Water and Wastewater Engineering

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9-1

CHAPTER

REVERSE OSMOSIS AND NANOFILTRATION

9-1 INTRODUCTION

9-2 THEORY

9-3 PROPERTIES OF RO AND NF MEMBRANES

9-4 RO AND NF PRACTICE

9-5 ELECTRODIALYSIS

9 -6 CHAPTER REVIEW

9 -7 PROBLEMS

9-8 DISCUSSION QUESTION

9 -9 REFERENCES

9-2 WATER AND WASTEWATER ENGINEERING

9-1 INTRODUCTION

Delineation of Membrane Processes

Reverse osmosis (RO), nanofiltration (NF), and electrodialysis are membrane processes that use

the differences in permeability of water constituents as a separation technique. The membrane is

a synthetic material that is semipermeable; that is, it is highly permeable to some constituents and

ss permeable to others. To remove a constituent from the water, the water is pumped against

the surfac e of a membrane resulting in a separation of product and waste streams as shown in

Figure 9-1.

Four types of pressure driven membranes are generally recognized: microfiltration (MF),

ultrafiltration (UF), nanofiltration (NF), and revers

e osmosis (RO). The hierarchy of the pro-

cesses is identified by the types of materials rejected, operating pressures, and nominal pore

sizes on an order-of-magnitude basis. These are shown schematically in Figure 9-2 . Unlike NF/

RO that are pressure driven, electrodialysis (ED) and electrodialys

is reversal (EDR) processes

are electrical voltage-driven.

The focus of the discussion for this chapter is RO and NF because they remove ions and have

been used widely in softening water. MF and UF separate suspended particles (colloidal matter,

microorganisms, and viruse

s) from the water. They are discussed in Chapters 12 and 26. ED/EDR

are reviewed briefly.

In the past, there was a distinction made between RO and NF membranes based on their

original manufactured properties and permeation capabilities. The differences have blurred

with the introduction of new RO membranes. The new RO membranes, c

alled “loose” RO,

“softening membranes,” and “low-pressure” RO, have discriminating characteristics similar to

the NF membranes. Although the distinctions are important from a theoretical point of view,

the remainder of the discussion will treat NF/RO systems together for design and operational

considerations.

FIGURE 9-1

S chematic of separation process through reverse osmosis or nanofiltration membrane.

Feed stream

Feed-concentrate channel

Permeate channel

Semipermeable

membrane

Waste stream containing

impermeable components

(concentrate or reject)

Product stream containing

permeable components

(permeate)

REVERSE OSMOSIS AND NANOFILTRATION 9-3

9 -2 THEORY

Osmosis

Osmosis i s defined as the spontaneous transport of a solvent (in this case, water) from a dilute solu-

tion to a concentrated solution across an ideal semipermeable membrane that impedes passage of

the solute (ions in solution) but allows the solvent (water) to flow. This is shown schematically in

Figure 9-3 . The syste

m will reach equilibrium when the hydrostatic pressure on the saline water side

balances the force moving the water through the membrane. This is noted as the osmotic pressure

in Figure 9-3 b. If pressure is exerted to overcome the osmotic pressure, the solvent (pure water) will

flow from the saline side to the fresh water side. The semipermeable m

embrane will not allow the

passage of molecules other than water and gases. This is noted as reverse osmosis in Figure 9-3 c.

Osmotic Pressure

The driving force for diffusion is typically described as a concentration gradient. A more rigorous

explanation is a gradient in Gibbs energy. The general form of the Gibbs function is

GPSTun









∂∂∂∂

(9-1)

where G  Gibbs energy, J



P  pressure, Pa

S  entropy, J/K

T  absolute temperature, K



 chemical potential of solute i, J/mole

 amount of solute i in solution, moles

FIGURE 9-2

Common constituents removed by membrane processes.

MF  microfiltration;

UF  ultrafiltration;

NF  nanofiltration;

RO  reverse osmosis.

Bacteria,

protozoa, algae,

particies  0.1 m

Viruses and small colloids

Dissolved organic matter

and divalent ions (Ca

2

; Mg

2

)

Monovalent species (Na



; Cl



)

Water

Pore

sizes

Nonporous

Con

ntrat

Con

ntrat

Con

ntrat

Con

ntrat

MF UF NF RO

 0.1 m  0.01 m  0.001 m

9-4 WATER AND WASTEWATER ENGINEERING

Chemical potential is defined as the change in Gibbs energy resulting from a change in the

amount of component i when the temperature and pressure are held constant

G

n





(9-2)

Thus, under constant temperature conditions, equilibrium ( G  0) will be achieved when

Pun









(9-3)

The pressure ( P ) to balance the difference in chemical potential of a solute is called the osmotic

pressure (MWH, 2005). By convention it is given the symbol  . The equation for osmotic pres-

sure can be derived thermodynamically using assumptions of incompressible and ideal solution

behavior:



iCRT

(9-4)

where i  number of ions produced during dissociation of solute

 osmoticcoefficient un itless,

C  concentration of all solutes, moles/L

R  universal gas constant, 8.314 kPa · m

/kg mole · K

T  absolute temperature, K

The number of ions per mole, i, for example would be 2 for NaCl. The osmotic coefficient, ,

depends on the nature of the substance and its concentration. For NaCl it ranges from 0.93 to

1.03 over a concentration range of 10 to 120 g/L of salt. Seawater has an osmotic coefficient that

varies

from 0.85 to 0.95 for the same concentration range. Robinson and Stokes (1959) provide

osmotic coefficients for a variety of electrolytes.

FIGURE 9-3

Direct and reverse osmosis. ( Source: Davis and Cornwell, 2008.)

Semipermeable

membrane

Fresh

water

Low

TDS

Saline

water

High

TDS

Low

TDS

High

TDS

Osmosis

(a)(b)(c)

Osmotic

equilibrium

Osmotic

pressure

Reverse

osmosis

Osmotic

pressure

WaterWater

REVERSE OSMOSIS AND NANOFILTRATION 9-5

Flux

Several models have been developed to describe the flux of water (m

/ d · m

of membrane sur-

face area) and solutes (kg/m

· d) through the RO membrane. Because there is some controversy

about the mechanics of permeation, thes e are presented in summary here. For more details see

MWH (2005). The models are:

• Solution-diffusion model: Permeation occurs through a dense membrane where the active

layer is permeable but nonporous. Water and

solutes dissolve into the solid membrane mate-

rial, diffuse through the solid, and reliquefy on the permeate side of the membrane. Separa-

tion occurs when the flux of the water is different from the flux of the solutes.

• Pore flow model: This model assumes the RO membranes have void spaces (pores) through

which the liquid water travels. It considers the water and solute fluxes to be coupled. Rejec-

tion occurs because the solute molecules are “strained” at the entrance to the pores. Because

the solute and water molecules are similar in size, the rejection mechanism i

s not a physical

sieving but rather a chemical effect such as electrostatic repulsion.

• Preferential sorption-capillary flow model: This model assumes the membrane has pores.

Separation occurs when one component of the feed solution (either solute or water) is pref-

erentially adsorbed on the pore walls and is transported through the me

mbrane by surface

diffusion.

Ultimately, these models express flux as the product of a mass transfer coefficient and a driv-

ing force. The water flux is

()





(9-5)

where J

 volumetric flux of water, m

/ d · m

 mass transfer coefficient for water flux, m

/ d · m

· kPa

 P  net transmembrane pressure, kPa

  difference in osmotic pressure between the feed and the permeate, kPa

The driving force for the solute flux is the concentration gradient. The solute flux is

k C

 ()



(9-6)

where J

 mass flux of solute, kg/d · m

 mass transfer coefficient for solute flux, m

/ d · m

 C  concentration gradient across the membrane, kg/m

The flux of solutes through the membrane is

JCJ

spw



(9-7)

where C

 solute concentration in the permeate, kg/m

The recovery ( r ) is the ratio of permeate flow to feed water flow:



(9-8)

9-6 WATER AND WASTEWATER ENGINEERING

The flow balance and mass balance are

QQQ

FPC



(9-9)

CQ CQ CQ

FF PP CC



(9-10)

where the subscripts refer to feed water ( F ), permeate ( P ), and concentrate ( C ).

Rejection is defined as

Rej

1

(9-11)

With an assumption that the rejection is close to 100 percent, these equations can be solved for

the concentrate concentration:



⎛

⎝

⎜

⎞

⎠

⎟

(9-12)

9-3 PROPERTIES OF RO AND NF MEMBRANES

Membrane Material

The materials most widely used in RO and NF are cellulosic derivatives and polyamide derivatives.

Cellulosic acetate (CA), the common commercial material, is not tolerant to temperatures

above 30  C and tends to hydrolyze when the pH is less than 3 or greater than 8. It is susceptible to

biologi

cal degradation and degrades with free chlorine concentrations above 1 mg/L. Most mem-

brane manufacturers guarantee integrity of membranes if the chlorine concentration and contact

time are within specified limits.

Polyamide (PA) membranes are generally resistant to biological degradation, are

stable over

a pH range of 3 to 11, and do not hydrolyze in water. Under similar pressure and temperature

conditions, PA membranes can produce higher water flux and higher salt rejection than CA mem-

branes. However, PA membranes are more susceptible to fouling and cannot tolerate free chlo-

rine at any concentration (MWH, 2005).

Membrane Configuration

The membrane units are fabricated in either a spiral-wound configuration or a hollow-fiber con-

figuration.

The spiral-wound configuration is shown in Figu re 9-4 . Two sheets of flat-sheet membrane

are joined along three sides with the active membrane layer facing ou t. A spacer is placed be-

tween the membrane sheets to keep them from to

uching. The open end of the envelope formed by

the two sheets is attached to a perforated central tube that collects the permeate. The spiral-wound

elements are typically 1 m long and 0.3 m in diameter. The area for a 1 m long element would be

about 30 m

. Individual elements have a permeate recovery of 5 to 15 percent. To achieve higher

recoveries, elements are placed in series. Typically, four to seven elements are arranged in series

in a pressure vessel (MWH, 2005).

REVERSE OSMOSIS AND NANOFILTRATION 9-7

The hollow-fiber element has several hundred thousand fibers with outside diameters on the

order of 0.085 mm suspended in a pressure vessel as shown in Figure 9-5 . Permeate recovery is

about 30 percent for each element.

The spiral-wound configuration is the most co

mmon for the production of drinking water

from grou ndwater and surface water. The hollow-fiber configuration is used extensively for

desalinization of seawater in the Middle Eas t (Taylor and Wiesner, 1999).

Temperature Effects

Temperature affects water viscosity and the membrane material. In general, the permeate flow

increases as the temperature rises and the viscosity decreases. The relationship between membrane

material, temperature, and flux is specific to individual products. Correction factors sho

uld be

obtained from manufacturers (AWWA, 1999).

Service Life

Membrane fouling generally occurs by one of the following mechanisms (AWWA, 1999):

• Deposition of silt or other suspended solids.

• Inorganic scale deposits.

• Biological fouling.

• Interaction of organic constituents with the membrane.

O xidation of the membrane from chlorination prior to ion exchange will significantly reduce

the s

ervice life. This is especially true for PA membranes. If prechlorination is essential, CA res-

ins are recommended (MWH, 2005).

Source water

Membrane (cast on fabric backing)

Source water & flow spacer

Source water &

flow spacer

Concentrate

Cutaway view of a spiral membrane module

Permeate

water

Processed water passes

through the membranes

on both sides of the porous

permeate carrier.

The

permeate flows through the

porous material in a spiral path

until it contacts and flows through

the holes in the permeate core tube.

Source

water

Source water

Membrane leaf

Adapted from Hydranautics

Water Systems diagram

Porous permeate carrier

FIGURE 9-4

T ypical spiral-wound RO membrane element.

( Source: U.S. AID, 1980.)

9-8 WATER AND WASTEWATER ENGINEERING

E xcessive concentrations of iron and manganese, if oxidized, will form precipitates that will

foul the membrane. Scaling will also occur as a result of increasing recovery because the concen-

tration of limiting salts will increase to their solubility limit, and they will precipitate. The most

common scales of limiting

salts are calcium carbonate and calcium sulfate. Others of concern

are calcium fluoride, calcium orthophosphate, strontium sulfate, barium sulfate, and amorphous

silica (MWH, 2005).

An additional negative impact is the result of the accumulation of solutes

that form a bound-

ary layer of high concentration at the membrane surface. The concentration at the surface of the

membrane becomes higher than the concentration in the bulk feed water. This effect is called

polarization. It has the following negative impacts (MWH, 2005):

• Water flux is lower because the osmotic pressure gradient is higher.

• Rejection is

lower.

• Solubility limits of solutes are exceeded leading to precipitation and scaling.

These issues may be ameliorated by pretreatment of the raw water and operational procedures.

9-4 RO AND NF PRACTICE

Process Description

The smallest physical unit of production capacity is the membrane element. The membrane

elements are enclosed in pressure vessels as shown in Figure 9-6 . A group of pressure vessels

operating in parallel is called a stage. The arrangement of one or more stages is called an array.

In a multistage process

the stages are arranged in series. The number of pressure vessels

decreases in each succeeding stage to maintain the water velocity in the feed channel as permeate is

Hollow

fibers

Feed

tube

Flow

screen

Concentrate

Pretreated

source

water

Permeate

FIGURE 9-5

T ypical hollow fiber NF/RO membrane module.

(Adapted from U.S. AID, 1980.)

REVERSE OSMOSIS AND NANOFILTRATION 9-9

extracted. A two-stage arrangement is shown in Figure 9-7 a and schematically in Figure 9-7 b. A

three-stage arrangement is shown schematically in Figure 9-7 c.

A schematic of a typical RO or NF facility is shown in Figure 9-8 .

Pretreatment

The first pretreatment is to prevent scaling by silica (SiO

) and sparingly soluble salts such as

calcium carbonate and calcium sulfate. Scale control consists of pH adjustment and/or addition of

an antiscalant. Typically, the antiscalants are proprietary polymeric compounds.

Unlike ion exc hange column

s, RO/NF systems are not backwashed. Therefore, the second

pretreatment process is filtration to remove particulate matter that will clog the feed channels or

accumulate on the membrane surface. For surface water sources granular filtration or membrane

filtration may be required. The minimum filtration requirement regardle

ss of the water source is

a cartridge filter rated in the 1 to 25  m range with a typical rating of 5  m. The maximum feed

water turbidity recommended by manufacturers is 1 NTU with a preferred turbidity of less than

0.2 NTU (Bergman, 2005).

Disinfection may also be required to prevent biologic

al fouling. Even though groundwater

is expected to have a very low microbial population, when the membrane is out of service, the

population on the membrane can quickly multiply. Chlorine solutions may be used for CA mem-

branes, but other techniques such as ultraviolet irradiation, or chlorination followed

by dechlori-

nation are used for PA membranes.

Post-treatment

Because the RO/NF membranes do not remove gases, these are stripped after the RO/NF unit.

The primary gases of concern are hydrogen sulfide and carbon dioxide. The removal of H

S is to

prevent odor complaints. CO

is removed because it forms carbonic acid.

The permeate has a low pH as a result of removal of alkalinity (buffering capacity) from

the water and the addition of acid to prevent scaling. This water is corrosive to the d istribution

system. In addition to stripping CO

, addition of a base and corrosion inhibitor is generally re-

quired. Split treatment and blending may aid in corrosion control. Maximum Contaminant Limits

(MCLs) for constituents such as arsenic must be considered if split treatment is part of the corro-

sion control strategy.

Concentrate Stream

The concentrate stream is under high pressure as it leaves the RO/NF unit. Energy recovery sys-

tems are often used in reducing the pressure.

Permeate

water

Pressure vessel

Brine seals O-ring

connector

Snap ring

End cap

Source water inle

Anti-telescoping

support

Concentrate (brine)

seal

Membrane

element

module

Membrane

element

module

Membrane

element

module

Concentrate

outlet

FIGURE 9-6

Cross section of pressure vessel with three spiral-wound RO elements.

( Source: U.S. AID, 1980.)

9-10 WATER AND WASTEWATER ENGINEERING

Plan view

(a)

(b)

(c)

End-perspective view

Permeate

(product)

Concentrate

(reject)

(brine)

Concentrate

Permeate

ConcentrateFeed

Stage 2:

12 Vessels

72 Elements

Permeate

Concentrate

Feed

Stage 1:

24 Vessels

144 Elements

FIGURE 9-7

E xample array arrangements: (a) a 4  2 stage arrangem ent showing 12 pres-

sure vessels, each with 6 elements per vessel; ( b ) a schematic of a 4  2 array;

( c ) a schematic of a 4  2  1 array.

Raw water

Acid

Antiscalant

Prefiltration

Feed

pumps

Membrane

arrays

Energy

recovery

Break tank

Concentrate

disposal

Corrosion

inhibitor

DisinfectantBase

Air

Treated water

to distribution

system

stripping

tower

FIGURE 9-8

S chematic of typical reverse osmosis facility.