bars.
UF throughput depends on such physical properties as permeability and thickness of the membrane and such system variables as feed consumption, feed concentration, system pressure, velocity and temperature.
UF membranes entrain molecular weights in the range of 300 to 500,000, with pore sizes ranging from 0.001 to 0.1 micron. Their nominal molecular weight cutoff is 1,000 to100,000. When processing a suspension, the solids collect as a porous layer over the membrane's surface.
Polymeric materials -- including polysulfone, polypropylene, nylon 6, polytetrafluoroethylene (PTFE), polyvinylchloride (PVC) and acrylic copolymer--have found successful use as UF membranes. Also, inorganic materials such as ceramics, carbon-based membranes and zirconia are commercially available. These have about the same chemical resistance of polysulfone but are more heat resistant.
Microfiltration (MF). By far the most widely used membrane process, MF essentially provides sterile filtration, restraining pass-through of microorganisms and material of colloidal size and larger. MF is being appraised as a replacement for diatomaceous earth, a rock known for it s absorptive capacity and chemical stability.
MF's pores are 0.1 to 10.0 microns -- two to five orders of magnitude larger than that for other membrane classes. (The smallest bacterium, pseudomonas diminuta has a size of 0.3 micron.)
MF membranes constitute natural or synthetic polymers, such as cellulose nitrate or acetate, polyvinylidene difluoride (PVDF), polyamides, polysulfone, polycarbonate, polypropylene and polytrifluoroethylene (PTFE). Membranes of inorganic materials, such as metal oxides (including alumina, glass and zirconia-coated carbon) also find use.
Material selection directly reflects end-application demands, such as mechanical strength, temperature resistance, chemical compatibility, hydrophobility, hydrophilicity, permeability, permselectivity and cost.
Dialysis. This membrane-transport process is driven primarily by concentration differences, rather than by pressure or electric-potential differences, across the thickness of a membrane.
Electrodialysis (ED). ED is useful for several general types of separations of salts, acids and bases from aqueous solutions. It also serves to separate and concentrate monovalent ions from multiple charged components or to separate ionic compounds from uncharged molecules.
An ED system constitutes both cation- and anion-selective membranes placed in an electric field. (The former passes only the cations; the latter only the anions.)
ED membranes usually consist of cross-linked sulfonated polystyrene. Anion membranes can contain quaternary ammonium groups. Usually, ED membranes are fabricated as flat sheets containing about 30 percent to 50 percent water. Membrane fabrication is done by applying the cation- and anion-selective polymer to a fabric material.
Electrodialysis reversal (EDR) makes use of ion-specific membranes arrayed between anodes and cathodes to drive salt ions in controlled migrations to the electrodes. Although not as prevalent as RO, it continues to be commonly used by the water industry for making potable product from sea or brackish water. EDR is exploiting new areas of application, including pH control without adding acid or base and regeneration of ion-exchange resins.
Extant since the 1960s, membrane water-treatment facilities have sustained drastic construction and operation cost reductions. These reductions have opened alternative sources of drinking water to U.S. communities that have had no previous practical role in supplying their own drinking water.
Enabling membranes' expanded use are advanced materials, improved membrane packaging and better overall technology. Membranes now can be formulated to remove virtually all dissolved solids and impurities or specific impurities from the water. This versatility allows almost any source of water to be used as an alternative water source.
Supporting evidence for membrane treatment over conventional treatment technology is the generally superior quality of the water it produces. Water providers have noted that they can add membrane facilities and blend the water they produce with that from conventional facilities. Some see this as one approach to forestalling expensive replacement of existing conventional facilities when new federal drinking-water standards make them non-compliant.
Because of increasing use of membranes (in areas other than water treatment), membrane capital costs are now nearly half of what they were a decade ago. Plus, the life of membranes now ranges from five to eight years where in the early '90s it was three. And the energy needed to drive the water through the membranes is rapidly dropping -- although energy still accounts for the technology's largest expense.
Payroll is membranes' second most significant operating cost. However, membrane processes have proven well suited to automation, and reputedly, automation allows the operation of large membrane facilities with part-time personnel. In fact, membrane personnel costs are seen as being less than those for traditional water-treatment methods.