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Membrane Separation

Figure 1: Typical NF/RO configuration.
Figure 1: Typical NF/RO configuration.
Figure 2: Typical ED/EDR configuration.
Figure 2: Typical ED/EDR configuration.

Included Processes: Nanofiltration, Reverse Osmosis, Electrodialysis, and Electrodialysis Reversal

Membrane separation processes include nanofiltration (NF), reverse osmosis (RO), electrodialysis (ED), and electrodialysis reversal (EDR).

NF and RO both operate based on the principle of reverse osmosis. Osmosis is the natural flow of a solvent, such as water, through a semi-permeable membrane (which acts as a barrier to dissolved contaminants) from a less concentrated solution to a more concentrated solution. This flow will continue until the concentrations on each side of the membrane are equal. The amount of pressure that must be applied to stop this flow of water is called the osmotic pressure. Reverse osmosis is the reversal of the natural osmotic process by applying pressure in excess of the osmotic pressure to the more concentrated solution. The feed pressure forces the water through the membrane against the natural osmotic gradient, thereby increasing the dissolved contaminant concentrations on one side of the membrane and increasing the volume of water with lower concentrations of dissolved contaminants on the other. As the desired level of removal increases, the feed pressure also generally increases. For NF membranes, the typical feed pressure range is between 50 to 150 psi. For RO membranes, typical feed pressures range between 125 to 300 psi for low pressure systems, 350 to 600 psi for standard pressure systems and 800 to 1,200 psi for seawater applications.

ED is an electrochemical separation process in which ions are transferred through membranes from a less concentrated to a more concentrated solution as a result of the flow of direct electric current. ED systems consist of alternating anion and cation ion exchange membranes placed between positive and negative electrodes. Applying a voltage across the electrodes causes the positively charged cations to move towards the negative electrode and negatively charged anions to move towards the positive electrode. Contaminants are removed from the solvent (water, in this case) through the membrane; as opposed to other membrane processes where the solvent passes through the membrane and the contaminants are rejected by the membranes. In the EDR process, the electrical polarity (anode and cathode) are periodically reversed to control membrane scaling and fouling. Polarity reversal typically occurs two to four times per hour. When the electrical polarity is reversed, the product and concentrate streams are also reversed. This prevents any of the flow compartments from seeing streams with high dissolved solids for extended periods of time and aids in controlling fouling of the membranes.

NF and RO processes include three basic flow streams: the feed, permeate or product, and concentrate or waste streams. NF and RO elements are generally spiral wound. A number of membrane elements (typically three to seven) are arranged in pressure vessels. A series of pressure vessels are arranged in stages, wherein the concentrate from the prior stage becomes the feed for the subsequent stage. The permeate from each stage is blended together for the final product stream. The concentrate from the final stage is usually wasted.

Like NF and RO, the primary flow streams consist of the feed, product, and concentrate. Instead of elements and pressure vessels, EDR consists of stacks of EDR membranes arranged in lines. These lines make up the stages in an EDR system. Unlike the NF and RO processes, the product from the prior stage is further treated in subsequent stages. The concentrate from each stage is blended and wasted.

The primary difference between NF and RO is the size of dissolved contaminants that can be removed. The molecular weight cut-off (MWCO) is a measure of the removal characteristics of a membrane (expressed in Daltons) in terms of atomic weight. The typical range of MWCO levels is generally less than 100 Daltons for RO membranes, and between 200 and 1000 Daltons for NF membranes. Consequently, NF membranes are typically used for hardness (Ca2+, Mg2+) and organics (e.g., DBP precursors) removal. RO membranes are typically used for total dissolved solid (TDS) and monovalent ion removal (e.g., seawater and brackish water desalting, F- and Cl- removal). ED/EDR are capable of removing any dissolved ionic contaminant, including hardness, but are most typically used for TDS reduction and inorganic ion removal (e.g., brackish water desalting, F- and Cl- removal) .

NF, RO, and ED/EDR membrane systems frequently require some type of pretreatment to: (1) condition the water for optimum membrane effectiveness, and (2) modify the feed water to prevent membrane fouling and plugging, and (3) maximize the time between cleanings and prolong membrane life. The type of pretreatment required depends on the feed water quality and membrane type. The feed water must be generally free of suspended matter for RO and NF systems, and to a lesser extent for ED and EDR systems. For this reason, surface waters may require more extensive pretreatment than groundwater. Turbidity and the silt density index (SDI) are typically used as indicators of feed water suspended solids. Acid and/or antiscalant addition is commonly used when scaling is the primary fouling concern.

Scaling occurs when the concentrations of dissolved contaminants in the concentrate exceed the solubility product of a particular compound. Calcium carbonate, calcium sulfate, barium sulfate, and/or silica are typically the limiting compounds. However, hydrogen sulfide, iron, manganese, organics, and microbial levels must also be carefully controlled minimize fouling. The concentration of these contaminants in the concentrate stream limits system recovery, which is the ratio of the system permeate and feed water flow rates expressed as a percentage. The concentration of dissolved contaminants in the concentrate stream can increase significantly compared to the feed concentration (up to tenfold) depending on the system recovery, Standard RO and NF individual membrane elements have recoveries ranging between 80-85%. Recovery for seawater desalination is usually in the range of 50%. ED and EDR recoveries are typically 85-95%.

Temperature can significantly impact membrane performance. Water temperature has a significant impact on water density and viscosity, which impacts NF and RO membrane flux - the rate of product flow through the membrane, typically expressed in gallons per day per square foot of membrane area (gfd). As the viscosity and density increase, the transmembrane pressure required to pass the water through the membrane also increases, resulting in an increase in the specific flux, which is the flux divided by the transmembrane pressure. Contaminant solubility also decreases as temperature decreases which may require operation at a lower recovery to prevent scaling. For ED and EDR systems, water temperature affects ion mobility. Higher temperatures can cause membrane degradation and compaction of RO and NF membranes.

NF, RO, and ED/EDR remove bicarbonate and alkalinity to varying degrees causing depression of the treated water pH. For this reason, pH and/or alkalinity adjustment may be necessary to provide effective corrosion control downstream of these processes.

Residuals generated from membrane separation systems include the concentrate from the membrane processes and the spent cleaning chemicals. Concentrate disposal can be challenging as it is typically a relatively high volume, high TDS waste stream and requires a comparatively large body of water for discharge or must be discharged to a wastewater treatment plant. Chemical cleaning is required to periodically remove scale build-up and biological fouling on the membrane surface. Spent cleaning solutions are generally acidic in nature and require neutralization prior to disposal.

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