Desalination Membrane related technology - ISS … & Membrane related technology ... Fundamentals of Reverse Osmosis Technology 3 ... Process Design of Spiral Wound RO Systems 170

Maria D. Kennedy, PhD LN0076/10/1 Sergio G. Salinas Rodríguez, MSc Prof. Jan C. Schippers, PhD, MSc

Desalination & Membrane related technology

Maria D. Kennedy, PhD Lecture Notes LN0076/10/1 Sergio G. Salinas Rodríguez, MSc Prof. Jan C. Schippers, PhD, MSc

Desalination & Membrane related technology

Table of contents

Title Page number

Fundamentals of Reverse Osmosis Technology 3

Overview reverse osmosis / nanofiltration membranes and elements

18

Fouling in RO and NF systems 35

Particulate fouling 48

Fouling due to Iron and Manganese 92

Organic fouling 111

Biofouling and Pretreatment 118

Scaling 151

Process Design of Spiral Wound RO Systems 170

Process Design calculations: RO 187

“Fundamentals of Reverse Osmosis Technology”

Maria D. Kennedy , PhDProf. Jan C. Schippers, PhD, MScJ pp

Delft – April 2010

dilutesolution

concentratesolution

appliedpressure

Osmotic Pressure

solution solution pressure

Semi‐permeablemembrane

The direction of water flow is determined by the pressure, temperature & concentration of dissolved solids

Osmosis Reverse Osmosis

2

Osmotic Pressure

Water Type TDS π

An approximation of πmay be made by assuming that ca. 1450 mg/L Total Dissolved Solids (TDS) equals 1 bar osmotic

Brackish Water

Sea Water

1000 mg/L

35000 mg/L

ca. 0.8 bar

ca. 28 bar

1450 mg/L Total Dissolved Solids (TDS) equals 1 bar osmotic pressure (1000 mg/L TDS ca. 0.8 Bar)

3

Water Flow

Theory suggests that the chemical nature of the membrane is such that it will absorb and pass water preferentially to dissolved salts at the solid/liquid interface.

This may occur by weak chemical bonding of the water to the membrane surface or by dissolution of the water within the membrane structure (Solution Diffusion Theory)

The chemical & physical nature of the membrane (e.g., surface charge & pore size) determines its ability to allowsurface charge & pore size) determines its ability to allow the preferential transport of water over salt ions.

4

=Q P K A -

Water Flow

= w wQ P K A -

Qw : Rate of flow through membrane [m3/s]

ΔP : Hydraulic pressure differential [bar] = Pf ‐ Pp

ΔΠ : Osmotic pressure differential [bar]

= Osmotic pressure feed water – Osmotic pressure product water

Kw : Membrane permeability coefficient for water [m3/(m2 s bar)]

A : Membrane area [m2]

5

Water Flux

= w

w

QJ P K

A -

J : Water flux [m3/(m2 s)] = Qw/A

ΔP : Hydraulic pressure differential [bar]ΔP : Hydraulic pressure differential [bar]

ΔΠ : Osmotic pressure differential [bar]

Kw : Membrane permeability coefficient for water [m3/(m2 s bar)]

6

Salt Flow

The salinity of the permeate (Cp) depends on the relative rates of water & salt transport through a membrane.

= s sQ C K A

Qs : Flow rate of salt through membrane [kg/s]

ΔC : Salt concentration differential across membrane [kg/m3] = Cf ‐ Cp

Ks : Membrane permeability coefficient for salt [m3/(m2 s)]

A : Membrane area [m2]

7

* *

* *( )

ssp

ww

Q C K AC =

Q P K A

Salt Passage (I)

*

*

( )

( )

f p

f

s

pp w

C C KC =

P P K

*(1 )

( )

Cpsp Cf KC

= C P P K

*( )ff p wC P P K

* 100% Salt Passage ( )p

f

C SP

C

8

Salt Passage (II)

Since Cp is small compared to Cf, Cp/Cf <<1, the salt transport (Qs) is constant at a certain Cp and is independent p

of the pressure

As a consequence, the salt passage (SP) is lower at high pressure and vice versa

Thi i b h i f l (Q ) ill b dil dThis is because the same quantity of salt (Qs) will be diluted by a larger volume of (product) water and vice versa.

9

*Salt Rejection (SR) 100%

pfC C

Salt Rejection

Salt Rejection (SR) 100%fC

*(1 ) 100%p

f

C

C

100% Salt Passage (SP)

Salt Rejection is the opposite of Salt Passage

10

Recovery/Conversion

100% * fQ

Qp

(R) Recovery

Q : Product water flow rate (m3/s)Qp : Product water flow rate (m3/s)

Qf : Feed water flow rate (m3/s)

11

Recovery affects salt passage & product flow.

What are the effects of Recovery?

As recovery increases, the salt concentration on the feed‐concentrate side of the membrane increases, which increases the salt transport and the permeate salinity (Cp).

High salt concentration in the feed‐concentrate solution i h i hi h l dincreases the osmotic pressure, which consequently reduces the Net Driving Pressure (NDP). As a result, the product water flow rate is reduced and the permeate salinity (Cp) increased.

12

feed permeate

Mass Balance Equations

f p c

f f p p c c

Q = Q Q

Q C = Q C Q C

feed p

concentrate

Q : Flow [m3/s]

C : Concentration [kg/m3]

f,p,c : feed, permeate, concentrate

13

feed permeate

Mass Balance Equations

R : Recovery

concentrate

100p

f

QR =

Q

R : Recovery

Q : Flow [m3/s]

C : Concentration [kg/m3]

f,p,c : feed, permeate, concentrate

14

Qf

C

Qp

C

Water Balance

f p c

f f p p c c

Q = Q Q

Q C = Q C Q C

Q Q

CfCp

Qc

Cc

= 1

: = = 1

p cf p c f p c

f f

f pc

f f

Q QC C C or C = R C + ‐ R C

Q Q

Q QQSince ‐ R

Q Q

15

c

f

CCF

C

Water Balance

Since : 1 ‐ Salt Rejection

substitution of (1 ‐ ) from 1 ‐

yields : 1 ‐ 1 ‐

1 1

f

f p c

p

p f

f

f f c

C

C R C R C f

CC C f f

C

C R C f R C

R fC

1 1and

1

( ) 1

c

f

R fCCF

C R

If salt rejection f th

1

: 1

en CFR

16

Concentration Factor

Recovery *Concentration Factor (CF)Recovery *Concentration Factor (CF)

50%

75%

80%

90%

2

4

5

10

*Concentration Factor (CF) is calculated assuming that Salt Rejection (f) = 1

17

Concentration Polarization (I)

As water flows through a membrane & solutes are rejected by the membrane, the retained solutes can accumulate at the membrane surface where their concentration will gradually increase.

The concentration build‐up at the membrane will generate a diffusive flow back to the bulk of the feed, but after a given period of time steady state conditions will be establishedestablished

Steady‐state conditions are reached when the convective solute flow to the membrane surface is balanced by the solute flux through the membrane plus the diffusive flow from the membrane to the bulk

18

Under steady‐state conditions, the concentration at the membrane surface (Cm) is constant

Concentration Polarization (II)

The cross flow along the membrane surface enhances back diffusion of solutes to the bulk.

This increase in salt concentration at the membrane surface i ll d i l i iis called concentration polarization.

19

MembraneFeed/Bulk Solution Boundary

LayerPermeate

Concentration Polarization (III)

Cbulk/feed

Cmembrane

J.cp

Feed Flow

J.c

bulk/feed

Cpermeate

(x)

D dc/dx

20

Concentration Polarization (IV)

At steady‐state the convective transport of solute to the membrane is equal to the sum of the permeate solute flow

Boundary conditions : 0

p

m

dcJ c D J c

dx

x c c

plus the diffusive back transport of solute:

After integration ln

b

J

m p m p D

b p b p

x c c

c c c cJor e

c c D c c

21

The ratio of the diffusion coefficient (D) and the thickness of the boundary layer (δ) is called the mass transfer coefficient

Concentration Polarization V

k;

int int1 ‐ ; intrinsic retention of the membrane

then / becomes;

p

m

m b

D k

CR R

C

C C

int int

/ ;

exp ( )

(1 ) exp ( )

m

b

JC k

JC R Rk

22

The ratio Cm/Cb is called the concentration polarisationfactor. This ratio increases (i.e., the concentration Cm at the

Concentration Polarization VI

membrane surfaces increases) with increasing flux (J), with increasing retention (Rint) and with decreasing mass transfer coefficient k.

When the solute is completely retained by the membrane

Dint( 1 0) ;

exp

p

Jm k

b

DR and C and k then

C

C

23

expJ

m kC

C

Concentration Polarization VII

pbC

This is the basic equation for concentration polarization which demonstrates the two factors (the flux J and the mass transfer coefficient k) and their origin (membrane part J, hydrodynamics k) responsible for concentration polarization.

The pure water flux (specific permeability) is determined by the b d d h b f h hmembrane used and this parameter is not subject to further change

once the membrane has been selected.

On the other hand, the mass transfer coefficient depends strongly on the hydrodynamics of the system and can therefore be varied and optimized

24

Mass Transfer Coefficients I

The mass transfer coefficient k, is related to the Sherwood number (Sh)

Re (a,b&c are constants)

With Re and

b ch

h h

k dSh a Sc

Dd v v d v Sc =

D

k : mass transfer coefficient [m/s]

dh : Hydraulic diameter [m]

v : Flow velocity [m/s]

n : Kinematic viscosity [m2/s]

: Dynamic viscosity [kg/(m s)]

D : Diffusion coefficient (m2/s)

25

The mass transfer coefficient k is mainly a function of the feed flow velocity (v), the diffusion coefficient of the solute (D), the density and the module shape & dimensions

Mass Transfer Coefficients II

shape & dimensions.

Of these parameters, flow velocity and diffusion coefficient are the most important

K = f(v,D).

Mass transfer coefficients in various flow regimes

l i t b l tlaminar turbulent

Tube Sh = k dh/D = 1.62 (Re Sc dh/L)0.33 Sh = 0.04 Re0.75 Sc0.33

Channel Sh = 1.85 (Re Sc dh/L)0.33 Sh = 0.04 Re0.75 Sc0.33

26

By decreasing the flux (J)

How can concentration polarization be controlled?

By increasing the mass transfer coeficient (k). k is mainly determined by the diffusion coefficient and the flow velocity. Because the diffusivity of solutes cannot be increased (only by changing the temperature), k can only be increased by increasing the feed velocity along the membrane or by changing the module shape andmembrane or by changing the module shape and dimensions (decreasing module length or increasing the hydraulic diameter)

27

Greater osmotic pressure at the membrane surface than in the bulk feed solution and reduced Net Driving Pressure

Effects of Concentration Polarization

differential across the membrane.

Reduced water transport through the membrane (Qw)

Increased salt transport through the membrane (Qs)

Increased probability of exceeding solubility of sparingly soluble salts at the membrane surface, and the distinct

ibilit f li ( i it ti )possibility of scaling (precipitation).

28

J

km

b

CConcentration Polarization Factor (β)= =exp

C

Concentration Polarization Factor (CPF)

Where: Kp is a proportionality constant depending on the module geometry

p

f,avg

b

Q

Q

p

C

In practice,the formula is simplified to:

β = K * e

geometry.

This simplification is justified by the fact that (i) Qp is proportional to J and (ii) Qfavg is proportional to k and k is almost proportional to the cross flow velocity (v)

29

Concentration Polarization Factor

Using the arithmetic average of feed and concentrate flow as average feed flow, the Concentration Polarization Factor

1

2

2 iR

R

pK e

can be expressed as a function of the permeate recovery rate of a membrane element Ri

Th l f h C i P l i i F f 1 2The value of the Concentration Polarization Factor of 1.2, which is the recommended Hydranautics limit, corresponds to 18 % permeate recovery for a 40” long membrane element.

30

“Overview reverse osmosis / nanofiltrationmembranes and elements”

Prof. Jan C. Schippers, PhD, MSc


RO and NF membranes/elements

A large number of companies are manufacturing RO and NF membranes

Leading companies are :

Dow Chemical Company (Filmtec) www.dow.com

General Electrics (Osmonics)

Hydranautics www.membranes.com

Koch www.kochmembrane.com

T t b Toray www.toray‐membrane.com

Sidmas

Toyobo www.toyobo.co.jp/e/

Trisep

Dupont (Permasep) ‐ only replacement2

Several materials and blends are used e.g.,

cellulose acetate (di‐, tri‐)

Membrane materials and structure

aromatic polyamide

aromatic polyamide urea

polyvinyl alcohol

Two types of membranes are manufactured

one polymer

l two or more polymers

Both have an asymmetric structure with an ultra thin active layer and one or more support layers.

3

Cellulose acetate membranes consists of one polymer with varying density (pore size) across its thickness.

Membrane materials and structure

Composite membranes are made of (at least) threedifferent polymers:

polyester support layer (woven or non woven)

polysulphone intermediate support layer

active layer on top e.g., polyamide.

4

0.2 microns

Asymmetrical membrane “cellulose acetate”

Dense layer

Cellulose acetate

Porous

100 microns

5

Fabric carrier material

matrix

“cellulose acetate”“top layer”

6

Asymmetrical composite membrane

0.04‐0.1 microns

Polymer A

Porous

75 micronsPolymer B

7

Fabric carrier material

support material

8

Why is top layer very thin?

top layers have very narrow pores, smaller than 0.001 µm (or 0.000001 mm);

as a consequence the hydraulic resistance is high;

making top layer very thin, keeps hydraulic resistance acceptable;

Remark: Very thin top layers are very vulnerable for damage e.g. mechanical and chemical (pH, cleaning solutions)

9

Several “coatings” can be applied to:

improve the salt rejection.

Coatings

The materials are deposited from a solution on the membrane surface.

Tannic acid solution was commonly applied to restore the rejection of Dupont’s permeators.

Remark: The application of these coatings on spiral wound elements is not common practice.

10

Membranes consists of organic polymers (e.g., cellulose acetate, polyamide) of thickness 0.1‐0.3 mm

Membrane configurations

Material of this thickness cannot usually withstand high pressures (5‐90 bar). To solve this problem a number of different membrane configurations are developed.

The four most important configurations are :

Tubular

h ll fib hollow fibre

spiral wound

plate and frame

11

Membrane devices

For desalination mainly:

hollow fibres,

and

spiral wound

membrane devices are applied

The capacity of an element or permeator is between :

a few liters per day to about

125 m3 per day

12

13

Tubular Nanofiltration membranes

14

Hollow fiber

Hollow fiber membranes are asymmetric in structure.

Triacetate is mainly applied as material.

Diameter ranges

from 42 – 85 µm (inside – outside)

from 90 – 300 µm (inside – outside)

Only one manufacture is making these devices

Toyobo

replacement in existing plants.

15

Hollow fine fiber

Toyobo’s fibers are about three times thicker than a human hair and are made of cellulose triacetate.

Pressure forces the (pure) water through the fiber walls into the bore of the fiber.

One or two elements are placed in one vessel

16

Cross winded bundles

17

Toyobo elements up to: length 2 m, diameter 0.28 m

18

Toyobo hollow fiber element

19

Two elements in one vessel

20

Sea water RO plant with Toyobo elements

21

Spiral Wound

Flat membrane envelop is formed around fabric spacers and closed on three sides.

The open side terminates at a perforated product water tube.

The envelope or leaf, together with an external spacer for the feed water stream, is rolled spirally around the product tube.

O (20 ) l f t d ith thOne or more (20 or even more) leafs are connected with the product water tube.

The element is then installed in a pressure vessel

22

The feed water flows axially through the channels (external spacer) between the spiral windings.

Spiral Wound

Water permeates through the membrane and flows radial inside the membrane envelope towards the product tube.

Systems have up to seven elements within one pressure l i ( d l )vessel in an array (module).

23

24

25

26

Question

Why are there 25 leaves instead of one only?

27

28

29

30

31

32

Spiral RO and NF Element 18 inch (45 cm) and 1.50 m high

33

Spiral wound.exe

spiralwound.exe

34

“Fouling in RO and NF systems”



Introduction

Fouling and Scaling in Reverse Osmosis Plants

Many Reverse Osmosis Plants run smoothly

Many Reverse Osmosis Plants suffered from;

membrane fouling and/or

scaling;

Many Reverse Osmosis Plants (new and old) still suffer from;

membrane fouling and/or;

scaling

What may cause fouling in RO and NF?

Five different types of fouling are identified:

Particulate fouling,

• due to suspended and colloidal matter;

Inorganic fouling,

• due to Fe(II) and Mn(II)

Biofouling,

• due to growth of bacteria;

Organic fouling Organic fouling,

• due to organic compounds;

Scaling,

• due to precipitation of sparingly soluble compounds.

Effect of fouling and scaling

Fouling and scaling may manifest in three ways:

Clogging of cross winded fibres (Toyobo) or spacer spiral sound elements

Increase hydraulic resistance of the membrane due to deposition and/or adsorption of material on the membrane surfacesurface

Decrease in rejection due to concentration polarization in the foul layer.

Clogging

Toyobo – Cross Winding

Clogging bundle, spacer (I)

Results in:

Higher differential pressure (head loss) across the bundle or spacer resulting in:

lower net driving pressure

• which requires a higher pressure to maintain the capacity.

damage to elements due to

t l i i i l d• telescoping in spiral wound

• channeling in spiral wound

• squeezing spiral wound

• breaking fibers

Clogging bundle, spacer (II)

Local clogging may occur as well, which results in:

Uneven flow distribution, with places with low or no flow at , pall.

Resulting in locally:

high conversion

high concentration polarization

Resulting in:

deposition of particles;

local precipitation of sparingly soluble compounds;

growth and attachment of bacteria;

Pilot plant Water Supply North‐Hollandwith spiral wound elements (1979)

Pilot plant with spiral wound elements

Capacity: 10 m3/h

Stage: 4g

Elements per vessel: 4 (of 1 m length)

Type of elements: Spiral Wound,

cellulose acetate 10 cm in diameter

Feed water: Pre‐treated river/lake water

Development pressure drop in:first stage spiral wound pilot plant (1979)

100 kPa = 1 bar

Telescoping

“Channeling”

Source: Dr. P. Sehn Dow/Filmtec

“Squeezed element”

Increasing hydraulic membrane resistance

Increasing hydraulic membrane resistance

Results in:

higher required pressure to maintain capacity

or

lower capacity when pressure is not increased.

As a result conversion decreases (same feed flow but less product)

ibl ( l ) l j i hi h li i i possibly (not always) lower rejection, so higher salinity in product, due to increased concentration polarisation

increased cleaning frequency which may result in shorter lifetime of membranes

Fouling spiral wound elements First stage (1979)

100 kPa = 1 bar

Concentration Polarization

Feed/Bulk Solution BoundaryLayer

Permeate

(high pressure) (low pressure)

Concentration Polarization

Cb lk/f d

Cmembrane

Feed Flow

water water

Cbulk/feed

Cpermeate

Membrane

Concentration polarization

Concentration polarization will increase, when cross flow velocity close to the membrane will decrease. Due to:

uneven flow distribution

foul layer

As a result

dissolved salts and organic compounds;

ll id l colloidal matter;

suspended matter

will accumulate at the surface of the membrane

As a result

Concentration polarization

sparingly soluble compounds may/will precipitate;

colloidal and suspended matter will deposit;

rejection for salts may decrease; due to higher concentration t th b fat the membrane surface.

Chemical cleaning

Rule of thumb: Cleaning is recommended when:

Mass transfer coefficient (MTC) or normalized flux drops by 10%.

“Normalized” salt content (TDS) of product water increase by 10%.

Normalized feed channel pressure drops (feed pressure‐concentrate pressure) increases by 15%.

Wide range of chemicals are used.Wide range of chemicals are used.

Compatibility with the membrane has to be secured.

Manufactures have lists available and some sell cleaning agents.

“Particulate Fouling”



Particulate fouling

Suspended/colloidal, e.g.,

clay minerals;

organic materials;

coagulants, e.g., Fe(OH)3, Al(OH)3;

algae;

bacteria, as such (not growing);

Extra cellular Polymer Substance (EPS)

d/ T t E l P ti l (TEP)and/or Transparent Exopolymer Particles (TEP);

2

Filtration Mechanisms (I)

In “Dead end” and “Cross flow” membrane filtration, two main mechanisms are identified:

Pore blocking;

Cake formation

Pore blocking is subdivided in:

Complete blocking;

Standard blocking;

I t di t bl ki Intermediate blocking

Cake formation is subdivided in:

Cake formation without compression cake.

Cake formation with compression cake.

3

Filtration Mechanisms (II)

In “Dead end” membrane filtration:

All rejected particles present in the feed water will deposit on the membrane surface.

No escape!!

In “Cross flow” membrane filtration:

A part of the rejected particles will deposit.

P ti l t i th t ti ill l th d l Particles present in the concentration will leave the module.

4

Filtration Mechanisms (III)

Remark:

not all particles present in the water that passes the p p pmembrane will deposit.

“Shear force” of the water flowing along the membrane surface is responsible.

The higher the cross flow velocity the less particles will deposit.

5

In Ultrafiltration and Microfiltration

“Dead end”

Filtration Mechanisms (IV)

and

“Cross flow”

filtration are applied.

In Reverse Osmosis and Nanofiltration

“Cross flow” filtration is applied only.

filtration is applied only.

6

Filtration Mechanisms (V)

Complete Blocking

Each particle in water completely blocks one p p ypore, with no superposition of particles.

Standard Blocking

Each particle at membrane is deposited on the internal pore walls, decreasing the pore volume.

7

Intermediate Blocking

The probability that particles can settle on

Filtration Mechanisms (VI)

p y pother particles previously deposited and already blocking the pores or particles can directly block membrane area.

Cake Filtration

E h i l l h i lEach particle can settle on other particles previously deposited and already blocking the pores, but there is no room for particles to directly block membrane area. The cake might be compressible.

8

Flow through a R.O. membrane is usually described with:

Q = dV/dt = (ΔP ΔΠ) K A

Particulate fouling equation (I)

Qw = dV/dt = (ΔP ‐ ΔΠ) Kw. A

Where:

Qw = permeate flow, e.g., m3/h

V = total filtered volume water (permeate) (L or m3)

t = time (e.g., hour, minute, second)

ΔP = differential pressure (pressure feed ‐ pressure permeate)p (p p p )

ΔΠ = difference osmotic pressure

(osmotic pressure feed – osmotic pressure permeate)

Kw = permeability constant for water

A = surface area of the membrane(s) (m2)

9

Qw/A = permeate flow through membrane

surface area (m3/m2h)

Particulate fouling equation (II)

= filtration rate (m3/m2h), used in rapid sand

filtration

= Flux (L/m2h), used in membrane filtration

Qw/A = J (flux)

J = 1/A * dV/dt

(ΔP ‐ ΔΠ) = net driving pressure NDP

To simplify the equations we assume:

ΔΠ is negligible. (OK for low salinity waters)

So:10

J = 1/A * dV/dt = ΔP . Kw

l h f d d f

Particulate fouling equations (III)

Frequently the concept of resistance R is used, instead of

permeability

Kw = 1/ η . Rt

Where:

η = viscosity of the waterη y

Rt = total resistance is sum of resistance membrane (Rm),

pore blocking (Rp) and cake formation (Rc)

Rt = Rm + Rp + Rc

11

m p C

1 ΔPJ

R R R

Particulate fouling equation (IV)

When we assume that pore blocking does not play a dominant

role in RO, then fouling is mainly due to cake formation.

m p C

t

1 ΔP

R

As a consequence:

m C

1 ΔPJ

R R

12

c

I VR

A

Particulate fouling equations (V)

Where:

I = is a measure of the fouling characteristics of the water.

The value of I is a function of the nature of the colloids and is proportional to the concentration.

A

I c r

c = concentration colloids.

rk = resistance cake per mg cake per m2 membrane (mg/m2).

kI c r

13

Reverse Osmosis plants use to operate at constant capacity and recovery. So, the flux (J) is constant.

Particulate fouling equation (VI)

When membranes foul, the pressure has to be increased, in order to keep the capacity (and flux) constant.

Rewriting the basic equation in:

tΔΡ1J

t

m c

Jη R R

Where:

ΔPt = pressure at time “t”. (will increase)

= constant

14

Substitution:

V 1 dV

Particulate fouling equation (VII)

c

V 1 dVJ t because J constant

A A dt

I Vin:R I J x t

A

t

m

results in:

ΔP1J

η R I J x t

15

ΔPt = ∙Rm∙J + ∙I∙J2∙t

Particulate fouling equation (VIII)

So ΔPt is linear proportional with time and proportional with

(flux)2 or J2.

As a consequence flux has a very dominant effect on the

development of ΔPt.

16

Particulate fouling equation

Remark: The equation is valid for “dead end” filtration.

In “cross flow” filtration only a part of the particles will y p pdeposit on the membrane surface. This is due to the shear force of the cross flowing water.

So “I” has to be corrected with the deposition factor “β”. This factor is the fraction of particles which really deposit on the membrane surface.

β ≤ 1β ≤ 1

The equation will change into:

17

Particulate fouling equation

∆Pt = η∙Rm∙J + η∙β∙I∙J2∙t

18

This phenomenon explains why manufactures of spiral wound

element recommend:

Particulate fouling

“Lower design fluxes at higher fouling potential of

feed waters”

Recommended flux (J) for different feed waters

Surface water 14 – 24 L/m2.h

Well water

RO permeate

24 – 31

34 – 51

Remark: Surface water use to have the highest fouling potential

19

Parameters like suspended matter (mg/L), turbidity and particle counts are unreliable.

Fouling potential due to particles

For this purpose the SDI (Silt Density Index) is commonly applied as a measure for fouling potential due to particles.

It is measured with membranes with 0.45 µm pores.

Al(III) is measured when, e.g., alum is used as a coagulant. Should preferably < 10 µg/L .

M i i f ll i di id l ll id l dMeasuring concentration of all individual colloidal and suspended particles is “A mission impossible” that is why a “sum parameter” is applied.

20

Silt Density Index

21

History

The Silt Density Index has been introduced by DuPont/Company Permasep Products at the request of the Bureau of Reclamation (USA).

Initially, the test was named Fouling Index.

It was intended to characterize the fouling potential of feed water of DuPont's hollow fine fiber RO permeators(membrane elements).

Th t t t i t d d d ll id lThe target contaminants were suspended and colloidal matter.

22

History and present

Later on manufacturers of spiral wound elements and different hollow fibers elements recommended this test as well.

In addition manufacturers formulated maximum levels for SDI to minimize suspended and colloidal fouling and to obtain long‐term performance.

Today, SDI < 2 to 3 has been set as a requirement for the performance of MF/UF systems used as pretreatment forperformance of MF/UF systems, used as pretreatment for RO/NF.

23

Present

SDI is applied worldwide;

The “Desalination Society” believes that when meeting SDI recommendations are not compromised, no fouling in RO/NF systems, due to suspended and colloidal matter, will occur;

SDI h h f “ l i ” i di i SDI has the status of “ultimate parameter” in predicting fouling in RO/NF systems

However doubts are growing worldwide.

24

Silt Density Index

Silt density index (SDI) is commonly applied as a parameter for the fouling potential of feed water for Reverse osmosis and Nanofiltration plants

Based on the measurement of plugging a membrane filter having 0.45 �m pores at a pressure of 210 kPa (30 psi).

25

SDI/MFI equipment

26

Simple SDI Measurement Device

27

Silt Density Index

The measurement is taken as follows:

The time (t1) is noted which is required to filter the first 500 ml.

15 minutes (T) after the start of this measurement time (t2) is noted which is required to filter 500 ml (V).

The index is calculated with the following formula:

28

1 1

2 2

t t1- 1-

t tSDI 100 100

T 15

SDI measures:

What is measured with SDI?

SDI measures:

decline in filtration rate expressed in

“percentage per minute”

This observation follows from:

29

Where:

∆t1, ∆t2 is time required to collect a first and second filtrate volume of ∆V, e.g., 500 ml, respectively;

T = T is time starting to collect a second volume ∆V, e.g., 15,10, 5 minutes;

T= 0 time starting to collect a first volume ∆V.

30

Questions:

Silt Density Index

Questions:

Has SDI value measured in about 15 minutes, a maximum?

If so, what is the maximum value?

When the fouling potential is high then for T a shorter periodWhen the fouling potential is high, then for T a shorter period

has to be taken, e.g., 10, 5 and 2 minutes. Why?

31

Silt Density Index

1 1

2 2

t t1- 1-

t tSDI 100 100

T 15

32

Recommendations maximum preferably

Recommendation SDI

Recommendations maximum preferably

Hollow fine fiber 3 <1

(Dupont)

Hollow fiber 4

(Toyobo)

Spiral wound 4 – 5 <3Spiral wound 4 5 <3

33

Equipment for measuring is simple and cheap;

Silt Density Index

Procedure is simple and can be done by operators;

Method is applied worldwide.

34

It is well know that, even when the recommendations are not compromised, serious fouling may occur.

Silt Density Index

This might have two principle reasons:

Other type(s) of fouling occurred. However not are noticed, e.g., biofouling, inorganic and organic fouling, fouling due to corrosion products;

SDI has no direct predictive value in fouling RO/NF membrane systems. However, it is sometimes an indirect indicator for the fouling potential of RO/NF feed waters.

35

In addition this index shows several deficiencies, e.g.,

Silt Density Index

it has no linear correlation with colloidal matter concentration;

it has no temperature correction;

it is not based on any filtration mechanism, which makes modeling rate of fouling in R.O. systems impossible;

it makes use of membranes with pores of 0 45 μm only while it makes use of membranes with pores of 0.45 μm only, while pore in RO/NF membrane are approx. 0.001 μm.

36

SDI and formazine concentration

37

Silt Density Index

Further on:

Erratic results are reported with water supersaturated with air;

Different results are obtained with membranes from different manufacturers;

Relatively high values are reported in effluents of Micro‐ and Ultrafiltration systems;

38

Modified Fouling Index

39

Modified Fouling Index

Has been developed to overcome the four main deficiencies of SDI;

Has been derived from the Fouling Index (SDI);

Makes use of the same equipment (when flat membranes are applied);

Based on the occurrence of cake filtration during a distinct part of the test, since cake filtration is most likely, the dominant filtration mechanism in RO and NF as well.

40

Principles MFI

MFI has been derived from the Fouling Index (SDI);

Makes use of the same equipment and membrane filters of the same pore size;

Takes into account the observation that:

during the initial stage of the filtration pore blocking occurs;

pore blocking is being followed by cake filtration;

finally depth filtration and/or cake blocking occurs.

41

Principles MFI

Is based on the stage of cake filtration that occurs during a distinct period of the test.

m b c

dV PJ

A.dt R R R

Make use of the general equation describing cake filtration:

c p

V VR . .C .I

A A

42

Where:

J flux R resistance membrane

Principles MFI

J = flux; Rm= resistance membrane

V = total filtered volume; Rb = resistance blocking

t = time; Rc = resistance cake

P = pressure; Rt = total resistance

η = viscosity;

α = specific cake resistance;α = specific cake resistance;

Cp = concentration particles;

I = fouling index

43

Substitution and rearranging results in:

Principles MFI

m2

.R .Idt.V

dV P.A P.Aint egration,gives :

m2

.R .It.V

V P.A 2P.A

44

MFI is defined as the slope in the graph tg α, during cake filtration: t/V versus V and equals;

Definition MFI

filtration: t/V versus V and equals;

Under the conditions:

P = 200 kPa

A = 13.8x10‐4 m2

2

IMFI

2 P A

V = volume in L

t = time in seconds

η = viscosity at 20 oC

Temperature 20 oC.

45

This definition and conditions has been chosen since:

Definition MFI

MFI = 1 s/L2 is equivalent to approximately SDI = 1

Conversion of MFI into I (fouling index) results in:

I = 3.8x108 ∙ MFI (m‐2) or MFI = 0.26x10‐8 ∙I

46

tg α = MFI

47

SDI, MFI(0.45) and formazine concentration

48

.

Automatic MFI equipment

49

Predicting flux decline in RO and NF systems

50

Predicting rate of fouling in RO systems

Assuming cake filtration, the rate of pressure increase due to particles at constant flux, follows from :

Pt = η∙Rm∙J + η∙β∙I∙J2∙t

Where:

Pt = pressure at time t to maintain constant flux;

β = deposition factor;

Remark:

Effects of osmotic pressure, head losses, etc. are not incorporated

51

In “cross flow” filtration only a part of the particles will deposit on the membrane surface. This is due to the shear

Deposition factor β

force of the cross flowing water.

So a correction has to be made by introducing the deposition factor β. This factor β is the fraction of particles –present in water passing the membrane – that really deposits on the membrane surfacedeposits on the membrane surface.

β ≤ 1, deposition factor.

52

Question: What is the operation time to get 15 % increase in pressure, due to particulate fouling?

Example: Predicting rate of fouling in spiral wound RO elements

Assumptions:

‐ Filtration mechanism: Cake filtration;

‐ Required pressure 10 bar (clean membranes);

‐ Average flux equals 20 L/m2h;

‐ MFI = 1 s/L2;

‐ Deposition factor β = 1.

Answer: 150,000 days or 400 years

Remark: Flux in hollow fiber elements is 10 to 15 times lower, so, rate of fouling is 100 to 225 times lower.

53

MFI values in the range of 1 to 1000 s/L2;

Consequences

Can not explain significant rates of fouling in spiral wound RO systems due deposition of particles on the membrane surface.

Remark: MFI = 1000 s/L2 might result in pressure increase of 15 % in 150 days (assuming β = 1)

SDI values up to 5 and higher;

Can not explain significant rates of fouling of RO systems due deposition of particles on the membrane surface.

54

MFI as a function of pore diameter in pretreated surface water

Pore diameter (µm) MFI (s/L2)

0.8 4

0.4 60

0.2 200

0.1 1800

0.05 4500

MFI depends strongly on pore diameter of membranes

55

Particles much smaller in size than 0.45 μm, are responsible for membrane fouling due to deposition on membrane surfaces

Conclusions

membrane fouling, due to deposition on membrane surfaces.

As a consequence SDI and MFI(0.45) can not predict rate of fouling due to this type of fouling.

R.O. feed waters contain large amounts of small particles, resulting in high MFI values, measured with membranes with much smaller pores.

These smaller particles are responsible for this type of fouling;

56

Predictive value SDI and MFI(0.45)

57

It is very unlikely that SDI and MFI(0.45) have a predictive value in RO/NF membrane fouling, due to deposition of

SDI and MFI(0.45)

particles on the membrane surfaces;

Unless they are correlated with MFI‐UF (MFI measured with ultrafiltration membrane filters)

B h i h h dd d l i di i Cl i fBoth might have an added value in predicting Clogging of:

Membrane bundles of Toyobo’s hollow fiber membranes;

Spacers in spiral wound elements;

58

Clogging due to particles may occur in:

Clogging non‐woven fabric, fiber bundle and spacers

Fiber bundles in Toyobo’s cross‐wound hollow fibers. Openings: 30 µm

Spacers in spiral wound elements.

Openings: 500 to 1000 µm

59

Clogging

Clogging will cause increase in differential pressure across feed brine channels.

Resulting in:

Damage of hollow fiber and spiral wound membrane elements;

F li d li i d t l fl di t ib ti i Fouling and scaling in due to unequal flow distribution in elements.

60

Fouling spacer spiral wound element

Source: H. Vrouwenvelder

61

Modified Fouling Index – Ultrafiltration

62

Why: MFI – UF?

MFI(0.45) and SDI fail to predict rate of fouling R.O. membranes due to too low levels;

Using membranes with smaller pores results in much higher levels.

Remark: Pores of RO/NF membranes are 0.001 µm (< 1 nm)

Newly developed test MFI‐UF makes use of ultrafiltrationmembranes (MWCO 13 kDa PAN is a candidate to bemembranes (MWCO 13 kDa PAN is a candidate to be applied as a standard), having pores close to the pore size of RO and NF membranes.

MFI‐UF has linear correlation with colloidal/suspended matter concentrations.

63

MFI‐UF and dilutions with RO permeate; of tap water (Δ), membrane concentrate (□) & RO feed water (○)

Source: Boerlage

64

SDI shows several deficiencies, e.g.,

No linear relation with concentration of suspended and

Summary and Recommendations

colloidal matter;

No correction for temperature;

Is not based on any filtration mechanism;

MFI (0.45) is a superior alternative since it:

Sh li l ti ith t ti Shows linear relation with concentration;

Is corrected for temperature;

Is based of cake filtration mechanism;

65

Summary and recommendations

Both SDI and MFI(0.45) have no value in predicting rate of fouling, due particle deposition on RO/NF membrane surfaces.

Unless they are correlated with MFI‐UF values.

Bothmight have predictive value in clogging, Toyobo hollow fibers and spacers of spiral wound elements;

MFI‐UF potential application in explaining and predicting rate of fouling at membrane surface due to particles

66

Particulate removal from Surface Water

67

Surface water: river, lake, sea water

The quality of surface water shows large differences, e.g., suspended and colloidal matter (SDI) and many show large variations in time.

A limited number of sources (locations) has low fouling potential and need only cartridge filtration.

T di i ll f f i f dTraditionally pre‐treatment of surface water is focused on reduction of SDI.

68


The majority of surface waters needs additional treatment:

A great variety of pre‐treatment methods are applied, e.g.,

artificial recharge, e.g.,

• through shore wells/ beach wells (sea water) or infiltration canals/ponds (river water).

R k Wh l d i d d t d l SDIRemark: When properly designed and operated low SDI values (~1) can be achieved.

However low concentrations of iron might be present, resulting in more frequent replacement of cartridges.

69


Rapid sand filtration

Coagulation / sedimentation/ rapid sand filtration

Ultra‐ and microfiltration

70

Permasep Engineering Manual (Dupont) recommends for removal of particulate matter:


In any case cartridge filtration (5‐20 µm) just preceding the high pressure pump

SDI<6:

M di filt ti ( id d filt ti ) Media filtration (rapid sand filtration)

Dual media filtration (anthracite/sand)

71


6<SDI<50:

In line‐coagulation (direct filtration)

(addition of a coagulant to water, mixing, passing through media or dual media filter).

SDI>50:

Coagulation, sedimentation (or floatation), rapid sand filtrationfiltration.

72

Surface water; river, lake, sea water

Remark 1:

These pre‐treatment processes reduce SDI and in addition p pbiofilm formation potential (except cartridge filtration) significantly.

Remark 2:

Chlorination combined with neutralization with sodium bi lfi l li dbisulfite was commonly applied.

However, it turned out that chlorination produces large quantities of assimilable organic carbon AOC), causing serious biofouling.

73

Rapid Sand Filtration

74

Location: Dhekelia, Cyprus

Status: In operation 1997/1998

Sea Water Reverse Osmosis

Status: In operation 1997/1998

Source: Mediterranean sea water

Capacity: 40,000 m3/day

Conversion: 50%

Energy consumption at start: 4.7 kWh/m3

BOOT contract: $ 1.00/m3 – 0.70/0.80

75

Rapid Sand Filters

76

Coagulation / Sedimentation / Rapid Sand Filtration

77

Coagulation/Sedimentation/ Rapid sand filtration

Coagulation is the addition of Fe(III) or Al(III) salts to the water to be treated.

Ferric and Alum form flocks in the water Fe(OH)3 and Al(OH)3, these flocks make small particles (colloids / suspended) larger.

As a result the settling velocity improves.

Sedimentation will remove large particles (flocks) in settling b i l ifibasins or clarifiers

Rapid Sand Filtration is polishing by removing small flocks.

Cartridge filtration is usually the final polishing step.

78

Location : Trinidad (Caribbean)

Coagulation/Sedimentation/Dual Media Filtration

Source : Atlantic Ocean (Sea water)Capacity : 100,000 m3/dayStatus : In operation May 2002Pre‐treatment:

■ Coagulation / Sedimentation / Dual media filtration;■ Coagulation / Sedimentation / Dual media filtration;

■ Addition of ferric chloride (coagulant) and coagulant aid;

■ Cartridge filtration (5 µm);

79

Turbidity Raw Seawater

Source: J. Kenneth (Ionics)

80

SDI after Pretreatment

Source: J. Kenneth (Ionics)

81

Micro‐ and Ultrafiltration

82

Micro‐ and Ultrafiltration

It is able to produce water with very low SDI values independent of the raw water quality.

Micro‐ and ultrafiltration is an emerging technology in surface water (river, and sea water) and treated domestic waste water pretreatment.

I i li d f d d d i It is applied on surface water and treated domestic waste water or as polishing step after conventional pretreatment.

83

Ultrafiltration/Reverse Osmosis, Heemskerk

General

Location: Heemskerk, The Netherlands

Status: In operation since 1999

Source: Pre‐treated River Rhine water

Capacity: 2300 m3/h

System

Ultrafiltration: XIGA

Reverse osmosis: Spiral wound, ultra low pressure

84

Process Scheme, Heemskerk

IJssel Lake

ultrafiltration

coagulation

sedimentation

rapid sand filtration

ultrafiltration

reverse osmosis

neutralization

85

Type: capillary membranes

C XIGA

Ultrafiltration

Concept: XIGA

Housing: fit in standard vessels up to 6 m length

86

UF and RO Units of Water Supply North Holland

87

MFI: IJssel Lake (Rhine River) 50 – 200 s/L2

RO units

MFI: R.O. feed water 0.15 s/L2

RO membranes were cleaned after 5 years of operation.

88

“Fouling due to iron and manganese”



Groundwater

2

Inorganic fouling in groundwater treatment

Membrane fouling due to iron and manganese primarily happens in (brackish) ground water treatment.

Where does iron and manganese come from in groundwater?

Answer: These compounds appear due to interaction b d d ilbetween ground water and soil.

3

Origin of iron and manganese

Most soils, from which water is being abstracted, contain iron and manganese.

Most relevant minerals are:

ferric hydroxide Fe(OH)3;

hematite Fe2O3;

goethite FeOOH;

magnetite Fe3O4;

manganese dioxide MnO2

4

These minerals are insoluble in water.

However under anaerobic conditions they will dissolve.


y

Anaerobic conditions may occur in ground water due to bacteria, consuming oxygen for oxidation organic matter and ammonium.

Organic matter is frequently present in soils, consisting of di hi h i l hsediments, which is commonly the case.

This organic matter originates from trees and plants to form humic substances e.g., peat.

5


2 C10H18O10 + 19 O2 + bacteria → 20 CO2 + 18 H2O

C10H18O10 represents a simplified formula for humicsubstances

Ammonium originates from humic substances (including humic acids), since these compounds usually contain

iammonium.

6

Bio‐oxidation of ammonium

nitrosomonas

2 NH4+ + 3 O2 → 2 NO2

‐ + 4 H+ + 2 H2O4 2 2 2

nitrobacter

2 NO2‐ + O2 → 2 NO3

‐

‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐

2 NH4+ + 4 O2 → 2 NO3

‐ + 4 H+ + 2 H2O

7

Iron leaching

Iron is leaching under anaerobic conditions in the soil

4Fe(OH)3 + 8H+ ↔ 4Fe3+ + 4OH‐ + 8H2O( )3 2

4Fe3+ + 4OH‐ ↔ 4Fe2+ + O2 + H2O‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐4Fe(OH)3 + 8H+ ↔ 4Fe2+ + O2 + H2O

Bacteria oxidize organic matter and use oxygen from Fe(OH)3 as a consequence iron (III) is reduced to iron (II), hi h i ll l bl Thi li b t iwhich is very well soluble. This process supplies bacteria

with energy.

Remark: When oxygen is present, it will oxidize Fe2+ to Fe3+

which will precipitate as Fe(OH)3.

8

Manganese leaching

Manganese, Mn2+

Is leached out under anaerobic conditions.

6MnO2 ↔ 2Mn3O4 + 2O2

2Mn3O4 + 12H+ ↔ 6Mn2+ + O2 + 6 H2O

‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐6 MnO2 + 12H+ ↔ 6 Mn2+ + 3O2 + 6H2O

Bacteria consume oxygen from MnO2.

R k Wh i M 2+ ill b idi dRemark:When oxygen is present Mn2+ will be oxidized slowly to subsequently Mn3O4 and MnO2 and precipitate;

Both Mn3O4 and MnO2 insoluble and are black;

Other manganese minerals play minor roles.

9

Groundwater / beach‐wells

Many ground waters have very low concentrations of particles (low turbidity and SDI).

Concentrations of iron and manganese are typically low e.g.,

iron < 0.05 mg/l

manganese < 0.01 mg/l

The rate of membrane fouling is low.

10

Many other ground waters/ beach wells have high turbidity.

In most cases turbidity appears after aeration, when oxygen is

Groundwater / beach‐wells

introduced. Oxygen will oxidise Fe(II) ferrous, which is very well soluble into in soluble Fe(OH)3. Resulting in high turbidity and high SDI.

Commonly manganese (Mn(II)) is present as well, which will not oxidize after the introduction of oxygen, due to the low yg ,reaction rate.

11

Groundwater samples: Immediately after sampling and after aeration and 30 minutes. Baq’a, Amman, Jordan

12

Iron and manganese

Iron and manganese are dissolved in water as Fe(II) – ions (ferrous) and Mn(II) – ions (manganous)

In presence of oxygen, Fe2+ and Mn2+ are oxidized to insoluble Fe(III) (ferric), Mn(IV) forms namely:

Fe(OH)3 MnO2

I b f ( bi di i ) In absence of oxygen (anaerobic conditions)

4Fe2+ + O2 + 10H2O ↔ 4Fe(OH)3 + 8H+

6Mn2+ + 3O2 + 6H2O ↔ 6MnO2 + 12H+

13

Iron and manganese in ground water

Oxygen presentNo Iron

No manganese

Unsaturated zone

Oxygen presentNo Iron

No manganeseSaturated zone

Groundwater level

No OxygenIron present

Manganese presentSaturated zone

14

Iron and manganese

Two zones in the soil are identified namely

Unsaturated (with air (oxygen) and water);

Saturated;

Water in unsaturated soil always contains

Oxygen

Water in upper layers of saturated zone contains

usually oxygen

Deeper layers not, e.g., due to oxidation of organic material.

15

Many ground waters contain iron (and manganese) and several don’t.

Iron and manganese

Whether iron (and manganese) is present or not, is governed by:

Presence of oxygen in the ground water

Presence of iron and manganese in soil

Remark: In general iron and manganese oxides are present in the soilpresent in the soil.

Oxygen plays a dominant role.

Iron and manganese concentrations tend to increase in course of time.

16

Fouling is due to:

oxidation of iron (II) in water to iron (III) followed by

Membrane Fouling due to iron and manganese

formation of insoluble Fe(OH)3, that will form flocs. These flocs might deposit in the RO/NF elements. Spacers and/or membrane surface;

adsorption of soluble iron (II) on membrane surface and spacers, and subsequent oxidation to iron (III), forming, a dense layer of insoluble ferric hydroxide (Fe(OH)3);

adsorption of soluble manganese (II) on membrane surface and spacers and subsequent oxidation to the insoluble Mn3O4 → MnO2 that forms a dense layer.

17

Rate of iron (II) oxidation with oxygen

Rate of oxidation of iron (II) by oxygen depends on:

pH ;

• the lower the pH, the lower the rate;

• the higher the pH, the higher the rate;

Oxygen concentration.

18

Sensitivity of Iron oxidation kinetics

19

Rate of manganese (II) oxidation with oxygen

The rate oxidation manganese oxidation is negligible at pH values below 9.

A catalyst, in the form of MnO2 and/or Mn3O4, is needed to speed up the rate of oxidation. This catalyst forms on the membrane surface in course of time.

B l H 6 9 h f id i ( i h l ) i Below pH 6.9 the rate of oxidation (even with catalyst) is very low.

20

Membrane fouling due iron

As long as no oxygen is present, no fouling will occur, since iron and/or manganese are not oxidized.

As soon as oxygen is introduced iron (II) will be oxidized and fouling starts;

Fouling mechanisms are:

formation of flocks of Fe(OH)3, resulting in high SDI;

adsorption of Fe(II) on membrane surface/spacer and subsequent oxidation of the adsorbed Fe(II) to Fe(III)→subsequent oxidation of the adsorbed Fe(II) to Fe(III) → Fe(OH)3.This creates new surface area for adsorption Fe(II).

In this process a dense layer of Fe(OH)3 is being formed with a high hydraulic resistance.

21

Manganese is oxidized with oxygen at an very slow rate if pH is below 9.

Fouling due manganese

The fouling mechanism is:

adsorption of Mn (II) on the membrane surface/spacer;

slow oxidation of the adsorbed Mn (II) to subsequently Mn3O4

and MnO2. forming new surface area with high adsorption capacity and catalytic properties.

It forms a dense layer with a high hydraulic resistance It forms a dense layer with a high hydraulic resistance

Remark: This is an autocatalytic process. As consequence to fouling starts slowly and speeds up gradually.

22

Rate of oxidation Fe (II) and Mn (II)

Source: Stumm & Morgan

23

Iron Fouling

Source: Dr. P. Sehn Dow / Filmtec

24

Iron and manganese fouling

Source: Dr. P. Sehn Dow / Filmtec

25

How to avoid fouling due to iron (II) and manganese (II)

Four options:

1. Abstract water that does not contain any iron or manganese.

2. Abstract water that does not contain any oxygen and exclude oxygen.

Remark: Several plants apply this option successfully.

3. When oxygen enters the system. Lowering the pH might be useful, to such a level that the rate of oxidation is low.

Remark: This approach might require large amounts of acidRemark: This approach might require large amounts of acid.

4. Abstract water (does not matter whether oxygen is present or not).

5. Treat the water by e.g., aeration followed by rapid (green) sand filtration.

26

Feed water abstracted from layers with oxygen (and consequently no iron and manganese) will not cause

Controlling membrane fouling due to iron and manganese

membrane fouling.

Feed water abstracted from layers without oxygen (and consequently iron and manganese present) will not cause membrane fouling.

Condition: Oxygen must be excluded completely from entering the feed water in the well and plant

1mg Fe(II) needs 0.14 mg O2

1mg Mn(II) needs 0.29 mg O2

Because iron and manganese need very little oxygen to oxidize.

27

Iron and manganese membrane fouling

Feed waters abstracted from layers with and without oxygen, in one well, will cause severe membrane fouling (and well clogging).

Because water with oxygen and no iron and manganese will mix in the well with water without oxygen and with iron and manganese.

The same situation occurs when water is abstracted anaerobe and oxygen is introduced e.g., in a storage tank.

As a result, dissolved iron and manganese will be adsorbed on the membrane surface/spacer and subsequently oxidized to:

F (OH)• Fe(OH)3• MnO2 (and Mn3O4)

Forming a dense layer with a high hydraulic resistance.

Remark: Fe(OH)3 forms flocks as well in the water.

28

Iron and manganese removal

29

Removal Iron and Manganese (1)

Aeration followed by rapid sand filtration is commonly and successfully applied to remove iron and manganese.

Several plants apply pre‐chlorination to enhance oxidation of Fe (II) and Mn (II).

Intermittent dosing potassium permanganate is applied as well to enhancing the oxidation of Mn (II) that is adsorbed on the surface of filter media.

Remark Chl i ti lt i f ti f i il blRemark: Chlorination results in formation of assimilable(biodegradable) organic matter.

Natural organic matter (humic substances) is oxidized to smaller organic compounds, which are food for bacteria.

30

Iron and Manganese Removal (2)

31

Polishing with Cartridge filtration (1)

Commonly cartridge filtration is applied as:

polishing step after pre‐treatment with e.g.,

rapid sand filtration;

main pre‐treatment in groundwater, when iron and manganese concentrations are very low.

protection of the high pressure pumps against sand. Originating from e gOriginating from e.g.,

• wells;

• rapid sand filter (damage filter nozzles)

32

Cartridge with pores ranging from 100 µm down to 1 µm are applied.

Polishing with Cartridge filtration (2)

In practice mainly 5 –20 µm cartridges are used.

Replacement frequencies vary from “once per week to once per year” depending on the water quality.

33

Rapid sand filter

34

Cartridge filter; Gran Canaria

35

Cartridges

36

Summarizing

Many ground waters contain iron (II) and manganese (II);

Some ground water don’t contain these compounds;g p ;

Iron (II) and manganese (II) appear in anaerobic ground water;

Membrane fouling can be controlled by:

abstracting strict anaerobic groundwater and keeping it strictly anaerobic; Lowering the pH will reduce the rate of fouling;

removing iron (II) and manganese (II) by aeration followed by removing iron (II) and manganese (II) by aeration followed by rapid (green) sand filtration.

37

“Organic fouling”



Organic contaminants

We distinguish three categories;

Natural contaminants;

Pollutants introduced due to human activities;

Introduced organic compounds by the process itself.

2

Natural contaminants

A wide range of natural contaminants is present in groundwater, surface water seawater and waste water. This type of organic matter carries the name Natural Organic Matter (NOM).

It is measured as Dissolved Organic Carbon or Total Organic Carbon with a Total Carbon Analyzer and expressed as mg DOC/L.

Recently different types of natural organic compounds can be measured with the: “Liquid Chromatography‐ Organic Carbon Detection” Method developed by Dr. Huber : Bio polymers; Humics; Building blocks; Acids; Neutral compounds with low molecular mass.

3

Liquid Chromatography‐Organic Carbon Detection (LC‐OCD)

Molecular mass in Dalton

Biopolymers (>>20,000 Da)

Humics (0 – 20,000 Da)

Building blocks (300 – 500 Da)

Acids (<350 Da)

LMW neutrals (<350 Da)

Biopolymers = Proteins + Polysaccharides

4

Typical chromatogram of NOM in surface water

5

Source: DOC‐Labor Dr. Huber

Biopolymers

A special category biopolymers are Transparent Exopolymer Particles (TEP) are:

invisible;

very sticky (glue);

present in sea water, waste water and rivers; (up to several mg/L)

Growing evidence that these “Transparent Exo polymer Particles” (TEP), which vary in size up to 100 – 200 µm, may cause serious membrane fouling in RO NF UF and MFcause serious membrane fouling in RO, NF, UF and MF

Directly (organic particulate fouling )

and

indirectly by inducing and/or causing biofouling

6

Transparent Exopolymer Particles (TEP)

Appear in many forms including:

amorphous blobs;

clouds;

sheets;

filaments;

debris from broken plankton

Also are produced from gelatinous mucous envelopes disurrounding:

bacterial cells

Diatoms

various other algae

7

GUM XANTHAN ALCIAN BLUEXanthan TEP

TEP Staining using Alcian Blue

Anionic carboxyl group

100 µm

Insoluble precipitate TEP’s of different origin bind differently with Alcian Blue

Staining power of Alcian Blue varies ‐ standardization required!

TEP concentration in terms of mg Xanthan equivalent per liter (mg Xeq.L‐1) Xanthan is used for calibration.

µ

8

Feed Concentrate

Biofilm initiation by TEP on membrane

Biofilm

Permeate

Adapted from Berman & Holenberg, 2005

9

Organic coagulant aids may adsorb on membrane surface e.g.,

Cationic polymer C573 adsorbs strongly on the old Permasep

Introduced organic compounds

Hollow Fine fibres

Cationic polymers may combine with antiscalant (poly acids) to form sticky mucous layers.

Strong indications that some antiscalants cause fouling (sticky)

membrane surface;

spacers;spacers;

This is due to:

• their nature;

• ‘over’ dosing or poor mixing.

10

Antiscalant fouled membrane

11

Antiscalant fouled spacer

12

Pollutants

Oil compounds discharged in:

sea water;

river/lake water;

domestic waste water;

(ground water);

May cause serious fouling in RO and NF membranes

Remark: Not much information available.

13

Removal organic foulants

Coagulation/Sedimentation/Rapid Sand Filtration

Is rather effective, however coagulant aids deserve special attention.

Artificial recharge/Beach and shore wells

Are very effective due to the very low rate of filtration and long residence time.

Ultra and Microfiltration are:

Very effective into the removal of a substantial part of: Very effective into the removal of a substantial part of: Transparent Exopolymer Particles (TEP) due to their size.

Remark: Not much information available. Research is ongoing.

14

“Biofouling and pre‐treatment”



Biofouling

Biofouling in R.O. and NF systems is caused by bacteria.

Bacteria are everywhere.y

Even in safe drinking water, many harmless bacteria can be present. Up to millions per ml.

Bacteria have the tendency to adhere to the surfaces, including membrane surfaces.

Bacteria produce/make “extra cellular polymeric b ” (EPS)substances” (EPS).

2

These are composed of:

polysaccharides;

Biofouling

proteins; glycoprotein's; lipoprotein’s. other macromolecules.

Extra cellular polymeric substances are outside the cells and act as “glue”. g

There are numerous types of bacteria in water and numerous types of EPS;

Bacteria are small in size 0.1‐10 µm;

Practically all membrane systems carry biofilms.3

Question: Do the bacteria, present in feed water of RO and NF systems cause membrane fouling?

Answer:

When bacteria don’t multiply in membrane elements they p y yare not able to cause membrane fouling.

Reason is that bacteria in water are too small in number:

to cause clogging of

• spacers in spiral wound elements;

fib b dl i ill b l (T b )• fibre bundles in capillary membrane elements (Toyobo);

• (non) woven fabric in hollow fine fibre membrane elements (Dupont).

to form layers on the membrane surface, with a significant hydraulic resistance.

4

Bacterial growth

Living bacteria need nutrients to survive.

Nutrients needed for respiration is the minimum they need.p y

When more nutrients are available, they will multiply until:

number of bacteria

and

available nutrients e.g., per day are balanced.

So when excess of nutrients is available they will increase in bnumber.

When lack of nutrients exist they will reduce in number.

5

Bacteria easily form thin layers on

spacers;

Biofilm, biofouling

(non) woven fabric;

membrane surface

This process is termed:

Biofilm formation

defined as accumulation of bacteria, including EPS on f d h d hsurface due to attachment and growth.

Accumulation of biomass/biofilm formation to such a level that operational problems occur, is termed:

Biofilm

6

Biofilm

7

Source: Harvey Winters

8

Biofilm

9

Operational problems

Biofouling will results in:

Fast increase in:

• pressure drop across the elements (ΔP feed‐brine channel) (spacer, bundles, (non) woven fabric) resulting in decrease of Net Driving Pressure.

Fast decrease in:

• mass transfer coefficient (permeability) of the membranes (higher feed pressure required).

10

Operational problems

Increase in:

salt passage due to concentration polarization in the biofilm(higher salinity in permeate)

Increased risk of :

scaling due to concentration polarization in biofilm

11

Autopsies of 45 membrane elements from 15 different (pilot and full

scale) plants showed:

Biomass in membrane elements

Total Direct Count (TDC)

Heterotrophic Plate Count (HPC)

Adenosine Tri Phosphate (ATP)

5 x 106 ‐ 2 x 109 cells/cm2

1 x 103 ‐ 3 x 107 CFU/cm2

3 – 45,000 pg ATP/cm2

Membrane elements taken from plants suffering from biofoulingMembrane elements taken from plants suffering from biofouling

demonstrated:

Biomass concentration > 1000 pg ATP/cm2

12

Increase in normalized pressure drop and ATP on membranes


13

Biomass in membrane elements

Total Direct Count:

number of bacteria (dead or alive) counted with microscope ( ) pper cm2 membrane surface

Heterotrophic colony forming units Plate Count:

(CFU) on special medium (alive) Biomass scraped from membrane surface.

Adenosine a measure for the Triphosphate:

amount of active biomass. (essential component in living bacteria)

14

What causes biofouling?

The fundamental reason for bio fouling is the presence of nutrients in the feed water.

Bacteria in water need organic matter to survive and multiply.

Only a very small part of organic matter in water can “be eaten”, is assimilable.

This part is termed:

i il bl i (AOC) assimilable organic matter (AOC), or

biodegradable dissolved organic carbon (BDOC)

Ammonium (NH4+) can be used as well.

15


Nutrients are present in:

raw and treated domestic/industrial waste water (high concentrations)

river water, due to discharge of waste water and growth of algae

seawater, due to discharge of waste water, river water and growth of algae

Remark: Some seawaters have very low concentrationsRemark: Some seawaters have very low concentrations

groundwater, due to presence of NH4+

Remark: Ground waters containing O2 have a very low concentrations ammonium.

16

Nutrients can be introduced by:

Contaminated acid e.g., sulphuric acid, hydrochloric acid due


to:

• not sufficiently cleaned, tanks, trucks;

acid already used elsewhere

• e.g., pickling process;

antiscalants, which are partly biodegradable

Remark: Some antiscalants contain substantial concentrationsRemark: Some antiscalants contain substantial concentrations of biodegradable compounds

17

Assimilable organic compounds formed by chlorination of feed water.


Chlorine reacts with natural organic matter (NOM) e.g., humic acids to form assimilable/ biodegradable organic carbon.

Chl i l h i id l l i ll iChlorine cuts large humic acid molecules in smaller pieces, which are assimilable (can be “eaten”) by bacteria e.g., organic acids, aldehydes

18

Humic Acid

19


Hydrogen Sulphide (H2S) is commonly present in anaerobic ground waters (oxygen is absent).

Under aerobic conditions (O2 is present) bacteria use H2S as an excellent nutrient and may grow very fast. Causing serious bio fouling.

20

Measuring

Assimilable Organic Carbon

AOC

21

AOC determination

The AOC determination is based upon the growth curve of the bacteria Pseudomonas fluorescens strain P17 and NOX.

Initial concentration of the added bacteria is about 1000 CFU per ml.

Growth curves of the organisms are derived from periodic lcolony counts.

Test is calibrated with acetic acid.

22

AOC is expressed as µg acetate Carbon/L.

AOC determination

Test is laborious, may take several weeks and is costly.

Only specialized laboratories are able to do this test reliably and accurately.

Only applicable for research purposes

23

The BDOC determination is based upon the degradation of dissolved organic carbon.

BDOC determination

A mixture of selected bacteria is added.

DOC concentration is measured during several days/weeks.

The final reduction in DOC equals the BDOC in mg/L.

Test is laborious may take several days/weeks and is costly.

Only specialized laboratories are able to do this test reliably and accurately.

Only applicable for research purposes

24

Effect of Chlorination

on

Biofouling

25

Chlorination

Biofouling can effectively be controlled by chlorination of the feed water.

Chlorine in the form of OCl‐ and preferably HOCl, must be present in the water entering the membrane elements.

Unfortunately

currently most successful thin film composite spiral wound membranes don’t tolerate chlorine (OCl‐ and HOCl);

in the past most successful polyamide hollow fine fibreelements (Dupont) don’t tolerate chlorine as well;

26

Polyamide hollow fine fibre and thin film composite spiral wound membranes exposed to chlorine will demonstrate:

Chlorination

reduced salt rejection (increased salt passage)

and

increased mass transfer (increased permeability)

Cellulose di‐ and tri acetate membranes (Toyobo) tolerate hl ichlorine.

However this type of membranes has a limited market share. Mainly in seawater RO.

27

To protect thin film composite and polyamide hollow fine fiber membranes against chlorine, sodium bisulphite

Chlorination

solution is added.

Bisulphite neutralizes chlorine

Cl2 + H2O + HSO3‐ → 2Cl‐ + SO4

2‐ + 3H+

This approach has been applied commonly. However severe bi f li d i lbio fouling occurred in most plants.

In 1984 Van der Kooij demonstrated that chlorination results in the formation of assimilable/ biodegradable organic carbon.

28

In 1989 Applegate published a paper dealing with biofoulingdue to pre‐chlorination

Chlorination

Several papers have been published dealing with successful elimination or reduction to intermittent chlorination

Today many plants still apply continuous “Chlorination/ bi l h li i ”bisulphate neutralization” to protect:

thin film composite membranes

29

Several plants

Chlorination

Lost

or

damaged their membranes severely due to failure of sodium bisulphite dosing equipment.

30

Al Zawra Sea Water RO plant in Ajman, United Emirates

Effect continuous and discontinuous chlorination

Capacity 4500 m3/day;

Availability:

• at continuous chlorination: 75% Due frequent cleaning because of biofouling;

• after change to discontinuous chlorination > 98%

Discontinuous chlorination exists of:

twice a week for 6 to 8 hours at a residual chlorine level of 1 mg/L.

Source: A.B. Hamida & I.Moch (1995)

31

Pressure drop across Hollow Fiber permeators with chlorination

32

Pressure drop across Hollow Fiber elements with intermittent chlorination

33

Intermittent Chlorination

Intermittent chlorination combined with sodium meta bisulfite (SMBS) neutralization is gaining ground.

1 ‐ 2 mg/L during 0.5 ‐ 1 h one or two times per day is successfully applied in seawater RO to control marine growth (shell fish/coral) in intake structures/ pipes.

Thi h i i i bi f li iThis approach minimizes biofouling in:

thin film composite;

cellulose triacetate

34

Intermittent chlorination/ bisulfate neutralization

Intermittent Chlorination

1 ‐ 2 mg/L during 0.5 ‐ 1 h one or two times per day might be useful in seawater RO to control marine growth (shell fish/coral) in intake structures/ pipes.

This approach will minimize biofouling in:

thin film composite;

cellulose tri acetate

35

Effect of Antiscalant

on

Biofouling

36

Antiscalants

Different organic compounds are applied as antiscalant e.g.,

Polycarboxylic acids;

Polymaleic acids;

Polyacrylic acids;

Phosphonates.

These products are brought on the market under different brand names.

S f th d t t d l l t bi Some of these demonstrated clearly to cause severe bio fouling e.g., RO pilot plant study.

Vrouwenvelder measured AOC of different antiscalantproducts.

37

RO pilot plant Amsterdam Water Supply

Equipped with Toray spiral wound elements

3 stages, 10 m3/h capacityg , / p y

Experiments with two different antiscalants

Ropur RPI 2000

Flocon 100

Both resulted in fast decline of:

MTC (Mass Transfer Coefficient)

or

Permeability membrane (Kw)

38

0.150

0.175

1 0

1.2

Cleanings

MTC of the AWS RO pilot plant (early experiments)

0.050

0.075

0.100

0.125

MT

C (

gsfd

/psi

)

0.4

0.6

0.8

1.0

MT

C (

10-8

m/s

.kP

a)

Stage 1

Stage 2

Stage 3

0.000

0.025

0 50 100 150 200 250 300 350 400 450 500 550 600 650 700 750

Operation Time (Days)

0.0

0.2

Ropur RPI 2000 Flocon 100 No Scale inhibitor Flocon 100

39


AOC content of some commercially available antiscalants

40

Effect Pre‐treatment Processes

on

Biofouling

41

Pretreatment Process (I)

In reverse osmosis and nanofiltration a variety of pretreatment techniques are applied. e.g.,

cartridge filtration (1‐25 µm);

rapid sand filtration (0.8 – 1.2 mm);

dual media filtration (2 –3 mm/ 0.8 – 1.2 mm);

coagulation / sedimentation / filtration

42

Pretreatment Process (II)

Beach wells are used as a pre‐treatment for sea water as well;

Ultra filtration is gradually introduced;

Addition of monochloramine in feed water with a high bio fouling potential is introduced;

Chlorination – sodium bisulphite neutralisation is applied to control main growth.

43


44

The main purpose of these techniques is to remove suspended and colloidal matter to ensure a low SDI value


(except for chlorination).

Rapid sand filtration/Dual media filtration are able to reduce the bio fouling potential substantially.

Fl i d d h h bi f li f i Flemming demonstrated that the bio fouling of river water treated by flocculation and sedimentation was significantly reduced by rapid sand filtration (RSF).

45

Before RSF After RSF

Rapid Sand Filtration (RSF)

Biofilm on RO membranes 27 µm 3 µm

Permeate production after 10 days 65% 95%

BDOC feed water 0.32 mg/L 0.12 mg/L

Source: H. C. Flemmingg

46

Coagulation

Sedimentation


and Ultrafiltration

47

Coag./Sed./RSF and UF

The effect of

coagulation/ sedimentation/rapid sand filtration

ultrafiltration;

has been demonstrated by Water Supply North Holland with Ijssel lake water. This water originating from the Rhine River and was treated subsequently by the above mentioned processes.

The graph shows a large reduction in AOC from > 100 down to 16 µg/L Ac.C/L by coagulation/ sedimentation/ RSF and further reduction by ultra filtration down to 7 µg/L Ac.C/L.

48

Remark:

A full scale R.O plant capacity 2000 m3/h operating with this

Pre‐treatment

p p y / p gpre‐treatment was running about 5 years without any need for membrane cleaning.

Process scheme:

IJssel Lake

Coagulation

S di t ti Sedimentation


Ultra Filtration

Reverse Osmosis (ULPRO)

49

20

> 100

Effect Coag/Sed/RSF, UF and ULPRO on AOC

16

7

10

15

AO

C (

µg

Ac-

C e

q/L

)

3

0

5

IJssel Lake Feed UF Feed ULPRO Product ULPRO

50

Feed water UF is treated lake water after coagulation, sedimentation and rapid sand filtration.

Feed water ULP‐RO is after ultrafiltration.

ULP RO is Ultra Low Pressure RO.

51

Beach Wells

52

Beach wells are expected to reduce the bio fouling potential to a very low level.

Beach Wells

Under the condition that oxygen is present in the abstracted water.

This is because the beach (sand) acts as a filter, running at a very low rate of filtration and as a consequence a long residence time.

Oxygen is required since bacteria need oxygen to oxidiseOxygen is required, since bacteria need oxygen to oxidiseorganic matter.

Cartridge filters don’t perform well in reducing the bio fouling potential. The residence time is too short to enable bacteria to do their job.

53

Beach wells with capacity 8,000 ‐ 20,000 m3/day

54

Ranney Radial Well

55

Large scale ultra filtrationas

pre‐treatment

b d hcombined with DBNPA

56

Ultra filtration is gradually gaining ground as pretreatment technique prior to RO, processing surface water and treated d i

Pre‐treatment Ultrafiltration

domestic waste water;

It is capable of handling water with varying water quality, ensuring SDI values below 3;

It is reducing bio fouling potential as well.

However, additional measures are usually required e.g., intermitted dosing mono chloramine or other biocides e.g., DBNPA

The largest Ultra filtration plant as pretreatment for RO has been installed in Sulaibiya, Kuwait (2004)

Capacity 370,000 m3/day (18,000 m3)

Source: Treated Domestic Waste Water

57

Sulaibiya: Water source and DBNPA dose

Treated domestic waste water;

High COD;

BOD/COD = 0.5

DBNPA dose

12 mg/l for 30 minutes every 2 to 3 days

58

Sulaibiya WWTP reuse scheme

Source: F. Knops

59

Sulaibiya Ultrafiltration plant ‐ 370,000 m3/day

Source: F. Knops

60

2,2‐dibromo‐3‐nitrilopropionamide

Br

DBNPA

H

H

N

O

CC

C

Br

Br

N

CAS # 10222‐01‐2EC # 233‐539‐7MW = 242Formula: C3H2Br2N2O

Oquick kill efficacy at low ppm levels

rapid decomposition to non‐toxic end products, i.e., CO2, NH3

and Br–

easily rejected by thin‐film composite RO membranes

non‐oxidizing ORP of 540 mv at 20‐ppm concentration

61

RO plant Sulaibiya, Kuwait Capacity ‐ 18,000 m3/day

Source: Dow

62

DBNPA injection every 3 days results in CIP every 7 weeks

Source: Dow

63

Source: Dow

DBNPA injection every 3 days results in CIP every 3 weeks

64

Deactivation by Sodium Bisulfite

2 moles of NaHSO3 per 1 mole of DBNPA

Source: Najmy (Dow)

Impact of Residual NaHSO3 on DBNPA Dosage

4

6

8

10

ual

DB

NP

A(p

pm

)

0.2 ppm

0.5 ppm

1.0 ppm

NaHSO3

0

2

0 2 4 6 8 10 12

Assumed DBNPA (ppm)

Act

u

65

“Scaling”



Scaling

Sparingly soluble inorganic compounds present in the feed water, increase in concentration in the brine (concentrate)

Precipitation may occur when the solubility will be exceeded

Examples:

calcium carbonate ‐ calcium fluoride

calcium sulfate ‐ calcium phosphate

silica (SiO2) ‐ barium sulfate

strontium sulfate

2

Na Ca Mg K Ba Sr

Solubilities of salts in pure water (18 oC) in g/L

Na Ca Mg K Ba Sr

Cl 360 730 560 330 370 510

SO4 170 2 350 110 0.002 0.11

NO3 840 1220 740 300 90 70

CO3 190 0.013 1 1080 0.02 0.011

F 45 0.016 0.076 930 1.6 0.1

3

Concentration factor

Ions concentrate in the brine.

Concentration Factor C

where:

Cc = concentration in concentrate (brine)

Cf = concentration in feed water

)f1(R1CF

f

c

CCCF

where: f = rejection

R = recovery

If rejection f = 1 (100%):

R1CF

R11CF

4

Recovery * Concentration Factor (CF)

Concentration factor

50 %

75 %

80 %

90 %

2

4

5

10

*Concentration Factor (CF) is calculated assuming that Salt Rejection (f) = 1

5

10

Concentration factor at 100% retention

What is scaling ?

Scaling:

Deposition of sparingly

4

p p g ysoluble compounds on membrane surface

0% 50% 75% 90%

1

2

4

conversion

6

RO systems comprise of:

Scaling in the last stage/element

one, two or three stages

e.g.,

8 vessels in parallel recovery 50 %

or

8 vessels – 4 vessels recovery 75 %

or

8 vessels – 4 vessels – 2 vessels recovery 85 %

7

Scaling in the last stage/element

The higher the recovery, the higher the concentration of salts and sparingly soluble compounds.

R11CF

Since is increasing

So scaling use to occur dominantly in the last stage (and last elements, in which the highest recovery exists)

Graph shows MTC (= Kw, normalised permeability) in two stages of a pilot plant, with recovery 80 %, scaling due to BaSO4.

8

Feed water: Pretreated lake water

Scaling in RO pilot plant

Type of membranes: spiral wound

Number of stages: 2

Recovery: 80 %

Antiscalant: no!

Supersaturation: barium sulfate9

Saturated:

water is just saturated with a compound (salt);

Definitions

Not more can dissolve

Under Saturated:

water can dissolve more than present.

Super saturated:

water contains more than can dissolve;

Sooner or later a part of the compound (salt) will precipitate.

10

0.16

121

2 3 6

7

4

MTC of stage 1 and stage 2 of RO pilot plant (numbers refer to cleanings)

0.08

0.12

MT

C (

gsfd

/psi

)

4

6

8

10

MT

C (

10-9

m/s

.kP

a)

Stage 1

Stage 2

Flux 13.9 gsfdFlux 17.5 gsfd (29.7 L/m

2.h)

2 3

5

64

0.00

0.04

0 50 100 150 200 250 300 350 400 450 500 550Operating Time (Days)

0

2

(23.6 L/m2.h)

Flux 17.5 gsfd (29.7 L/m .h)

11

Scaling BaSO4

12

CaCO3 scaling

Source: Dr. P. Sehn Dow/Filmtec

13

Scaling potential

When water is supersaturated with one or more compounds, scaling is expected to occur.

Calculation methods to establish whether compounds become super saturated in RO/NF systems are available.

However rather complicated and time consuming.

Several computer programs are available. In this course the “4Aqua” program will be demonstrated.

14

How to avoid scaling?

not exceeding the solubility of any compound

Remark: This approach likely limits the recovery to a large extent, which results in higher pretreatment and energy cost and wastage of water

dosing an acid to eliminate super saturation

Remark: This only applicable for calcium carbonate.

dosing antiscalant (in combination with acid)

Remark: Antiscalant allow significant supersaturation of specific sparingly soluble inorganic compounds

15

Saturated, Under Saturated, Super Saturated

Determining whether a compound e.g., calcium sulphate is Saturated, Under Saturated or Super Saturated is complicated.

Simply adding together the calcium concentration and sulphate concentration and comparing with solubility of CaSO4 is completely wrong .

because:

Concentration of calcium and sulphate are in general not Concentration of calcium and sulphate are in general not matching.

Or calcium is in excess or sulphate is in excess.

Solubility depends on temperature

Solubility depends on presence of other ions (salinity).16

Saturated, Under Saturated, Super Saturated

Nernst discovered in 1889 that solubility is governed by the:

Solubility Product Principley p

In a saturated solution of e.g., CaSO4 the solid is completely ionised in water.

CaSO42‐ → Ca2+ + SO4

2‐

Solid not in water ions in water

The ionic product in a Saturated solution is called “Solubility Product” and is constant.

It is the product of:

Concentration Ca2+ × concentration SO42‐

expressed in mol/L.

17

e.g., [Ca2+] x [SO42‐] = Ksp(CaSO4)

Solubility Product

[Ca2+] = calcium concentration in mol/l

[SO42‐] = sulphate concentration in mol/l

For CaF2 the formula is

[Ca2+] x [F‐]2 = Ksp(CaF2)p

Ksp = solubility constant (at constant temperature and salinity).

Ksp is different for different compounds.

18

When the “ionic product” e.g., [Ca2+] [SO42‐] is:

Higher than Ksp(CaSO4);

Supersaturation, equilibrium, under saturation

g sp( 4);

water is supersaturated.

As a consequence calcium sulfate will precipitate.

Equals to Ksp(CaSO4);

equilibrium exists.

As a consequence calcium sulfate will not precipitate and not di ldissolve.

Lower than Ksp(CaSO4);

water is undersaturated.

As a consequence more calcium sulfate can dissolve

19

Supersaturation Ratio (Sr)

How much is the supersaturation?

Supersaturation is commonly expressed in two ways

2 24[ ][ ]

rsp

Ca SOSK

p y p y

Supersaturation ratio, Saturation Index or Ratio

Supersaturation ratio is e.g.,

When:

Sr < 1 water is ……………….. with CaSO4

Sr > 1 water is ……………….. with CaSO4

Sr = 1 water is ……………….. with CaSO4

sp

20

2+ 2-[Ca ][SO ]

Supersaturation Index (SI)

When:

SI < 0 water is ……………….. with CaSO4

4

sp

[Ca ][SO ]SI = logK

SI > 0 water is ………………. with CaSO4

SI = 0 water is ………………. with CaSO4

21

Ratio

Some computer programmes use simply the:

2+ 2-4

sp

Ca SORatio = K

p p g p y

which is quite confusingq g

22

Calcium carbonate

23

Calcium Carbonate SI

For super saturation of calcium carbonate SI is commonly applied.

For low salinity:

Langelier approach is applied

For higher salinity:

Stiff and Davis is applied.

Remark: Theory is complex

24

Precipitation/dissolving calcium carbonate

Calcium carbonate is a solid.

Its solubility in pure water is: 13 mg CaCO3/L which is quite y p g 3/ qlow.

The quantity of CaCO3 that can dissolve in water is governed by the equilibrium

CaCO3 + H+ Ca2+ + HCO3

‐

This equilibrium is the result of two basic equilibria

CaCO3 Ca2+ + CO32‐

CO32‐ + H+ HCO3

‐

CaCO3 + H+ Ca2+ + HCO3

‐ overall

25

Solubility governing parameters

Accordingly the equations governing parameters are:

Ca2+ concentration (the higher Ca2+, the lower solubility)

HCO3‐ concentration (the higher HCO3

‐, the lower solubility)

H+ concentration or pH (the lower, the higher solubility).

Remark:

Role of CO32‐ concentration is governed by:

• HCO3‐ concentration

• pH

26

Commonly the “Langelier Saturation Index (LSI)” is used for low salinity waters.

Langelier Saturation Index (I)

For high salinity waters a modified approach is applied, called “Stiff and Davies Index” (StDI).

LSI and StDI are by definition:

LSI, StDI = Actual pH – pH at saturation (saturation pHs)

LSI = pH ‐ pHs

27

Langelier Saturation Index(II)

Langelier designed a nomograph

He defined the LSI (Langelier Saturation Index)

LSI = pHact ‐ pHs = 0 just saturated or equilibrium

LSI = pHact ‐ pHs > 0 supersaturated with calcium carbonate or precipitative

d d

( g )

LSI = pHact ‐ pHs < 0 undersaturated or aggressive against calcium carbonate. It will dissolve.

28

Langelier Saturation Index (III)

CaCO3 + H+ Ca2+ + HCO3‐

At low pH the reaction is pushed to the right side, so CaCO3

dissolves.

At high pH the reaction is pushed to the left side, so CaCO3

precipitates.

29

mg/L mg/L

Example: Groundwater

Na+

K+

Ca2+

Mg2+

Ba2+

134

19

212

133

0.05

Cl‐

SO42‐

HCO3‐

NO3‐

F‐

198

877

240

9

0.3

Sr2+

pH

5

7.0

PO43‐

SiO2

4

41

Temp 20C TDS = 1831

30

Example: Groundwater

Reverse Osmosis plant it treating ground water.

Recovery is 75 % and constant.

Questions:

Is scaling of one of more compounds expected?

If so which?

What is the degree of (super) saturation?

How to avoid scaling?

31

Calculations with:

4 Aqua program

32

Calculation with 4 Aqua (I)

This program calculates:

Saturation for:

CaSO4 * Ca3(PO4)2 BaSO4 * SiO2

CaF2

and for:

CaCO3

33

Calculation with 4 Aqua (II)

You can change:

Recovery;

Temperature;

pH feed water;

It gives a recommendation for antiscalant dose.

Remark: It is not known which calculation methods are applied.

34

‐ We take the same example for R = 75 %

‐ Results: Feed water

Feed water: Calculations 4 Aqua

Saturation

4 Aqua

CaSO4

BaSO4

S SO

9 %

598 %

42 %SrSO4

CaF2SiO2

CaCO3*

42 %

3 %

36 %

‐ 0.02

35

‐ Results: Concentrate 75 %

Concentrate: Calculations 4 Aqua

Saturation

4 Aqua

CaSO4

BaSO4

SrSO4

87 %

3016 %

171 %SrSO4

CaF2SiO2

CaCO3*

171 %

205 %

138 %

+ 1.64

36

Concentrate: Results Calculations 4 Aqua

Scaling is expected to occur of:

BaSO4 (barium sulphate)

SrSO4 (strontium sulphate)

CaF2 (calcium fluoride)

SiO2 (silica)

CaCO3 (calcium carbonate)

Since saturation is above 100 %

37

Scaling can be prevented of all compounds by dosing antiscalant.

Concentrate: Results Calculations 4 Aqua

Remark: Phosphate requires acid dose as well

Scaling due to Calcium carbonate can be prevented by addition of acid (lowering pH)

38

Maria Kennedy, PhDProf. Jan C. Schippers, MSc, PhD

“Process Design of RO systems with spiral wound elements”

J pp


Parameters in Process Design to be chosen/known

Permeate flow / Plant capacity to be determined by costumer

Conversion / Recovery

Salinity of the feed water

Salinity of the permeate

Temperature

Fouling / scaling potential feed water

2

Feed flow

Feed pressure

Parameters to be calculated / determined / chosen / verified

p

Average membrane flux

Total number of elements / vessels

Arrangement of the vessels / array

Configuration / staging

Salinity product water element/ plant

3

Conversion / Recovery

Needs to be chosen / determined / verified;

Brackish water plants use to operate at 75%, some up to p p , p90%

maximum is mainly governed by scaling potential of feed water

Sea water plants use to operate at 30 – 50%

maximum is governed by osmotic pressure

4


Two categories are identified in practice

Brackish, up to 10,000 mg/L

R1C

C fc

Seawater, higher than 30,000 mg/L

Salinity, determines together with conversion the osmotic pressure

(assumption SR ~100%)R1

(mg/L)C100.8(bar) π 3 ; Rule of thumb

5


Salinity determines the permeate salinity, together with membrane performance (Ks); flux and conversion

J

CC sKfc

p

Where:

Cc = Concentration concentrate (brine)

Cf = Concentration feed

Cp = Concentration permeate

Cfc = Concentration feed/concentrate

A great variety of membranes are on the market, with different salt rejections (Ks) and fluxes (Kw)

6


Two different types of membranes are identified namely:

Brackish water membranes up to about 99 % rejection.

Seawater membranes with rejections >99.5 %.

In general:

High salt rejection (low Ks) combines with low Kw value (due to smaller pores).

7

Salinity of the permeate

Can be chosen to a certain extend by the choice of:

Type of membrane;

Conversion: lower conversion results in lower salinity;

Flux: higher flux gives lower salinity.

For drinking water, 500 mg/L is usually the guideline;

For industrial waters much lower guidelines are often adopted;

8

Temperature

Has an effect on Kw and Ks, both increase at higher temperatures

25)(t1.03ww KK

C25ºat ww KK

Kw is linked with the viscosity of water;

Where:

t = temperature in ºC

Ks is linked with the diffusion coefficients of ions and pore size in membranes

9

Temperature

Both tend to increase with temperature

Diffusion coefficient

rηπ6Tk

D

Where:

k = Boltzmann constant

T = 273 + t ºC

η = viscosity

r = radius ion

Only empirical formula for Ks are available, with limited value.

10

Fouling potential

Fouling potential due to

Particles

Bacterial growth

Organic matter

determines in practice the allowable flux.

In practice rate of membrane fouling is expected to be l hproportional with:

Flux;

Fouling potential

11

Fouling potential

Membrane fouling is compensated by increasing feed pressure, because plants usually operate at constant capacity.

At 15% pressure increase, chemical membrane cleaning is recommended, to avoid irriversible fouling.

12

Recommended flux for different feed waters

Design flux

(L/m2 h)

Surface water 14 – 24

Well water 24 – 31

RO permeate 34 – 51

In practice pilot plant studies are conducted to verify the design flux rate.

13

Scaling potential

C

In the concentrate, concentrations of salts are increased,

R1C

C fc

Including sparingly soluble compounds e.g.,

Calcium carbonate;

Calcium sulphate;

(assumption SR ~ 100%)

Strontium sulphate;

Barium sulphate;

Silica (SiO2)

14

Scaling potential

As soon as the solubility is exceed, precipitation / scaling might occur. Results in lower Kw.

In brackish waters, the scaling potential determines the

Precipitation of calcium carbonate can be avoided by acid dosing.

Supersaturation to a certain extend is allowable by antiscalant dosing.

, g pmaximum conversion.

In seawater, the osmotic pressure determines the maximum conversion.

15

How to calculate the feed flow?

The feed flow of the plant is linked with the product/permeate flow and conversion (R)

f

p

Q

QR

R

QQ p

f

The maximum feed flow of an element is given by the manufacturer, to avoid membrane damage.

16

How to calculate the feed pressure?

The feed pressure of an element follows from:

pavgf P--P/2-PNDP

Where:

NDP = net driving pressure;

Pf = feed pressure;

ΔP = head loss across one element (~ 0.2 bar);

Δπavg = average difference osmotic pressure feed ‐permeate;

Pp = product pressure

17

How to determine NDP?

NDP follows from:

wKNDPJ

Flux needs to be chosen, based on expected fouling potential feed water

Kw is not directly available from manufactures information. So Kw have to be calculated from test results under standard

di iconditions.

18

Calculation of Kw

Formula:w

w KNDPA

QJ

Example:Data sheet SWC 3 membrane element (L=1 m, d=20 cm)Cf = 32,000 mg/L

SR 99 7 %

A

NDPJ

Kw

SR = 99.7 %

Qw = 930 L/hr

Ae = 34.37 m2 (membrane area)

R = 10%

Pf = 55 bar

19

Calculation of Kw

Pf = 55 bar

pavgfavg P-∆π - ∆P/2-P NDP Pf 55 bar

P/2 = 0.2/2 = 0.1 bar

(assumption SR ~100%)

30.8 10 bar2

f cavg

C C

R1C

C fc

πavg = 27 bar

Pp = 0

NDP = 27.9 bar Kw = 0.97 L/m2.hr.bar

20

Calculation of Kw

Remark:

In a RO plant NDP is not constant in a vessel. It gradually decreases as process proceeds, because:

• Osmotic pressure increases in concentrate.

Pressure decrease due to head loss across elements (spacers), 0.2 bar/element. As a consequence the flux will gradually decrease.

Osmotic pressure have to be corrected for concentration

So more extended calculations, taking into account these effects, are needed.

ppolarization, which depends on concentrate and permeate flow.

21

Average membrane flux

To be chosen based on the expected fouling potential feed water (See table).

Check maximum flux in first elements of vessels as well.

22

Total number of elements/vessels

A

Total number of elements follows from:

ee A

An

J

QA p

Where:

ne = number of elements

A = total required membrane area

Ae = membrane area per element (~35m2)

Qp = permeate flow/capacity

J = flux

23

Total number of elements/vessels

Spiral wound elements are placed in pressure vessels.

In large plants 6 to 7 elements of 1 m length are placed.

In smaller plants 1 to 4 elements of 1 m length are placed in vessels.

24

Arrangement vessels

Usually vessels are placed in parallel position, with 6 to 7 elements in a vessel.

However obtained conversion in one bank (group of parallel vessels) equipped with spiral wound elements is not more than 50 %.

Reason is that higher conversions result in too low ratio's concentrate to permeate flow per element. Minimum 5:1 (β = 1 21) As a result too high concentration polarization

exp PP

fc

QK Q

= 1.21). As a result too high concentration polarization factor β .

Should be < 1.2

25

Arrangement vessels

When conversion has to be higher than 50% a second (and third) stage/bank is necessary.

The number of vessels in the next stage is about 50% of the previous one.

Because the ratio feed flow to permeate flow at the entrance of the next stage is the same.

In the second stage about 50% is converted in product. This b i th t t l i t b t 75%brings the total conversion at about 75%.

The total conversion with a third stage will be about …%.

26

Arrangement vessels

1 2 3 4 5 6

Pump

Vessels Vessels

Valve

Staging: 4 vessels in parallel

2 vessels in parallel

27

Salinity product water

The salinity product/permeate follows from:

SR1CC

Or more accurately

since SR depends on flux

Where:

SR1CCfcp

JKC

C sfcp

Since SR depends on flux, Ks is not directly available from manufacturers information.

So Ks have to be calculated from test results under standard conditions.

2cf

fcCC

C

28

Calculation Ks

Formula:

JKC

C sfcp

Example:Data sheet SWC 3 membrane element (L=1 m, d=20 cm)Cf = 32,000 mg/L

Jp

JCC

Kfc

ps

f

SR = 99.7 %

Qp = 22.3 m3/day = 930 L/h

Ae = 370 ft2 = 34.37 m2

R = 10%

29

Calculation Ks

Calculation:

S1CC

(Assumption SR100%)

SR1CC

fcp

2cf

fcCC

C

R1C

C fc

Ks = 0.081 L/m2 h

227 L/m hrP

e

QJ

A

30

Calculation Ks

Remark:

Different ions have different Ks values so the rejection (SR) s j ( )is different.

In general

SR: Mg2+ > Ca2+ > Na+

SO42‐ > Cl‐

So calculations should be done for different ions, which makes the whole set of calculations very complicated.

31

Calculation full scale plant

In a RO plant the flux is not constant in a vessel. It gradually decreases as the process proceeds. First elements have higher flux.

Reasons are:

Osmotic pressure increases in the concentrate;

Pressure decreases due to head loss across elements (spacer, 0.2 bar/element)

As a consequence C will be higher in the last elements

2cf

fcCC

C

As a consequence Cp will be higher in the last elements.

Osmotic pressure has to be corrected for each element for concentration polarization (β) has to be corrected for concentration polarization (β) as well.

32

Calculation full scale plant

So extended and detailed calculations, taking in account these effects, are necessary.

33

“Process Design of a Seawater Reverse OsmosisProcess Design of a Seawater Reverse Osmosis Plant with Spiral Wound Membrane Elements”

Manual Calculations

Maria D. Kennedy, PhD Prof. Jan C. Schippers, PhD, MSc


Assignment: Design a SWRO Plant

Capacity of the plant Qp (m3/h) 45

Salt concentration C (mg/l) 34380( g/ )

Temperature 25C Seawater well

Total recovery R (%) 35%

Membrane Element used SWC3 ‐ Spiral Wound Element

Membrane area per element

8 x 40 inch Ae (m2) 34.37

Six elements in a vessel

Salt rejection (SR) % 99.7%

(Under standard conditions)

(See Element Specification Sheet for other details)

2

Calculate the following:

1. What is the total number of elements and pressure pvessels?

2. What is the array staging?

3. What is the feed pressure?

4. What is the salinity of the product water per element and the plant?

3

Guidelines from the Membrane Manufacturer

– Maximum feed flow per element (from the data sheet) 17 m3/h.

– Concentration polarization factor < 1.2

Or

– Minimum ratio of concentrate to permeate flow for any element 5:1

4

Assumptions

1. Water temperature = 25C2. Effect of concentration polarization on osmotic pressure can

be neglected for this assignment

3. Rejection is same for all different ions (which is not the case; however if this effect is taken into account, calculations will be very complicated)

4. Kp in the formula for equals 0.994. Kp in the formula for equals 0.99

5

Approach

Step 1:

Calculate approximately the number of elements and pressure vessels. We assume an average flux of 13 L/m2.h (This figure comes from practice)

Check whether the maximum feed flow per element of 17 m3/h will not be exceeded.

Step 2:

6

Step 2:

Make a very simplified calculation of the expected permeate concentration (Cp) Ignore the effect of flux on the salt rejection (SR) in this stage and assume an average concentration in the concentrate.

Steps 3 & 4:

Make a (preliminary) calculation of the required feed pressure. For this purpose, the Kw (membrane permeability for water) needs to be calculated from the data of the element specification sheet.

Calculate flows and recovery for each element in a vessel.

In order to be able to verify:

the feed pressure

th d h k the recovery, and check

the concentration polarization factor β for each element and/or

the ratio of concentrate to permeate flow per element and the permeate quality per element.

7

Steps 5 & 6

Step 5:

Calculate permeate quality for each element and per vessel. To simplify the calculations, it is assumed that salt rejection is constant, namely 99.7% (standard conditions)

Calculate the β factor per element

Step 6:

8

Step 6:

Calculate the salt rejection and permeate quality per element, taking into account the effect of flux

Steps 7‐9

Step 7:

Summarize calculations

Step 8:

Compare the answers of different approaches and calculations with Hydranautics computer program

9

Step 9:

Explain the differences

Formulae used

f

p

Q

QR

cpf QQQ

(bar) = 0.8 x 10-3 * C (mg/l) [1000 mg/l 0.8 bar ]

2

QQQ cf

fc

R-1

C=C f

c

2

CCC cf

fc

10

Formula used:

SP*CSR) -1 ( C=C fcfcp

A*K*NDPQ ww

ww K*NDP

A

QJ

NDP = PF - (p / 2) - avg - PP

J

K*CC sfc

p

= Kp * exp (Qp /( (Qf + Qc) /2)) = Kp * exp(Qp/Qfc)

11

Assuming average flux (l/m2/h) 13 (from practice) Q per element Qpel (m3/h) 0.447

Step 1: Calculation number of elements and pressure vessels

No of elements required 100.7say 101 Assuming 6 Elements per vessel (from practice)

No of pressure vessels 16.8 Provide 17pressure vessels

Total number of elements 102 (17 * 6)

Permeate flow per vessel

Qp = flux * membrane area/element * nr. of elements/vessel

= 13 l/m2 h * 34 37 m2 / element * 6 /vessel 13 l/m h 34.37 m / element 6 /vessel

= 2.65 m3/h.vessel

Feed flow per vessel

R = Qp/Qf = 0.35

Qf = Qp/0.35

12

Feed flow per pressure vessel (m3/h) 7.56

3

Step 1: Calculation number of elements and pressure vessels

Permeate flow per pressure vessel (m3/h) 2.65 Concentrate flow per pressure vessel (m3/h) 4.92

Qf = 7.56 m3/h Qp = 2.65 m3/h

Qc = 4.92 m3/h

‐ Checkmaximum feed flow

Maximum Feed Flow m3/h ( from the data sheet) 17 Hence OK

13

Feed flow Qf (m3/h) of the plant 128.6

f

p

Q

Q=R

Step 2: Simplified calculation of permeate concentration (Cp)

Concentrate flow Qc (m3/h) 83.6

Concentrate concentration Cc (mg/l) 52892 (assumption SR = 100%, instead of 99.7%)

A f d b i t ti C ( /l) 43636

R-1

C=C f

c

2

C+C=C cf

fc

pfc Q-Q=Q

Average feed brine concentration Cfc (mg/l) 43636

Permeate concentration Cp (mg/l) 131

2fc

SR) -1 ( C=C fcp

14

Step 2:Simplified calculation of permeate concentration (Cp)

Qp = 45 m3/h

Cp = 131 mg/l

Qf = 128.6 m3/h

Cf = 34380 mg/l

Qc = 83.6 m3/h

Cc = 52892 mg/l

15

Calculation of membrane permeability coefficient for water NDP = PF - (p / 2) - avg - PP A*K*NDP=Q ww

Step 3: Preliminary calculation of feed pressure. For this purpose membrane permeability will be calculated, based on standard conditions.

From the element specification sheet Pf (bar) 55 Area of one element A (m2) 34.37

Note: s refer to standard conditions

Assuming head loss p of 0.2 bar 0.2 Cfs (mg/l) 32000 R 10% Ccs (mg/l) 35556

fs (bar) = 0 8/1000 * 32000 25 60fs (bar) 0.8/1000 32000 25.60

cs (bar) 28.44

avg (bar) = avg (assumption p = negligible) 27.02

NDPs (bar) 27.88 Nominal Capacity Qs (m3/d) 22.30 (Specification sheet) Nominal Capacity Qs (m3/h) 0.93 Flux under standard conditions (l/m2h) 27

16

Membrane permeability Kw (m3/m2.bar.h) 9.70E-04 Kw (l/m2.bar.h) 0.97

Membrane productivity Kw A (m3/h.bar) 0.033 Estimation of Required Feed Pressure

Step 3: Preliminary calculation of feed pressure. For this purpose membrane permeability will be calculated, based on standard conditions.

Estimation of Required Feed Pressure Flux (l/m2/h) = Qw/A 13.00 Qpe (m3/h) = capacity/102 elements = 45 m3/h / 102 0.45 NDP (bar) 13.41 For a pressure vessel Cf (mg/l) 34380 R 35% Cc (mg/l) 52892

f (bar) 27.50f ( )

c (bar) 42.31

avg (bar) 34.91 Assuming head loss p (bar) of 0.2 bar/element 1.20

NDP = PF - (p / 2) - avg - PP Estimated feed pressure Pf (bar) - assume Pp = 0 48.91 say 50bar

17

Qc

Qf

Step 4: Calculations of flows and recovery for each element

Qp1

Qp2

Qp3

Qp4

Qp5

Qp6

Qp

Element 1 Feed Pressure Pf1 (bar) 50.00 Assuming head loss per element p (bar) of 0.20 Qf1 (m3/h) = 7.56 Cf1 (mg/l) = 34380 Assuming R % 6% To calculate osmotic pressure Cc1 (mg/l) = 36574

f1 (bar) 27.50

c1 (bar) 29.26

avg (bar) 28.38 NDP1 (bar) 21.52

Qp1 (m3/h) 0.72 R = Qf1/Qp1 9.5%

A*K*NDP=Q ww

18

II Iteration For R = 9.5% Cc1 (mg/l) = 37982

Step 4:Calculations of flows and recovery for each element

f1 (bar) 27.50

c1 (bar) 30.39

avg (bar) 28.94 NDP1 (bar) 20.96 Qp1 (m3/h) 0.70 R 9.23% Check: For R = 9.23%

Cc1 (mg/l) = 37878 Close Cc1 (mg/l) = 37878 Close

f1 (bar) 27.50

c1 (bar) 30.30 Close Qp1 (m3/h) 0.70 OK Qc1 (m3/h) 6.86 Cfc1 (mg/l) = 36129

19

Element 2


Element 2 Feed Pressure Pf2 (bar) 49.80 Qf2 (m3/h) = Qc1 6.86 Cf2 (mg/l) = Cc1 37878 Assuming R % 9% Cc2 (mg/l) = 41624

f2 (bar) 30.30

c2 (bar) 33.30

avg (bar) 31.80avg (bar) 31.80 NDP2 (bar) 17.90 Qp2 (m3/h) 0.60 R 8.7%

20


(b ) 30 30


f1 (bar) 30.30

c1 (bar) 33.19

avg (bar) 31.74 NDP2 (bar) 17.96 Qp2 (m3/h) 0.60 R 8.72%

Check: For R = 8.72%


f2 (bar) 30.30


21

El t 3


Element 3 Feed Pressure Pf3 (bar) 49.60 Qf3 (m3/h) = Qc2 6.27 Cf3 (mg/l) = Cc2 41496 Assuming R % 8.2% Cc3 (mg/l) = 45202

f3 (bar) 33.20

c3 (bar) 36.16

avg (bar) 34.68 NDP3 (b ) 14 82 NDP3 (bar) 14.82

Qp3 (m3/h) 0.49 R 7.9%

22



( g )

f3 (bar) 33.20

c3 (bar) 36.04


Check: For R = 7.92%


f3 (bar) 33.20


23

Element 4


Element 4 Feed Pressure Pf4 (bar) Qf4 (m3/h) = Qc3 Cf4 (mg/l) = Cc3 Assuming R % Cc4 (mg/l) =

f4 (bar)c4 (bar)avg (bar)

NDP4 (bar)

49.40 5.77

45063 7.5%

48717

36.05

38.97

37.51 11.79 NDP4 (bar)

Qp4 (m3/h) R

11.79 0.39

6.8%

24

II Iteration For R = 6.8%


Cc4 (mg/l) = 48351

f4 (bar) 36.05

c4 (bar) 38.68


Check: For R = 6.89% Cc4 (mg/l) = 48400 Close

f4 (bar) 36.05


25

Element 5



f5 (bar) 38.72

c5 (bar) 41.41

avg (bar) 40.07 NDP5 (b ) 9 03 NDP5 (bar) 9.03

Qp5 (m3/h) 0.30 R 5.6%

26


Cc5 (mg/l) = 51271


Cc5 (mg/l) = 51271

f5 (bar) 38.72

c5 (bar) 41.02


Check: For R = 5.73% Cc5 (mg/l) = 51340 Close

f5 (bar) 38.72


27

Element 6



f6 (bar) 41.07

c6 (bar) 43.37


28



For R 4.4% Cc6 (mg/l) = 53703

f6 (bar) 41.07

c6 (bar) 42.96


Check: F R 4 53%For R = 4.53% Cc6 (mg/l) = 53776 Close

f6 (bar) 41.07


29

A) Assuming a constant salt rejection of 99.7% (Standard conditions)

so independent of flux

Step 5: Calculations of permeate quality (at constant rejection) and β factor for each element

so independent of flux

Cfc1 (mg/L) = 36129

Cp1 (mg/L) = 108.4

SR) -1 ( C=C fcp

1 = Kp * exp (Qp /( (Qf + Qc) /2)) C 2 ( /l)

1.09

Cp2 (mg/l) =

2 = Cp3 (mg/l) =

3 =

119.1

1.08

129.8

1.08

30

Cp4 (mg/l) =

4 = 140.2

1 06

Step 5: Calculations of permeate quality (at constant rejection) and β factor for each element

4 = Cp5 (mg/l) =

5 = Cp6 (mg/l) =

6 =

1.06

149.6

1.05

157.7

1.04

1 1 2 2 3 3 4 4 5 5 6 6* * * * * *p p p p p p p p p p p pC Q C Q C Q C Q C Q C QC

Remark: First estimate was 131 mg/l (Step 2)

1 1 2 2 3 3 4 4 5 5 6 6

1 2 3 4 5 6

128 /

p p p p p p p p p p p pproduct

p p p p p p

CQ Q Q Q Q Q

mg l

31

B) Salt rejection depends on the flux

Under standard conditions

Step 6: Calculations of salt rejection taking into account the effect of flux

Under standard conditions

p fcC =C ( 1- SR) *fc sp

C KC

J

Flux J (l/m2h) Cf (mg/l) R Cc (mg/l)

27 32000

10% 35556 Cc (mg/l)

Cfc(mg/l) SR Cp (mg/l)

Membrane permeability for salt Ks (l/m2h)

33778 99.70%

101.3

0.081

32

Cfc1 (mg/l) = 2

36129


Flux J1 (l/m2h) Cp1 (mg/l) Cfc2 (mg/l) Flux J2 (l/m2h) Cp2 (mg/l) Cfc3 (mg/l) Flux J3 (l/m

2h)

20.32144.0

3968717.412184.6

4327914.433( )

Cp3 (mg/l) Cfc4 (mg/l) Flux J3 (l/m2h) Cp4 (mg/l)

242.9

4673111.573246.2

33

Cfc5 (mg/l) Flux J5 (l/m

2h)

498708 953


Flux J5 (l/m h)Cp5 (mg/l) Cfc6 (mg/l) Flux J6 (l/m2h) Cp6 (mg/l)

8.953339.6

525586.675480.1

1 1 2 2 3 3 4 4 5 5 6 6

1 2 3 4 5 6

* * * * * *

236 /

p p p p p pproduct

C J C J C J C J C J C JC

J J J J J J

mg l

34

Capacity of the plant = 45 m3/h Assuming average flux = 13 L/m2.hNo of elements = 17 *6 = 102 SWC3

Process Design of SWRO Plant

Element Recovery P NDP Q C QElement Recovery Pf NDP Qf Cf Qp

bar bar m3/h mg/L m3/h1 9.2% 50.0 21.0 7.56 34380 0.702 8.7% 49.8 18.0 6.86 37878 0.603 7.9% 49.6 14.9 6.27 41496 0.504 6.9% 49.4 11.9 5.77 45063 0.405 5.7% 49.2 9.2 5.37 48400 0.316 4.5% 49.0 6.9 5.07 51340 0.23

Total 36% 2.73

Element Qc Cc Qfc Beta C:P Fluxm3/h mg/L m3/h L/m2hm /h mg/L m /h L/m h

1 6.86 37878 7.21 1.09 9.8 20.32 6.27 41496 6.57 1.08 10.5 17.43 5.77 45063 6.02 1.07 11.7 14.44 5.37 48400 5.57 1.06 13.5 11.65 5.07 51340 5.22 1.05 16.5 9.06 4.84 53776 4.95 1.04 21.2 6.6

Total 13.2

35

Comparison of permeate concentrations calculated by different approachesSalt Rejection at Standard Conditions = 99.7%

Process Design of SWRO Plant

Cp

From Hydranautics

computer program**

ElementQp

m3/h

Flux

L/m2h

Constant

SR = 99.7%

mg/L

Flux dependent

SR

mg/L

Flux CpL/m2h mg/L

1 0.70 20.3 108.4 144.0 20.0 158.5

2 0.60 17.4 119.1 184.6 16.7 209.1

3 0.50 14.4 129.8 242.9 13.7 273.0

4 0.40 11.6 140.2 246.2 11.1 357.6

5 0 31 9 0 149 6 339 6 9 0 474 5

Differences due to:

‐ concentration polarization

‐ osmotic pressure formula used

‐ different Ks value ?

‐ different flux (3%)

** Based on salt concentration of 34380 mg NaCl/L and membrane age of 0 years

5 0.31 9.0 149.6 339.6 9.0 474.5

6 0.23 6.6 157.7 480.1 6.7 677.4

Total 2.73 13.20 128.1 236.0 12.8 298.6

(average)

36

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