Water Treatment Membrane Processes - Introduction -

Water Treatment Membrane Processes - Introduction -
CEEN 572 Environmental Pilot Lab Water Treatment Membrane Processes - Introduction -

Membrane Separation Membrane Feed Permeate Particle or Solute Molecule
Membrane processes are used in water treatment for solid/liquid separation. RO and NF are pressure-driven membrane processes. In this figure, the orange circles represent the particles or solute molecules and the blue circles represents the solvent (water). The feed solution is forced under pressure through the membrane (animate) and the result is a relatively clean permeate solution. Separation occurs because the membrane transports one component (the water) more readily than any other components. Particle or Solute Molecule Solvent

Definition of Membrane Process
In a membrane separation process, a feed consisting of a mixture of two or more components is partially separated by means of a semipermeable barrier through which one or more species move faster than the other species In water and wastewater treatment applications, membrane processes are used as a solid/liquid separation process. In this case, water is more readily transported through the membrane than solids (both suspended and dissolved)

Classification of Membrane Operations
Driving force Mechanism of separation Membrane structure Phases in contact

Classification of Membrane Operations
Pressure-driven membrane operations Permeation operations Dialysis operations

Pressure-driven Operations
Microfiltration (MF) Ultrafiltration (UF) Nanofiltration (NF) Reverse Osmosis (RO)

Pressure-driven Membrane Processes

The solvent is transferred through a dense membrane tailored to retain salts and low- molecular-weight solutes To produce “pure” water from saline solution, feed pressure must exceed the osmotic pressure of the feed solution In order to obtain economically viable flows, at least twice the osmotic pressure must be exerted as hydraulic pressure (e.g., bars (700-1,100 psi) for seawater) organic molecules MW nm inorganic ions MW nm water MW nm

NF Sometimes referred to as low-pressure RO or membrane softening process Lies between RO and UF in terms of selectivity of the membrane Designed to remove multivalent ions but can remove sodium and chloride fairly well Looser NF membranes are more like UF and tighter NF membranes more closely resemble RO Recently has been employed for organic control Typical operating pressure: 5-14 bar ( psi) organic molecules MW nm inorganic ions MW nm water MW nm

UF Considered as a clarification and disinfection operation Membrane is porous and rejects most macromolecules, microorganisms, and all types of particles Osmotic pressure effects are negligible Typical operating pressure: bar (7-70 psi) MF Major difference between MF and UF is pore size – micron for MF Primary application is particulate removal (clarification) Typical pressures like UF organic molecules MW nm inorganic ions MW nm water MW nm

Selection of Membrane Processes
‘loose’ and ‘tight’

Membranes in Treatment of Drinking Water
The application of specific pressure-driven membrane process is highly dependent on the characteristics and quality of the source water Surface water: MF, UF, NF Groundwater (fresh): MF, UF Groundwater (brackish): MF/UF pretreatment, NF, RO Seawater: MF/UF pretreatment, NF, RO

Membranes in Treatment of Wastewater
The application of specific pressure-driven membrane process in wastewater treatment is highly dependent on the characteristics/quality of the source water and the pretreatment process/es used Raw wastewater: MF/UF, MBR, FO (not mainstream…yet) Effluent: MF/UF pretreatment, NF, RO

Membrane Technologies and their Traditional Counterparts
Membrane Separation Technology Constituents Removed Comparable traditional Water Treatment Method MF Bacteria and large colloids; precipitates and coagulates Ozonation-UV, chlorination, sand filtration, bioreactors, coagulation-sedimentation UF All of the above + viruses, high MW proteins, organics Sand filter, bioreactor, activated carbon NF All of the above + divalent ions, large monovalent ions, color, odor Lime-soda softening, ion exchange RO All of the above + monovalent ions Distillation, evaporation, ion exchange ED/EDR Dissolved ionic salts Ion exchange

Target Solutes MF: Microbes (protozoa and bacteria)
Turbidity (particles and colloids) UF: Same as MF + viruses, “some” NOM NF: Same as UF + NOM, SOCs (e.g., Atrazine), Divalent cations (Ca2+, Mg2+, Zn2+, Cd2+, etc.), Polyvalent anions (SO42-, PO43-, AsO43-, CrO42-, etc.) RO: Same as NF + simple ions (TDS, NO3-, ClO4-) MF + Coagulant: viruses, NOM (also fouling reduction) UF + PAC: SOCs, NOM (also fouling reduction) Submerged MF and UF: Fe and Mn (aeration), NOM (with coagulant), SOCs (with PAC)

Ranges of Pressure and Flux
Membrane Class Pore Size or MWCO (μm or Dalton) Pressure Flux psi kPa gfd (gal/ft2·day) LMH (L/m2·hr) MF 0.1 – 0.5 mm 10-100 60 – 120 100 – 200 UF 1 – 100 kD 50-500 30 – 60 50 – 100 NF 100 – 500 D 700-2,800 15 – 30 20 – 50 RO n/a ,000 1400-7,000 MF (Immersed) 0.2 mm -1.4 -10 UF (Immersed) 0.04 mm -7.0 -50 35 – 85 conversions 1 atm = kPa (kN/m2) = 14.7 psi 1 kPa = psi or 1 psi = 6.90 kPa 1 psi = atm gfd = LMH x 1.7

Ranges of Energy Consumption
Membrane Class Recovery Pressure Energy consumption kWh per psi kPa 1,000 gal m3 MF 94-98 15 100 0.1 0.4 UF 70-80 75 525 0.8 3.0 NF 80-85 125 875 1.4 5.3 LPRO 70-85 225 1,575 2.7 10.2 RO 400 2,800 4.8 18.2 ED 75-85 2.5 9.5

Permeation Operations
Gas Permeation (GP) Gas Diffusion Pervaporation (PV) Membrane Stripping (MS) Membrane Distillation (MD) Engineered Osmosis (EO)

Gas Separation Industrial Applications of GP:
Separation of H2 from CH4, (H2 permeation rate through dense membrane is very high) Adjustment of H2-to-CO ratio in synthesis of gas O2 enrichment of air Removal of CO2 Drying of natural gas and air Removal of He and organic solvents from air

Gas Separation AIR N2 VENT

Pervaporation (PV) Liquid/vapor separation – liquid partially vaporized through a dense membrane Solvent dissolves in the polymeric membrane, diffuses, and evaporates on the permeate side Rate of transfer of a constituent depends on its solubility in the membrane Activity difference maintained by partial vacuum on the permeate side Separation of solvents PV is used to separate dissolved volatile organic compounds (VOCs) from aqueous solutions

Pervaporation (PV) PV membrane separation systems are used in food processing, gas separation, and water treatment. Examples of applications include: Recovery of flavor compounds from food industry process streams Recovery of ethanol from fermentation and food industry process streams Removal of organic contaminants from waste streams C2H4/C3H8

Membrane Stripping and Membrane Distillation (MD)
Separation of volatile constituents from solution: Removal of volatile solutes or volatile solvent Microporous hydrophobic membrane Polypropylene, PTFE, PVDF, nylon Heated aqueous feed solution is brought into contact with feed side of the membrane The hydrophobic nature of membrane prevents penetration of aqueous solution into the pores Lower vapor pressure on the permeate side of the membrane induces evaporation through the pores Pores remain dry throughout the process !

Membrane Stripping and Membrane Distillation – Basic Configurations
Feed Solution Clean Water Feed Solution Sweep Gas Air Gap Feed Solution Direct Contact MD (DCMD) Sweeping Gas MD Air Gap MD Feed Solution Vacuum = Membrane Stripping Vacuum MD (VMD)

Brine Reconcentration
Engineered Osmosis Forward osmosis (applications: wastewater treatment, pretreatment, desalination, concentration) Brine Reconcentration Draw Solution Feed DP=Dp Brine Feed Osmosis Forward Osmosis (FO) “engineered osmosis”

UFO-MBR System Wastewater Feed Stream Diluted DS DS Tank Anoxic Tank
RO Permeate Tank Concentrated DS UF Permeate and Backwash Tank

Dialysis (DIA) Donnan Dialysis Electrodialysis (ED)
Dialysis Operations Dialysis (DIA) Donnan Dialysis Electrodialysis (ED)

Dialysis Operations The solute is transferred through the membrane, not the solvent The driving force is activity or an electrical potential difference Dialysis (DIA) The driving force is a transmembrane concentration difference Selective passage of ions and low-molecular-weight solutes and rejection of larger colloidal and high-molecular-weight solutes Main application is hemodialysis

Electrodialysis (ED) Operation by which ions diffuse through ion- exchange membrane under the influence of electric potential Ion exchange membranes AEM: quaternary amines CEM: carboxylates, sulfonates Plate and frame stacks Efficient for desalination of brackish water

Electrodialysis Reversal (EDR)
EDR is a variant of ED The same membranes are used to provide a continuous self-cleaning electrodialysis process which uses periodic reversal of the DC polarity to allow systems to run at higher recovery rates Polarity reversal causes the concentrating and diluting flow streams to switch after every cycle Any fouling or scaling constituents are removed when the process reverses, sending fresh product water through compartments previously filled with concentrated waste streams EDR systems operate with higher concentrations in the brine or concentrate streams with less flow to waste

Electrodialysis Reversal (EDR)
Applications for EDR technology include municipal drinking water, industrial process water, and wastewater reuse projects Municipalities find EDR successful for the removal of radium, arsenic, and perchlorate as well as desalination of well and surface waters

Other Classifications

Separating Mechanisms
Separation based on difference in size (sieving) MF, UF, DIA Separation based on difference in solubility and diffusivity of material in the membrane (solution- diffusion mechanism) GP, PV, RO, FO Separation based on difference in charges of the species to be separated (electrochemical effects) ED, EDR

Rejection Capabilities (pressure-driven processes)
RO membranes are typically characterized by manufacturers in terms of NaCl rejection, e.g., 96% or 99.9% NaCl rejection NF membranes may be characterized in terms of NaCl or MgSO4 rejection or they may be characterized in terms of molecular weight cut-off (MWCO)*, e.g., 98% MgSO4 and 80% NaCl rejection UF membranes are typically characterized using MWCO, e.g., 13,000 or 80,000 MWCO MF membranes are typically characterized by pore size, e.g., 0.1 or 1 µm * MWCO is determined by fitting rejection data of acromolecules (e.g., dextrans or proteins) * MWCO determined by fitting rejection data of non-polar organics

Porosity Porous membranes (MF, UF, NF, DIA) Nanoporous membranes
Macroporous: > 50 nm Mesoporous: 2 – 50 nm Microporous: < 2 nm Nanoporous membranes Dense media Diffusion of species takes place in the free volume present between the macromolecules chains of the membrane material IX membranes Specific type of nanoporous membranes

Morphology Symmetric Asymmetric – Single Material
Cylindrical Porous Porous Homogeneous (resistance to mass transfer is determined by total membrane thickness) Asymmetric – Single Material top layer Porous Porous with Top Layer Asymmetric – Composite dense skin layer (0.1 to 0.5 µm) porous membrane (50 to 150 µm) (resistance to mass transfer determined by skin layer thickness)

Structure of Membranes

Geometry / Packaging Flat-sheet membranes (spiral wound, plate-and- frame) Tubular membranes (shell-and-tube, immersed) Tubes Capillaries Hollow fibers

Geometry / Packaging

Spiral Wound Membrane Manufacturing (video + audio)

Spiral Wound Module Single Element vs. Bank or Array
Roga Module 1 ca. 1964

Spiral Wound Module Installation

Hollow Fiber Membrane Single Fiber (left) vs. Module (right)

Hollow Fiber Membrane Module (left) vs. bank (right)

Submerged Membrane – MF/UF
Uses Surface water treatment Pretreatment for RO Membrane bio reactors (MBR) Filtration for non-potable reuse (add MF after secondary WW treatment and produce water for irrigation) Operation Membranes are immersed in basin of feed water Operate under suction Advantages Operate at lower pressures than pressurized systems Less fouling potential - good for wastewater treatment Membrane cleaning and fixing

Submerged Membranes Filtrate Feed Water To RO Air Bubble Scouring of
Basin Filtrate Feed Water To RO Air Bubble Scouring of Membrane Surface MF/UF Hollow Fiber Membranes

System Configuration Mix Fill and React (1 hr) BR1 MT BR2
WELCOME TO MINES PARK BR2 React Draw (1 hr) SEPTIC TANK Permeate Tank

Immersed Membrane (GE / Zenon) Cassette vs. System

Immersed Membrane Backwash (USFilter)

Submerged MBR - Zenon

Submerged Membrane Designs

Flow Configuration Cross Flow vs. Dead End Filtration
Cross Flow Operation Feed Concentrate Permeate Dead End Operation Feed concentrate/retentate/reject/brine permeate/product Permeate

Cross Flow Operation Feed flow is parallel to membrane surface
Concentrate vt vd Permeate Feed flow is parallel to membrane surface Retained particles are scoured Have concentrate stream Preferable for concentrated solutions to control thickness of deposit on membrane (fouling)

Dead End Operation Feed flow is perpendicular to membrane surface
Permeate Feed flow is perpendicular to membrane surface Retained particles form a cake layer on surface No concentrate stream Preferable for dilute solutions due to lower energy requirements (pumping)

Comparison of Cross-flow Membrane Configurations
Cost Packing Density Operating Pressure Capacity Membrane Types Fouling Resistance Cleanability Traditional Spiral-Wound Low High Many Fair Hollow Fiber UF-High RO-Very High UF-Low RO-High Few UF-Good RO-Poor Tubular UF-Moderate Very Good Plate & Frame Moderate Adapted from "Select Engineering Principles of Crossflow Membrane Technology" Osmonics Inc. Technical Paper, P/N 56821

Membrane Materials Polymeric membranes: Polysulfone Polyethersulfone
Polyphenylsulfone Polyvinylidene Fluoride (PVDF) Polypropylene (PP) Polyethylene (PE) Cellulose and Cellulose acetates (CA) Polyamide (PA) Polyacrylonitrile (PAN) Polytetrafluoroethylene (PTFE) Polycarbonate (PC) Polymethylmethacrylate (PMMA) Ceramic membranes: Aalumina Titania Zirconia ATZ mix chemical, mechanical and thermal stability ability of steam sterilization and back flushing high abrasion resistance high fluxes durable bacteria resistance possibility of regeneration dry storage after cleaning

Membrane Properties Pure water permeability (PWP) Pore size
Molecular Weight Cut-Off (MWCO) Hydrophobicity/hydrophilicity Surface/pore charge Surface roughness Chemical stability / chlorine tolerance

Principles of Mass Transport and Rejection in Pressure-Driven Membrane Processes
particles macromolecules ions microbes

Overview RO, NF, UF, and MF have many similarities (geometry, flow configuration, material…) Principals of rejection differ substantially In RO, function of relative affinity of solute and solvent to the membrane In MF, mainly due to physical sieving

Membrane Performance The performance of a membrane is determined by mainly two parameters, flux and rejection: Flux (J), or permeation rate, is the volume flowing through the membrane per unit area per time (Q/A) Rejection (R), refers to a local relationship between upstream and downstream concentrations Another important parameter is recovery (r), which is defined as the amount of material collected as a useful product divided by the total amount of the material entering the process: in membrane separations, the useful product is most often the permeate water

Water and Solute Flux Water flux (Jw), or permeation rate, is the volume flowing through the membrane per unit area per time (Q/A) In membrane processes it is a function of driving force, membrane properties, and feed quality Specific permeate flux is the water flux calculated above normalized to the applied driving force Note: in MF/UF, DP = net applied pressure (NAP)

Water and Solute Flux Solute Flux is the mass of solute flowing through the membrane per unit of area per time In membrane processes it is a function of driving force (concentration), membrane properties, and solute/particle properties

Water Recovery Rate (r)
The ratio of the useful product (permeate) flow rate and the flow rate of feed to the process Global recovery rate: Where Qp is the product (permeate) flow rate and Qf is the feed flow rate In membrane processes, because of the modularity and various configurations, it is important to distinguish between membrane/module recovery and system recovery

Material Balance in Membrane Separation
A, B Concentrate Qc, Cc, Pc Feed Qf, Cf, Pf Permeate Qp, Cp, Pp

Mass balance for water flow Qf = Qc + Qp Mass balance for solute flux QfCf = QcCc + QpCp Product recovery r = (Qp/Qf)·100%

Example of Process Recovery
Assuming each membrane (or each stage) operated at 20% recovery, what is the total system recovery R1 gal R2 gal 100 gal R3 gal P1 gal P2 gal P3 gal

Example of Process Recovery
Repeat the exercise with 50% recovery per stage, what is the production rate of the third stage? R1 gal R2 gal 100 gal R3 gal P1 gal P2 gal P3 gal

Staging In RO and NF operations, membranes are often staged
tapered design compensates for loss of feed volume through system

Recirculation In MF and UF, some concentrate is often recirculated to the inlet Flexible (can control degree of recirculation) Economics (tradeoff between power for recirculation pump and additional recovery)

Mass balance for water flow Qf = Qc + Qp Mass balance for solute flux QfCf = QcCc + QpCp Product recovery r = (Qp/Qf)·100% Global Rejection

Rejection (R) Location-specific ratio of product concentration and feed concentration Global rejection… where cp is the solute concentration in the permeate and cf is the solute concentration in the feed Global system rejection… May yield different value as function of time Variability of feed, permeate {Cp=f(R)}, membrane condition…

Rejection (R) Local rejection due to change in bulk feed concentration in the flow channel cwall ≥ cbulk ≥ cfeed If we know the permeate flux and mass transfer coefficient (we will talk about it later), the concentration at the membrane surface can be predicted by calculating a polarization factor (PF)… cwall = PF · cbulk Apparent rejection calculated based on bulk concentration:

Temperature Effects Higher transmembrane pressure in the winter, or
More membranes in the winter to prevent fouling/cleaning Water demand difference between summer and winter may offset loss in membrane productivity Membrane fouling

Temperature Effects Change in temperature may result in a wide range of effects that go beyond the viscosity of the permeate alone Different ways to model effects of temperature: Arrhenius equation: JT = J20 exp (s/T) J20 = permeate flux at reference temperature of 20 °C s = empirical constant, membrane specific T = temperature For MF and UF: Flux20 = FluxT (μT/μ20); or Temperature Corrected Flux (TCF)

Temperature Effects Primary effect due to influence on viscosity
For temperatures in the range of 0-35 °C μ (centipoise) = – T x10-4 T2 centipoises = Pa-sec x 1,000 μ (Pa-sec) = 3.797x10-11 T4 – 9.963*10-9 T x10-6 T2 – 5.589x10-5 T x10-3

Membrane Fouling Deposition Scaling Biofouling Organic fouling
Silt and suspended solids Scaling Inorganic deposits formed due to concentration of sparingly soluble salts beyond the chemical solubility limit Biofouling Microbiological growth entering or within element Organic fouling Interactions of natural or synthetic organics Organic compounds can have a much greater effect on permeate flux than clays or other inorganic colloids, even at lower mass concentration.

Scaling SEM

Silt Density Index (SDI)
Empirical test of filterability Measures the tendency of a raw water to foul a membrane Use 0.45 μm filter in a dead-end filtration cell ti – time required to filter a fixed volume of raw water through a clean membrane (~500 ml) tf – time required to filter the same volume after the membrane has been used for a defined length of time Standard conditions: 47 mm filter, 2 bar (30 psi) transmembrane pressure, total time (tt) of 900 sec

Water Treatment Membrane Processes Microfiltration and Ultrafiltration
CEEN 470 Water and Wastewater Unit Operations Water Treatment Membrane Processes Microfiltration and Ultrafiltration

Overview Initial use of deep filtration microfilters…disposable, not sustainable… MF membranes provide removal by retention of contaminants on the membrane surface Lowest pressure membrane process Pore size of 0.05 – 5 micron Cake filtration provides additional removal capabilities …smaller particles than pore size can be removed

Current Status MF and UF generally accepted as being capable of meeting filtration requirements for drinking water production Turbidity removal / disinfection MF can resolve the conflict between need to provide primary disinfection and DBP formation LT2ESWTR identified membranes as treatment technique for higher level removal of cryptosporidium Substantial diversification of membrane processes and configurations

Filtration Spectrum

Treatment Capabilities
Removal of particulate matter Turbidity Particles Microbial control Removal of organic and inorganic species when feed water is pretreated (coagulation, adsorption) DOC/DBP precursors color / taste / odor Pesticides Iron / manganese (aeration / chemical oxidation) Arsenic

Treatment Capabilities
Parameter Pretreatment needed for substantial removal MF UF Particulate/microbial Turbidity None Protozoa Bacteria Viruses Coagulation Organic TOC Coagulation / PAC DBP precursor Color T&O Pesticides PAC Inorganic Iron & manganese Oxidation Arsenic Hydrogen sulfide

Modes of Application

Turbidity Removal

Particle Removal

Water Permeation Across Clean MF/UF Membranes
Pure water transport through clean porous membrane is: Directly proportional to transmembrane pressure (ΔP) Inversely proportional to viscosity (μ) Modeled using modified form of Darcy’s Law: Rm ≡ hydraulic resistance of the clean membrane to water permeation (units?) (mu (Pa·sec)) Rm = 1/m

Example: Membrane resistance
An MF membrane is tested in the lab by filtering clean, deionized water and the flux is found to be 2,000 LMH (L/m2-hr) at 20 C and 0.7 bar. Calculate the membrane resistance coefficient.

Water Permeation Across Clean MF/UF Membranes
Absolute transmembrane pressure vs. pressure gradient Typical units of water flux gfd (gal/ft2-day) LMH (l/m2-hr) LMH x 1.7 = gfd

Flow Through a Cylindrical Pore (Poiseuille’s Law)
ΔP/Δz is pressure gradient In real membranes pores are not perfectly cylindrical  Dimensionless tortuosity factor (t) is often introduced

Flow Through Membrane Pores
rpore = #pores/A

Significant Parameters
Pore size has the highest effect on resistance to water flow Pore size distribution Specific flux for membrane comparison Calculated based on area on feed side

Reduction in Membrane Productivity
Flux Decline Mechanisms Fouling Concentration polarization Resistance in Series

Reductions in Permeate Flux
Crossflow Filtration Pressure Feed Concentrate Membrane Permeate

Reduction in Permeate Flux over Time
Reversible vs. irreversible fouling

Increase in Transmembrane Pressure Over Time
Reversible vs. irreversible fouling

Classwork: UF Recovery
Assuming the following operating scenario: UF treatment plant with 48 HydraCap60 membrane elements 20 min operation with average productivity of 55 gfd backwash with product water for 45 sec uses 1500 gal of permeate What additional information is needed? What is the water recovery rate? What is the backwashing flux? 500 ft2 per element  total 24,000 ft2 120 gfd backwash  120gfd x 24,000 ft2 x d/1440 min x min/60 sec x 45 sec = 1500 gal

High-Pressure Membrane Technologies: Nanofiltration and Reverse Osmosis

Membrane Separation Nanofiltration (NF) “Membrane Softening”
Reverse Osmosis (RO) “Hyperfiltration”

RO and NF Membrane Applications
Desalting/total dissolved solids 1967 first spiral-wound configurations (General Atomics) for brackish water desalination Disinfection by-product precursors Leading to TTHMs and HAA formation Hardness, color, and turbidity After introducing NF membranes in 1986 (Filmtec Corp.) Inorganic chemicals For heavy metal, nitrate and fluoride removal

RO and NF Membrane Applications
Synthetic organic chemicals Pesticide removal since the 1990s Pathogens Very effective, but usually no driver for implementation Indirect Potable Reuse since the 1980s using reclaimed water for direct injection

Colligative Properties of Ionic Solutions
Properties of solution that depend on the number of solute molecules present, but not on the nature of the solute (number of particles in a given volume of solvent and not the mass of the particles) Osmotic pressure, vapor pressure, freezing point depression, and boiling point elevation are examples of colligative properties Osmotic pressure and vapor pressure are two properties of solution that play a major role in various membrane processes

Ionic vs. Covalent Electrolyte vs. Nonelectrolyte
Substance when dissolve can break up to ions or stay intact (i.e., NaCl vs. sugar) Type % Ionization Solubility Electrolyte: conduct electricity Strong electrolyte 100% Very soluble Weak electrolyte < 100% Slight-to-very soluble Nonelectrolyte: does not conduct electricity No conduction 0% Insoluble or soluble

Semipermeable membrane
Solute molecule Solvent molecule Osmosis Semipermeable membrane: permits passage of some components of a solution, example: cell membranes and cellophane Osmosis: the movement of a solvent from low solute concentration to high solute concentration There is movement in both directions across a semipermeable membrane (!) As solvent moves across the membrane, the fluid levels in the arms of a U-tube becomes uneven Eventually the pressure difference between the arms stops osmosis

Experiment in Osmosis

Examples of Osmosis Cucumber placed in NaCl solution loses water to shrivel up and become a pickle Limp carrot placed in water becomes firm because water enters via osmosis Salty food causes retention of water and swelling of tissues (edema) Water moves into plants through osmosis Salt added to meat or sugar to fruit prevents bacterial infection (a bacterium placed on the salt will lose water through osmosis and die)

Osmotic Pressure Osmotic pressure, π, is the pressure required to stop osmosis π = osmotic pressure M = molarity (mol/L) R = Ideal Gas Constant = L·atm/mol·K T = temperature (K)

Osmotic Pressure Applied pressure needed to stop osmosis STEP 1
Pure Solvent Solution Semipermeable Membrane

Osmotic Pressure Osmotic Pressure is a VERY sensitive measure of Molarity Seawater contains 34 g NaCl per liter M = 34 g/L / 58.5 g/mol = M π 25 °C) = (0.582 mol/L)·2·( L·atm/mol·K )(298 K) = 28.4 atm 1 atm supports column of water m high (28.4 atm)(10.34 m/atm) = 294 m (294 m)(3.28 ft/m) ~ 963 feet Hoover Dam (726.4 ft (221.4 m))

Van’t Hoff Factor (i) Factor which accounts for deviation of colligative properties due to electrolytic nature of solutes Van’t Hoff Factor (i) can be measured by Typical nonelectrolyte (i.e., urea, sucrose, glucose): i = 1 Electrolytic solute: Electrolyte Ideal i Measured i NaCl 2 1.9 HIO3 1.7 MgCl2 3 2.7 AlCl3 4 3.2

Discrepancy between Real and Ideal Van’t Hoff Factor i
Consider: AlCl3  Al Clˉ Ideally i = 4 because four ‘particles’ in solution, but actually… Real i = 3.2 The reason measured value (3.2) is lower than ideal: strong electrolytic attraction forces between oppositely charged ions causes some of the ions to be held together through ion-pair Van’t Hoff factor follows ideal value best when the concentration is very low. Large deviation from Van’t Hoff factor with very high concentrations

Exercise A mixture of solid NaCl and solid sucrose, C12H22O11 has an unknown composition. When 15.0 g of the mixture is dissolved in enough water to make 500 mL of solution, the solution exhibits an osmotic pressure of 6.41 atm at 25 °C. Determine the mass percentage of NaCl in the mixture. What mass of NaCl and sucrose must be added so that the osmotic pressure is 10 atm at 25 °C ?

Reverse Osmosis / Nanofiltration
Pressure greater than Δπsoln STEP 2 Pure Solvent Solution Semipermeable Membrane

Mass Transfer and Permeate Flux
Water (permeate) Flux Jw = Qp/A = Aw(ΔP – Δπ) [gfd, LMH] Js = BDCs Kw = water mass transfer coefficient ΔP = transmembrane pressure differential  = reflection coefficient (unitless) Δπ = transmembrane osmotic pressure differential A = effective membrane area B = salt permeability Specific Permeate Flux Jw = Qp/A*P [gfd/psi, LMH/kPa] Note: ΔP-Δp = net applied pressure (NAP) Kw: also pure water permeability Explain reflection coefficient for one pore, 0 or 1. S is than the percent of pores small enough to reject The solute

The Reflection Coefficient, σ
1 < σ < 0 σ = 0, non-selective σ = 1, semipermeable solute water pore solute water pore Kw: also pure water permeability Explain reflection coefficient for one pore, 0 or 1. S is than the percent of pores small enough to reject The solute

The Reflection Coefficient, σ
1 < σ < 0, partial selective solute water pore Kw: also pure water permeability Explain reflection coefficient for one pore, 0 or 1. S is than the percent of pores small enough to reject The solute

Mass transfer limited Permeate flux independent of transmembrane pressure Film Layer Model Permeate flux k’: mass transfer coefficient Linton-Sherwood Correlation: Film layer model is adaquate to simulate permeate flux when true solutes accumulate near the membrane surface. Clean Water Flux Transmembrane pressure

Concentration Polarization and Membrane Scaling
Cp Product Cb Bulk Boundary Layer Cm Cs Membrane Scale J J = Qp/Ah K = 1.62(UD2/2bL), U=Qt/Av J is convective flux toward membrane Ddc/dx is diffusive flux away from membrane cb is concentration of bulk solution cs is saturation concentration cm is concentration at membrane surface cp is concentration of permeate δ is concentration boundary layer – in this layer cb increases to cm if cm>cs then precipitation/scaling occurs

Temperature Effects JT = J20 exp (s/T) J20 = permeate flux at reference temperature of 20 °C s = empirical constant, membrane specific T = Temperature Temperature Corrected Flux (TCF)

Temperature Effects Water Flux (L/m2-hr) NaCl Conc. (g/L) 2 4 6 8 10
12 14 16 18 20 22 25 35 45 55 65 75 NaCl Conc. (g/L) Water Flux (L/m2-hr) Feed 20 degC - Permeate 20 degC Feed 25 degC - Permeate 20 degC Feed 30 degC - Permeate 20 degC No Temp Control Feed 30 degC - Permeate deg30C

Influence of Operating Parameters on RO
Important implication of SD model is that rejection increases with feed pressure Water flux  (DP - Dp) Salt flux  dcs/dz Increasing feed pressure doesn’t change salt flux! Flux and recovery inversely related due to reduction in driving force from feed to retentate in the module

Impact on Permeate Flux
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 50 100 150 200 250 Runtime (hours) Normailzed Permeate Flux (J/Jo) NF-90 NF-270 NF-4040 TFC-S The importance of fouling is further underscored by its effect on the permeate flux of four different membranes with very different initial permeabilities.

Water Treatment Membrane Processes - Introduction -

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