Water Treatment: Membrane Processes

MEMBRANE PROCESSES

Common membrane processes include ultrafiltration (UF), reverse osmosis (RO ), electrodialysis (ED), and electrodialysis rever sal (EDR). These processes (with the exceptio n of UF) reduce most ions; RO and UF system s also provide efficient reduction of nonionizzed organics and particulates. Because UF mem brane porosity is too large for ion rejection, t he UF process is used to reduce contaminants, such as oil and grease, and suspended solids.

Reverse Osmosis

Osmosis is the flow of solvent through a semi-permeable membrane, from a di lute solution to a concentrated solution. This flow results from the driving force created by the difference in pressure between the two soluti ons. Osmotic pressure is the pressure that must be added to the concentrated solution side in o rder to stop the solvent flow through the membrane. Reverse osmosis is the process of rev ersing the flow, forcing water through a membrane from a concentrated solution to a dilute solution to produce filtered water.

Reverse osmosis is created when sufficient pressure is applied to the concen trated solution to overcome the osmotic pressure. This pressure is provided by feedwater pumps. Concentrated contaminants (brine) are reduced from the high-pressure side of the RO membrane, and filtered water (permeate) is reduced from the low-pressure side. Figure 9-2 is a simpli fied schematic of an RO process. Membrane mod ules may be staged in various design configur ations, producing the highest-quality permeate wi th the least amount of waste.

Typically, 95% of dissolved salts are reduced from the brine. All particulates are removed. However, due to their molecular porosity, RO membranes do not remove dissolved gases, such as Cl₂, CO₂, and O₂.

RO Membranes. The two most common RO membranes used in industrial water treatment are cellulose acetate (CA) and polyamide (PA) composite. Currently, most membranes are spiral wound; however, hollow fiber configurations are available. In the spiral wound configuration, a flat sheet membrane and spacers are wound around the permeate collection tube to produce flow channels for permeate and feed water. This design maximizes flow while minimizing the membrane module size.

Hollow fiber systems are bundles of tiny, hair-like membrane tubes. Ions are rejected when the feedwater permeates the walls of these tubes, and permeate is collected through the hollow center of the fibers. Concentrated brine is produced on the outside of the fibers contained by the module housing.

Electrodialysis

Electrodialysis (ED) processes transfer ions of dissolved salts across membranes, leaving purified water behind. Ion movement is induced by direct current electrical fields. A negative elec-trode (cathode) attracts cations, and a positive electrode (anode) attracts anions. Systems are compartmentalized in stacks by alternating cation and anion transfer membranes. Alternating compartments carry concentrated brine and filtered permeate. Typically, 40-60% of dissolved ions are removed or rejected. Further improvement in water quality is obtained by staging (operation of stacks in series). ED processes do not remove particulate contaminants or weakly ionized contaminants, such as silica. Figure 9-6 is a simplified schematic of an ED process.

Electrodialysis Reversal

Electrodialysis reversal (EDR) processes operate on the same principles as ED; however, EDR operation reverses system polarity (typically 3-4 times per hour). This reversal stops the buildup of concentrated solutions on the membrane and thereby reduces the accumulation of inorganic and organic deposition on the membrane surface. EDR systems are similar to ED systems, designed with adequate chamber area to collect both product water and brine. EDR produces water of the same quality as ED.

Ultrafiltration

In many process and wastewater applications, reduction of dissolved ions is not required but efficient reduction of colloidal inorganic or organic molecules is. Ultrafiltration (UF) membrane configurations and system designs are similar to those used in the single-stage RO process. Because the large molecules removed by UF exhibit negligible osmotic pressure, operating pressures are usually much lower than in RO systems. Figure 9-7 illustrates the performance of ultrafiltration membranes. Typical applications include reduction of oil and grease and recovery of valuable contaminants in process waste streams.

PRETREATMENT

Processes that rely on microporous membranes must be protected from fouling. Membrane foul- ing causes a loss of water production (flux), reduced permeate quality, and increased trans-membrane pressure drop.

Membrane fouling is typically caused by precipitation of inorganic salts, particulates of metal oxides, colloidal silt, and the accumulation or growth of microbiological organisms on the membrane surface. These fouling problems can lead to serious damage and necessitate more frequent replacement of membranes.

SOLIDS REDUCTION

Membrane feedwater should be relatively free from colloidal particulates. The most common particulates encountered in industrial membrane systems are silt, iron oxides, and manganese oxides.

Silt Density Index (SDI) testing should be used to confirm sufficient water quality for the specific membrane system employed. SDI evaluates the potential of feedwater to foul a 0.45 µm filter. Unacceptable SDI measurements can be produced even when water quality is relatively high by most industrial water treatment standards. Where pretreatment is inadequate or ineffective, chemical dispersants may be used to permit operation at higher-than-recommended SDI values. RO systems are highly susceptible to particulate fouling, ED and EDR systems are more forgiving, and UF systems are designed to handle dirty waters.

SCALE CONTROL

Membrane processes produce a concentration gradient of dissolved salts approaching the membrane surfaces. The concentration at the membrane may exceed the solubility limits of certain species. Calcium carbonate (CaCO₃) and calcium sulfate (CaSO₄) are typical precipitates formed. Silica, barium, and strontium salts are also frequently identified in membrane deposits. Because of their low solubility, very low levels of feedwater barium or strontium can cause membrane fouling.

Various saturation indexes, such as the Stiff-Davis and Langelier, should be maintained below precipitating values in the brine (through pH control or deposit control agents) to prevent calcium carbonate fouling. Other precipitates may be controlled by the proper application of deposit control agents.

MICROBIOLOGICAL FOULING

Cellulose acetate membranes can be degraded by microbiological activity. Proper maintenance of chlorine residuals can prevent microbiological attack of these membranes.

Polyacrylamide membranes are resistant to microbiological degradation; however, they are susceptible to chemical oxidation. Therefore, chlorination is not an acceptable treatment. If inoculation occurs, microbiological fouling can become a problem. Nonoxidizing antimicrobials and biodispersants should be used if serious microbiological fouling potential exists.