Introduction
Ultra filtration (UF) is primarily a size exclusion based separation. Typical rejected species include sugars, bio-molecules, polymers and colloidal particles. The driving force for transport across the membrane is a pressure differential. UF processes operate at 2-10 bars though in some cases upto 25-30 bars have been used. UF processes perform feed clarification, concentration of rejected solutes and fractionation of solutes.
UF can also retain colloids and organic macromolecules, but only RO can remove inorganic ions.A UF membrane can be defined essentially as a barrier, which separates two phases and restricts transport of various chemicals in a selective manner. A membrane can be homogenous or heterogeneous, symmetric or asymmetric in structure, solid or liquid, can carry a positive or negative charge or be neutral or bipolar. The membrane thickness may vary from as small as 100 micron to several mms.
Membrane Types
The following types of membranes are commonly used:
Microporous Membranes: the membrane behaves almost like a fibre filter and separates by a sieving mechanism determined by the pore diameter and particle size.
Homogenous Membranes: This is a dense film through which a mixture of molecules is transported by pressure, conc. Or electrical potential gradient.
Asymmetric Membranes: An asymmetric membrane comprises a very thin (0.1 – 1 micron) skin layer on a highly porous (100 – 200 micron) thick substructure.The thin skin acts as the selective membranes.
Electrically Charged Membranes: These are necessary ion-exchange membranes consisting of highly swollen gels carrying fixed positive or negative charges.
Liquid Membrane: A liquid membrane utilizes a carrier to selectively transport components such as metal ions at relatively high rate across the membrane
Ultrafiltration Spiral Membrane
UF removes high molecular weight organics, turbidity, suspended solids, color bodies, microorganisms and most viruses, so that almost any natural water supply or waste water resource can be used as a feedwater stock for an industrial process or irrigation.
UF can eliminate or minimize raw water pretreatment requirements. Compared to conventional filtration and backwash systems, UF units are compact and deliver consistent treated water quality from variable feeds.
Pathogens, organics, turbidity and color can be ultra filtered from surface water supplies to meet the needs of communities and municipalities for safe drinking water. UF can remove Giardia, Crytosporidium and viruses to meet or exceed the Surface Water Treatment Rules of the EPA.
Ultrafiltration For Purified Water Application
Ultrafiltration (UF) is a crossflow membrane process . A pressurized feed stream flow parallel to a porous membrane filtration surface. A pressure differential forces water through the membrane. The membrane rejects particulate, organic, microbes, pyrogens and other contaminants that are too large to pass through the membrane. UF does not reject ionic contaminants as does reverse osmosis. Filtration is typically from 0.5 – 0.001 micron.
Membrane is available in polymeric and ceramic materials. The ceramic filtration elements will not delaminate, swell or compact even under elevated temperature, high operating pressure, or reverse flow conditions, Ceramic membranes can operate continuously at 900C and be repeatedly steam sterilized, with no change in structure, but with some apparent loss in permeability. Ceramic membrane also remains unchanged in solutions over the full 0-14 pH range.
Application
Pyrogen removal is commonly done by UF. UF is usor the manufacture of pyrogen- free Water For Injection (WFI).
Pretreatment Requirements
Typically pretreatment includes multimedia and activated carbon filters followed by deionization.
Performance
UF Membrane is used to produce less than 0.25 EU /ml. Water. When fed with USP Purified Water a 4 log10 reduction in endotoxins can be expected. Typical system operating conditions for this UF membrane includes temperature from 250 to 900 C and 70 psi feed pressure.
Advantages And Disadvantages
UF can produce low-endotoxin water at significant savings over distillation and polymeric membrane process. Additionally, there is a cost savings over the cartridge filtration.
Friday, September 19, 2008
Electrodeionisation
Introduction
Electrodeionization removes ionized or ionizable species from water using electrically active media and an electrical potential to effect ion transport. Electrodeionization is distinguished from electro dialysis or oxidation/ reduction processes by the use electrically active media and is distinguished from other ion exchange processes by the use of an electrical potential.
The electrically active media in electrode ionization devices functions to alternately collect and discharge ionizable species and to facilitate the transport of ions continuously by ionic or electronic substitution mechanism.
Electro deionization devices may comprise media of permanent or temporary charge and may be operated batchwise, intermittently, or continuously. The devices can be operated so as to cause electro-chemical reactions specifically designed to achieve or enhance performance and may comprise electrically active membranes such as, semi-permeable ion exchange or bipolar membranes.
Semi-permeable ion exchange membranes, permanently charged media, and a power supply that can create a DC electrical field.
A continuous electrodeionization cell is formed by two adjacent ion exchange membranes or by a membrane and an adjacent electrode, CEDI units typically have alternating ion depleting (purifying) and ion concentrating cells that can be fed from the same water source, or different water sources.
Water is purified in CEDI devices through ion transfer. Ionized or ionizable species are drawn from the water passing through the ion depleting (purifying) cells into the concentrate water stream passing through the ion concentration cells.The water that is purified in CEDI units passes only through the electrically charged ion exchange media, and not through the ion exchange membranes. The ion exchange membranes are permeable to ionized or ionizable species, but not permeable to water.
The purifying cells typically have permanently charged ion exchange media between a pair of ion exchange membranes. Some units incorporate mixed (cationic and anionic) ion exchange media between a cationic membrane and an anionic membrane to form the purifying cell. Some units incorporate layers of cation and anion ion exchange media between ion exchange membranes to form the purifying cell.
The power supply creates a DC electric field between the cathode and anode of the CEDI device. Cations in the feedwater steam passing through the purifying cell are drawn to the cathode. Cations are transported through the cation exchange media and either pass through the cation permeable membrane or are rejected by the anion permeable membrane. Anions are drawn to the anode and are transferred through anion exchanged media and either pass through the anion permeable membrane or are rejected by the cation permeable membrane.
The ion exchange membranes are oriented in a manner which contains the cations and anions removed from the purifying cells in the concentrating cells so that the contaminants are removed from the CEDI unit. Some CEDI units utilize ion exchange media in the concentrating cells, while others do not.
Removal of Weakly-Ionized Species by EDI
Electrodeionization (EDI), a membrane process used in the production of ultrapure water.
The economic benefits of EDI which continuously remove weakly-ionized species. Normally, weakly-ionized species, such as carbon dioxide, boron, and ammonia are difficult to remove via such membrane processes as reverse osmosis (RO) and electrodialysis reversal (EDR). EDI offers the benefit of continuous removal of these species to a very high degree.
Electrodeionization removes ionized or ionizable species from water using electrically active media and an electrical potential to effect ion transport. Electrodeionization is distinguished from electro dialysis or oxidation/ reduction processes by the use electrically active media and is distinguished from other ion exchange processes by the use of an electrical potential.
The electrically active media in electrode ionization devices functions to alternately collect and discharge ionizable species and to facilitate the transport of ions continuously by ionic or electronic substitution mechanism.
Electro deionization devices may comprise media of permanent or temporary charge and may be operated batchwise, intermittently, or continuously. The devices can be operated so as to cause electro-chemical reactions specifically designed to achieve or enhance performance and may comprise electrically active membranes such as, semi-permeable ion exchange or bipolar membranes.
Semi-permeable ion exchange membranes, permanently charged media, and a power supply that can create a DC electrical field.
A continuous electrodeionization cell is formed by two adjacent ion exchange membranes or by a membrane and an adjacent electrode, CEDI units typically have alternating ion depleting (purifying) and ion concentrating cells that can be fed from the same water source, or different water sources.
Water is purified in CEDI devices through ion transfer. Ionized or ionizable species are drawn from the water passing through the ion depleting (purifying) cells into the concentrate water stream passing through the ion concentration cells.The water that is purified in CEDI units passes only through the electrically charged ion exchange media, and not through the ion exchange membranes. The ion exchange membranes are permeable to ionized or ionizable species, but not permeable to water.
The purifying cells typically have permanently charged ion exchange media between a pair of ion exchange membranes. Some units incorporate mixed (cationic and anionic) ion exchange media between a cationic membrane and an anionic membrane to form the purifying cell. Some units incorporate layers of cation and anion ion exchange media between ion exchange membranes to form the purifying cell.
The power supply creates a DC electric field between the cathode and anode of the CEDI device. Cations in the feedwater steam passing through the purifying cell are drawn to the cathode. Cations are transported through the cation exchange media and either pass through the cation permeable membrane or are rejected by the anion permeable membrane. Anions are drawn to the anode and are transferred through anion exchanged media and either pass through the anion permeable membrane or are rejected by the cation permeable membrane.
The ion exchange membranes are oriented in a manner which contains the cations and anions removed from the purifying cells in the concentrating cells so that the contaminants are removed from the CEDI unit. Some CEDI units utilize ion exchange media in the concentrating cells, while others do not.
Removal of Weakly-Ionized Species by EDI
Electrodeionization (EDI), a membrane process used in the production of ultrapure water.
The economic benefits of EDI which continuously remove weakly-ionized species. Normally, weakly-ionized species, such as carbon dioxide, boron, and ammonia are difficult to remove via such membrane processes as reverse osmosis (RO) and electrodialysis reversal (EDR). EDI offers the benefit of continuous removal of these species to a very high degree.
EDI Process
EDI is an electrically-driven membranes process. EDI combines ion-exchange resins, ion-exchange membranes, and a DC electrical field. In the diluting cell, the DC electrical field splits water at the surface of the ion-exchange beads, producing hydrogen and hydroxyl ions which act as continuous regenerates of the ion-exchange resins.
Different ions are removed as water travels through the EDI diluting cell, strongly-ionized ions being removed first in the flowpath and weakly ionized species removed as the water moves down the flowpath. Removal of ionized species such as sodium, chloride, sulfate, and calcium by EDI is usually well over 99%.
A number of operating EDI plants were closely reviewed for weak ion removal efficiency. EDI is an integral part of a multi-step process to produce ultrapure water, but the feedwater sources are quite different, subject to seasonal variations in quality, such as temperature, pH, DS, and organic and suspended solids. The pretreatment processes and EDI are designed to handle these variations.
ADVANTAGES:
High chemical purity
Economic with high salt feed systems
Economic from a usage point of view
Self-disinfection
No chemicals
DISADVANTAGES:
Must be used in tandem with RO
Reverse Osmosis (Ro)
Reverse Osmosis (RO) was the first crossflow membrane separation process to be widely commercialized. RO removes most organic compounds and up to 99% of all ions. A selection of RO membranes is available to address varying water conditions and requirements.
RO can meet most water standards with a single-pass system and the highest standards with a double-pass system. This process achieves rejections of 99.9+% of viruses, bacteria and pyrogens. Pressure in the range of 50 to 1000 psig is the driving force of the RO purification process. It is much more energy-efficient compared to phase change processes (distillation) and more efficient than the strong chemicals required for ion exchange regeneration.
In order to remove a high percentage of the total dissolved solids (TDS), which may be present in the feed water, a reverse osmosis system should be considered. A single pass reverse osmosis unit will reject up to 95% of dissolved solids, and 99% of pyrogens and bacteria. Reverse osmosis is a process that removes dissolved solids from the feed water by using pressure to pump the feed water through a semi permeable membrane which retains the dissolved impurities and allows only the water to pass through. This creates a purer quality water on the outlet side of the membrane. To utilize the reverse osmosis membrane to produce a purifies water stream, the natural process of osmosis is reversed by applying an external pressure on the feedwater that is higher than the osmotic pressure of the feedwater. The pressure will force water back through the membrane and the membrane will retain the dissolved solids, organic and bacteria.
The reverse osmosis membranes are constructed of complex polymer formulations. There are three different formulations: Cellulose Acetate (CA), Polyamide (PA) and a thin film composite (TFC) polymer formulation. For most applications in the pharmaceutical industry, the most popular membrane has usually been the “CA” type in the spiral wound configuration. This is because of its ability to be sanitized and depyrogenated with chlorine or other chemicals.
Membrane Types:
The initial commercial development and subsequent advancements in reverse osmosis can be directly linked to the availability of membrane materials in appropriate forms. Semi-permeable membranes, suitable for commercial water purification by RO, consist of thin films of polymeric materials which first became available around 1960. The aim, for desalination purposes, is to produce a membrane which is highly permeable to water whilst being highly impermeable to salts and the basic difficulty is that attempts to close the paths for salt generally tend also to cause increased resistance to water flow. The breakthrough in membrane technology came when methods were found to produce a cellulose acetate membrane, which combined the two required properties by having a very thin dense skin (the active `layer’) on top of a porous substrate. The thin skin (of this so-called `asymmetric membrane) had good salt rejecting properties and, being only around 1 mm thick, also had reasonable water flux - the porous substrate providing mechanical support for the thin layer without hindering the passage of water.
The asymmetric cellulose acetate (CA) membranes typically exhibit salt rejections between 92% and 97% and water fluxes in the range 0.4 - 0.8 m3/m2 per day. More recent membrane development has led to improved fluxes and separation characteristics and has also been aimed at utilizing materials which can be used over a wider pH range and have lower tendencies towards compaction of the porous layer (which causes reduced flux) than the CA membranes. Of the range of materials, which have been studied and developed, the most successful alternatives to cellulose acetate have been polyamides. However, it is important to recognize that polyamide membranes are vulnerable to irreversible damage when exposed, for even short periods, to water containing chlorine. Thus feedwater must be effectively dechlorinated prior to supply to a RO module utilizing polyamide membranes.
Many of the recently developed reverse-osmosis membranes have been produced as thin-film `composite membranes’. The essential difference between such a membrane and a conventional sheet (CA) membrane is that, whereas the latter contains an active and support layer made in one series of operations, in composite membranes these two layers are made in separate stages and often consist of different polymers. Hence it is much easier to secure improvements in the performance of each layer without the risk of reducing the performance of the other layer. One particularly useful feature of composite-membrane construction is the ability to produce much thinner active payers than is possible with the asymmetric form and the overall effect is the availability now a days of composite membranes with better salt rejection without any rejection in the water flux compared to that obtained with cellulose acetate. Different versions of such thin-film composite membranes are manufactured for desalination of seawater or for treatment of brackish water of say a few hundred to a few thousand mg/1 TDS. For example, such brackish water RO membranes operating at perhaps 8 bar pressure achieve rejections of 98%-99% for most salts.
Fouling Of Ro Membranes
A RO Plant can be considered almost to be an ultimate filter for impurities in a water. Whilst this feature is, on the one hand, an attribute to the process, on the other hand it represents a weakness. This is because a RO membrane is prone to becoming fouled with a number of feed constituents. Despite the sweeping action of the feed flowing through and along the membrane, a feed containing a high burden of suspended matter, biological organisms or dissolved salts near to their solubility limits will result in the settlement of these constituents on the membrane surface. This fouling layer drastically degrades the performance of the membrane in ways that are `signaled’ by a progressive increase in the trans-membrane pressure, increase in product TDS and decrease in product flux to an extent that frequent intermittent cleaning operations will at least bee necessary and, in extreme cases, complete membrane replacement will be required.
Salt Concentration Effects During RO Operation:
Two consequences of the action of a RO membrane in allowing preferential passage through it of H2O molecules from a saline solution, are :
As water flows along a RO membrane, it is progressively increasing salt content in the bulk stream.
Additionally, there is a tendency for the rejected salt to accumulate in the concentrate boundary layer adjacent to the membrane surface. This phenomenon is known as concentration polarization.
Both these phenomena cause a reduction in the driving pressure, (Dr - Dp), equation (1), and an increase in the salt flux, see equation (2), i.e. they contribute to a lowering of the average performance of the unit compared with the performance calculated on the basis of the feed composition. An important feature of a detailed RO plant design is to minimize the extent of concentration polarization.
Membrane Configuration
Spiral Wound Systems : This type utilizes membranes in a convenient flat form. The membrane is cast onto a fabric support and then two of these fabric supported membranes are glued together with a porous material between them. This porous material supplies the subsequent route for the product water after its passage through the membrane. The resulting `sandwich’ is then glued together on three of its edges with its fourth edge being glued to a central tube which, containing appropriately spaced holes in its wall, acts as a collector for the product water. A flexible spacing mesh is then laid on top of the sealed membrane sandwich and the whole lot rolled up around the product collection pipe to form a spiral wound membrane unit. The unit is then mounted in a cylindrical pressure vessel usually constructed from glass-fibre reinforced polymer. The feed flows axially along the spacing mesh and the permeate via the porous backing material to the central collector. All kinds of membranes have been used in the spiral wound configuration - cellulose acetate, polyamide and other composite types.
Module arrangements : In either spiral wound or hollow-fine fibre form, high production rats are obtained by supplying the pressurized feed to an appropriate number of membrane modules arranged in parallel. Brackish water membranes can often achieve a suitable degree of desalting in one pass of feed through a module. However, if, as is often the case, a high plant recovery is important, then `brine staging’ is used. This involves using the reject brine stream from the first set of parallel modules as feed to a second set of parallel modules and so on.
Energy Recovery in Reverse-Osmosis Units : The main component of the energy requirement for a RO plant is in the high pressure pump for the feeedwater and an example of a way of saving energy is to utilize the power available in the concentrate flowstream to drive a turbine which, in turn, drives the main high pressure pump through a common shaft. Savings of around 30% on pumping costs have been claimed for sea water-RO units but for RO plant operating on relatively low TDS feed, the use of energy recovery is less attractive on account of much longer pay-back time on the capital cost of the turbine etc.
Operating Principle
If a solution of a non-vioatile solute (such as sodium chloride) is separated either from a sample of pure solvent (such as water) or from a more dilute solution by a membrane which is permeable to the solvent but impermeable to the solute, there will be tendency for the spontaneous passage of solvent through the membrane into the more concentrated solution under a driving force known as the osmotic pressure. This transport of water molecules, diluting an aqueous solution, will continue until the excess pressure thus produced by this inflow on the concentrated solution side has built up to a magnitude equal to the osmotic pressure at which the flow ceases due to the establishment of thermo dynamic equilibrium across the membrane.
Desalination can be accomplished by reversing the naturally-occurring process. Thus, if a pressure greater than the osmotic pressure is applied to the concentration solution side, then pure solvent will pass through the membrane producing a purer water product. This is the basic of reverse osmosis (RO) and a simplified representation of a RO system is shown.
The value of osmotic pressure of any solution, is directly proportional to the concentration of the solute in the solvent, that is, to the TDS of an aqueous solution. Thus, the minimum required pressure (and hence the energy cost) for reverse osmosis increase with the salinity of the water being treated. As examples, the osmotic pressures of solutions of 100 mg/l, 0.8% and 3.2% sodium chloride are about 0.08, 6.8 and 27 bar, respectively. Whilst the osmotic pressure represents the minimum required pressure to carry out reverse osmosis, commercial units require optional pressures significantly in excess of this because the water flux obtained is directly proportional to the magnitude by which the operational pressure exceeds the osmotic pressure :
water flux (in, say, m3/m2.day) = k1 ( Dp - Dp),
where Dp and Dp are the operation pressure and the osmotic pressure differentials, respectively, across the membrane and k1 is the pure water permeability of the particular membrane utilized.
Another important requirement of RO unit is the attainment of a high salt rejection by minimizing the salt flux across the membrane.
Salt flux = k2 DC,
where k2 is the salt permeability constant and DC is the difference in salt concentration in the feed and the product waters. The salt rejection is basically defined as follows :
salt rejection = ( Cf - Cd) Cf,
where Cf is the salt concentration in the feed and Cd is the salt concentration in the product. In the above relationship the salt concentrations are usually expressed in terms of the total salt content, TDS, but it should be noted that RO membranes possess somewhat varying abilities to reject different ionic species. In general, superior rejections are obtained for univalent ions.
Notice, incidentally, that an indirect consequence of achieving a higher H2O flux is to increase, the salt rejection. Hence increasing the operational pressure yields two benefits, namely increased productivity and a purer product.
Another important operational factor is the ` recovery ‘ which is defined as :
recovery = (product flow) / (feed flow)
and is obviously a measure of the proportion of product water extracted from the feed. It is clearly desirable to maximize the recovery, which is often around 70%-80%. The upper limits to achievable recovery are dictated by the TDS of the feed (increases in which tend to reduce obtainable recovery) and by the detailed ionic composition of the feed (which, for example, can limit recovery on account of a higher susceptibility to fouling of the RO membrane by precipitation of low-solubility salts such as calcium carbonate).
Another relevant operational parameter is the temperature. Increases in temperature tend to reduce salt rejection but more importantly, facilitate increased flux by reducing the water viscosity and hence promoting enhanced fluid flow through thee membrane.
RO can meet most water standards with a single-pass system and the highest standards with a double-pass system. This process achieves rejections of 99.9+% of viruses, bacteria and pyrogens. Pressure in the range of 50 to 1000 psig is the driving force of the RO purification process. It is much more energy-efficient compared to phase change processes (distillation) and more efficient than the strong chemicals required for ion exchange regeneration.
In order to remove a high percentage of the total dissolved solids (TDS), which may be present in the feed water, a reverse osmosis system should be considered. A single pass reverse osmosis unit will reject up to 95% of dissolved solids, and 99% of pyrogens and bacteria. Reverse osmosis is a process that removes dissolved solids from the feed water by using pressure to pump the feed water through a semi permeable membrane which retains the dissolved impurities and allows only the water to pass through. This creates a purer quality water on the outlet side of the membrane. To utilize the reverse osmosis membrane to produce a purifies water stream, the natural process of osmosis is reversed by applying an external pressure on the feedwater that is higher than the osmotic pressure of the feedwater. The pressure will force water back through the membrane and the membrane will retain the dissolved solids, organic and bacteria.
The reverse osmosis membranes are constructed of complex polymer formulations. There are three different formulations: Cellulose Acetate (CA), Polyamide (PA) and a thin film composite (TFC) polymer formulation. For most applications in the pharmaceutical industry, the most popular membrane has usually been the “CA” type in the spiral wound configuration. This is because of its ability to be sanitized and depyrogenated with chlorine or other chemicals.
Membrane Types:
The initial commercial development and subsequent advancements in reverse osmosis can be directly linked to the availability of membrane materials in appropriate forms. Semi-permeable membranes, suitable for commercial water purification by RO, consist of thin films of polymeric materials which first became available around 1960. The aim, for desalination purposes, is to produce a membrane which is highly permeable to water whilst being highly impermeable to salts and the basic difficulty is that attempts to close the paths for salt generally tend also to cause increased resistance to water flow. The breakthrough in membrane technology came when methods were found to produce a cellulose acetate membrane, which combined the two required properties by having a very thin dense skin (the active `layer’) on top of a porous substrate. The thin skin (of this so-called `asymmetric membrane) had good salt rejecting properties and, being only around 1 mm thick, also had reasonable water flux - the porous substrate providing mechanical support for the thin layer without hindering the passage of water.
The asymmetric cellulose acetate (CA) membranes typically exhibit salt rejections between 92% and 97% and water fluxes in the range 0.4 - 0.8 m3/m2 per day. More recent membrane development has led to improved fluxes and separation characteristics and has also been aimed at utilizing materials which can be used over a wider pH range and have lower tendencies towards compaction of the porous layer (which causes reduced flux) than the CA membranes. Of the range of materials, which have been studied and developed, the most successful alternatives to cellulose acetate have been polyamides. However, it is important to recognize that polyamide membranes are vulnerable to irreversible damage when exposed, for even short periods, to water containing chlorine. Thus feedwater must be effectively dechlorinated prior to supply to a RO module utilizing polyamide membranes.
Many of the recently developed reverse-osmosis membranes have been produced as thin-film `composite membranes’. The essential difference between such a membrane and a conventional sheet (CA) membrane is that, whereas the latter contains an active and support layer made in one series of operations, in composite membranes these two layers are made in separate stages and often consist of different polymers. Hence it is much easier to secure improvements in the performance of each layer without the risk of reducing the performance of the other layer. One particularly useful feature of composite-membrane construction is the ability to produce much thinner active payers than is possible with the asymmetric form and the overall effect is the availability now a days of composite membranes with better salt rejection without any rejection in the water flux compared to that obtained with cellulose acetate. Different versions of such thin-film composite membranes are manufactured for desalination of seawater or for treatment of brackish water of say a few hundred to a few thousand mg/1 TDS. For example, such brackish water RO membranes operating at perhaps 8 bar pressure achieve rejections of 98%-99% for most salts.
Fouling Of Ro Membranes
A RO Plant can be considered almost to be an ultimate filter for impurities in a water. Whilst this feature is, on the one hand, an attribute to the process, on the other hand it represents a weakness. This is because a RO membrane is prone to becoming fouled with a number of feed constituents. Despite the sweeping action of the feed flowing through and along the membrane, a feed containing a high burden of suspended matter, biological organisms or dissolved salts near to their solubility limits will result in the settlement of these constituents on the membrane surface. This fouling layer drastically degrades the performance of the membrane in ways that are `signaled’ by a progressive increase in the trans-membrane pressure, increase in product TDS and decrease in product flux to an extent that frequent intermittent cleaning operations will at least bee necessary and, in extreme cases, complete membrane replacement will be required.
Salt Concentration Effects During RO Operation:
Two consequences of the action of a RO membrane in allowing preferential passage through it of H2O molecules from a saline solution, are :
As water flows along a RO membrane, it is progressively increasing salt content in the bulk stream.
Additionally, there is a tendency for the rejected salt to accumulate in the concentrate boundary layer adjacent to the membrane surface. This phenomenon is known as concentration polarization.
Both these phenomena cause a reduction in the driving pressure, (Dr - Dp), equation (1), and an increase in the salt flux, see equation (2), i.e. they contribute to a lowering of the average performance of the unit compared with the performance calculated on the basis of the feed composition. An important feature of a detailed RO plant design is to minimize the extent of concentration polarization.
Membrane Configuration
Spiral Wound Systems : This type utilizes membranes in a convenient flat form. The membrane is cast onto a fabric support and then two of these fabric supported membranes are glued together with a porous material between them. This porous material supplies the subsequent route for the product water after its passage through the membrane. The resulting `sandwich’ is then glued together on three of its edges with its fourth edge being glued to a central tube which, containing appropriately spaced holes in its wall, acts as a collector for the product water. A flexible spacing mesh is then laid on top of the sealed membrane sandwich and the whole lot rolled up around the product collection pipe to form a spiral wound membrane unit. The unit is then mounted in a cylindrical pressure vessel usually constructed from glass-fibre reinforced polymer. The feed flows axially along the spacing mesh and the permeate via the porous backing material to the central collector. All kinds of membranes have been used in the spiral wound configuration - cellulose acetate, polyamide and other composite types.
Module arrangements : In either spiral wound or hollow-fine fibre form, high production rats are obtained by supplying the pressurized feed to an appropriate number of membrane modules arranged in parallel. Brackish water membranes can often achieve a suitable degree of desalting in one pass of feed through a module. However, if, as is often the case, a high plant recovery is important, then `brine staging’ is used. This involves using the reject brine stream from the first set of parallel modules as feed to a second set of parallel modules and so on.
Energy Recovery in Reverse-Osmosis Units : The main component of the energy requirement for a RO plant is in the high pressure pump for the feeedwater and an example of a way of saving energy is to utilize the power available in the concentrate flowstream to drive a turbine which, in turn, drives the main high pressure pump through a common shaft. Savings of around 30% on pumping costs have been claimed for sea water-RO units but for RO plant operating on relatively low TDS feed, the use of energy recovery is less attractive on account of much longer pay-back time on the capital cost of the turbine etc.
Operating Principle
If a solution of a non-vioatile solute (such as sodium chloride) is separated either from a sample of pure solvent (such as water) or from a more dilute solution by a membrane which is permeable to the solvent but impermeable to the solute, there will be tendency for the spontaneous passage of solvent through the membrane into the more concentrated solution under a driving force known as the osmotic pressure. This transport of water molecules, diluting an aqueous solution, will continue until the excess pressure thus produced by this inflow on the concentrated solution side has built up to a magnitude equal to the osmotic pressure at which the flow ceases due to the establishment of thermo dynamic equilibrium across the membrane.
Desalination can be accomplished by reversing the naturally-occurring process. Thus, if a pressure greater than the osmotic pressure is applied to the concentration solution side, then pure solvent will pass through the membrane producing a purer water product. This is the basic of reverse osmosis (RO) and a simplified representation of a RO system is shown.
The value of osmotic pressure of any solution, is directly proportional to the concentration of the solute in the solvent, that is, to the TDS of an aqueous solution. Thus, the minimum required pressure (and hence the energy cost) for reverse osmosis increase with the salinity of the water being treated. As examples, the osmotic pressures of solutions of 100 mg/l, 0.8% and 3.2% sodium chloride are about 0.08, 6.8 and 27 bar, respectively. Whilst the osmotic pressure represents the minimum required pressure to carry out reverse osmosis, commercial units require optional pressures significantly in excess of this because the water flux obtained is directly proportional to the magnitude by which the operational pressure exceeds the osmotic pressure :
water flux (in, say, m3/m2.day) = k1 ( Dp - Dp),
where Dp and Dp are the operation pressure and the osmotic pressure differentials, respectively, across the membrane and k1 is the pure water permeability of the particular membrane utilized.
Another important requirement of RO unit is the attainment of a high salt rejection by minimizing the salt flux across the membrane.
Salt flux = k2 DC,
where k2 is the salt permeability constant and DC is the difference in salt concentration in the feed and the product waters. The salt rejection is basically defined as follows :
salt rejection = ( Cf - Cd) Cf,
where Cf is the salt concentration in the feed and Cd is the salt concentration in the product. In the above relationship the salt concentrations are usually expressed in terms of the total salt content, TDS, but it should be noted that RO membranes possess somewhat varying abilities to reject different ionic species. In general, superior rejections are obtained for univalent ions.
Notice, incidentally, that an indirect consequence of achieving a higher H2O flux is to increase, the salt rejection. Hence increasing the operational pressure yields two benefits, namely increased productivity and a purer product.
Another important operational factor is the ` recovery ‘ which is defined as :
recovery = (product flow) / (feed flow)
and is obviously a measure of the proportion of product water extracted from the feed. It is clearly desirable to maximize the recovery, which is often around 70%-80%. The upper limits to achievable recovery are dictated by the TDS of the feed (increases in which tend to reduce obtainable recovery) and by the detailed ionic composition of the feed (which, for example, can limit recovery on account of a higher susceptibility to fouling of the RO membrane by precipitation of low-solubility salts such as calcium carbonate).
Another relevant operational parameter is the temperature. Increases in temperature tend to reduce salt rejection but more importantly, facilitate increased flux by reducing the water viscosity and hence promoting enhanced fluid flow through thee membrane.
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