cc(x)cc(x) cp(x)cp(x) Feed (Q f, c f ) Permeate (Q p, c p,out ) Concentrate (Q c, c c,out ), Retentate, Rejectate

Membrane Applications in Drinking Water Treatment

Pressure-Driven Membrane Processes Separate by size and chemistry Concentration, Porosity Effects

OTHER DRIVING FORCES Charge Gradient (Electrodialysis) Concentration Gradient (Dialysis) Temperature Gradient (Thermoosmosis)

PRESSURE GRADIENT PORE DIAMETER MEMBRANE DESIGNATION REMOVAL EFFICIENCY

Relative Sizes Separation Process Molecular Weight (approx..) Size, Microns Ionic Range (nanometer) Molecular Range 0.01 Macro Molecular Range Micro Particle Range Macro Particle Range , ,000500,000 Bacteria Viruses Dissolved Salts (ions) Algae Clays Silt Asbestos Fibers Cysts Sand Conventional Filtration (granular media) Organics (e.g., Color, NOM, SOCs) Microfiltration Ultrafiltration Nano filtration Reverse Osmosis Membrane Separations for Application to Drinking Water Treatment

The Two Meanings of Filtration: 2. Porous Membrane Filtration 1  m

40% PDMAEMA-60% PFOMA Thin-film Composite NF Membrane (Polysulfone Support Layer)

Membrane Geometry Spiral Wound NF/RO Hollow Fibers MF/UF

Tubular Elements

Spiral Elements

INORGANIC SYNTHETICS Ceramics Glass Metallic Excellent thermal stability Withstands chemical attack

PLATE AND FRAME

Two MF/UF Configurations Encased membrane system Submerged membrane system Pump supplies positive pressure to PUSH water through membrane media. Feed Water Filtrate Pump Pressure Vessel(s) Membrane Pump suction PULLS water through membrane media. Feed Water Filtrate Pump Open Tank Membrane

Permeate HF Air Raw Water Pump 2-12 psi Wasting Immersed Membranes with Gentle Crossflow

NF & RO Scottsdale Water Campus

CASCADE SYSTEM RETENTATE PERMEATE FEED

PERMEATEFEED RETENTATE QfQf CfCf A QPQP CPCP QRQR CRCR TMP = “Transmembrane pressure (difference)” Q p / A Flux (“LMH” or “GFD”) = Q p / A C p /C f (Contaminant) Rejection (%) = 1  C p /C f Recovery (%) = Q p /Q f

Membrane Geometry Approximate Packing Density (m 2 /m 3 ) Capillary Spiral wound Hollow fiber Flat (plate and frame) Tubular Membrane ProcessTransmembrane Pressure, ∆P tot (kPa) System Recovery (%) (a) Microfiltration10 to to 99+ Ultrafiltration50 to to 95+ Nanofiltration200 to to 90+ Reverse Osmosis500 to to 90 (a) Defined as the ratio of permeate flow rate to feed flow rate

Example. What height would a column of water have to be to exert a pressure equal to 15 kPa? 4500 kPa? Solution. From fluid mechanics: Therefore:

Example. What is the average velocity of solution toward a membrane, if the flux is 50 LMH?

Flow Through Porous Membranes For Laminar Flow: Darcy-Weisbach Eqn: For Steady Flow Through a Pore: Hagen-Poiseuille Eqn:

Flow Through Porous Membranes Resistance (kg/m 2 -s): Membrane Resistance (m   : Process Typical Volumetric Flux, (L/m 2 -h) Typical Membrane Resistance, R m (m  1 ) Microfiltration x10 11 – 1x10 12 Ultrafiltration x10 12 – 1x10 13 Nanofiltration20-501x10 13 – 1x10 14 Reverse osmosis5-405x10 13 – 1x10 15

Flow Through Porous Membranes Resistivity: Permeability for overall flow: Permeability for individual species:

1  m Contaminant Rejection by Open Pores (Clean Membrane)

AB Membrane Pore Contaminant Rejection by Open Pores (Clean Membrane) Increasing driving force increases flux of both water and contaminants. So, rejection of a given type of particle by a clean membrane is predicted to be independent of  P or J.

Membrane Fouling

Problems Caused by NOM Membrane Fouling DBPs +Cl 2 Interference w/Activated Carbon

NOM Fouling of an MF Membrane Note: <3% Removal of NOM from Feed Gel Surface Membrane Gel Cross-Section Membrane support

Heated Aluminum Oxide Particles (HAOPs) Al 2 (SO 4 ) 3 +NaOH  pH o C, 24 hrs Particle Size Range:  m, mean ~5  m Point of Zero Charge: pH 7.7 BET Surface Area: 116 m 2 /g Aluminum Content: ~25% (Al(OH) 3  H 2 O)

Transmembrane pressure with varying HAOPs surface loadings

DOC Concentrations in Permeate

Progressive NOM Deposition on the HAOPs Layer V sp : 0 L/m 2 1,200 L/m 2 3,600 L/m 2 4,700 L/m 2 7,000 L/m 2

Summary: Performance and Modeling of Porous Membranes Solution flux proportional to  P, inversely proportional to resistance Resistance of clean membrane can be estimated from basic fluid mechanics If contaminant rejection is primarily due to geometrical factors, it is expected to be insensitive to applied pressure and flux In practice, resistance of accumulated rejected species quickly overwhelms that of membrane (fouling) Frequent backwashing reduces, but does not eliminate fouling In drinking water systems treating surface water, NOM is often a major fouling species, even though only a small fraction of the NOM is rejected Approaches to reduce fouling by NOM and other species are the focus of active research

Transport Through Water-Selective, Dense (“Non-Porous”) Membranes With no  P, the concentration gradients drive water toward the feed and contaminants toward the permeate. cw,fcw,f Solute, 90% rejection Osmosis of water Pressure profile for P=0 everywhere cw,pcw,p cs,fcs,f cs,pcs,p

Increasing pressure increases the “effective” concentration of any species. For an increase of  P, the effective concentration is: At  P= 3000 kPa: For water: At 25 o C: Result: Even a large  P increases effective concentrations by only a few percent.

The pressure required to bring the effective concentration of water up to the concentration of pure water (and thereby stop diffusion) is the osmotic pressure, . Permeate is often approximated as pure water. In this example,  is a pressure that increases c eff by ~1%. Note that c eff of the solute also increases by ~1%. cw,fcw,f cw,pcw,p Solute, 90% rejection Osmosis eliminated c w,eff,f 55.5 P = 0 P =  0.56 cs,fcs,f cs,pcs,p c s,eff,f

Applying a  P >  causes water to move in the opposite direction from passive osmosis, hence is called reverse osmosis. For P ~3000 kPa, c eff increases by ~3%, so: cw,fcw,f cw,pcw,p Solute, 90% rejection Reverse osmosis c w,eff,f 56.5 P = 0 P >  0.57 Although increasing  P causes the same percentage increase in c eff for water and solute, it has a much bigger effect on  c eff for water than for solute. cs,fcs,f cs,pcs,p c s,eff,f

Performance and Modeling of Dense Membranes Water flux occurs by diffusion, and is ~proportional to  P , because changing  P has big effect on  c w,eff Solute flux occurs by diffusion, and is ~proportional to  c i, because changing  P has small effect on  c i,eff Conclusion: changing  P increases water transport more than solute transport, and so increases rejection (different from porous membranes) Fouling also occurs on dense membranes, mostly by NOM and precipitation (scaling); reduced by “anti-scalants” Dense membranes can’t be backwashed, because required pressures would be too high; therefore, major effort is usually devoted to pre-treatment to remove foulants Approaches to reduce fouling are the focus of active research