Seawater reverse osmosis plant using the pressure ... · X × =⋅ × (2) The corresponding...

Desalination 165 (2004) 289–298

0011-9164/04/$– See front matter © 2004 Elsevier B.V. All rights reserved

Presented at the EuroMed 2004 conference on Desalination Strategies in South Mediterranean Countries: Cooperationbetween Mediterranean Countries of Europe and the Southern Rim of the Mediterranean. Sponsored by the EuropeanDesalination Society and Office National de l’Eau Potable, Marrakech, Morocco, 30 May–2 June, 2004.

*Corresponding author.

Seawater reverse osmosis plant using the pressure exchanger forenergy recovery: a calculation model

Giorgio Migliorini*, Elena LuzzoFisia Italimpianti, Via De Marini 16, 16149 Genoa, Italy

Tel. +39 (010) 6096-429; Fax +39 (010) 6096-410; email: [email protected]

Received 16 February 2004; accepted 25 February 2004

Abstract

An RO water desalination system consists of the pre treatment section, the desalination section and the posttreatment section. In the new design plants different energy recovery systems are adopted to reduce the energyconsumption of desalination section. Among the various energy recovery systems the pressure exchanger (PX)system produces notable benefits not only to the energy consumption but also regarding the high pressure pumpsize and the consequent scale-up of the system. A secondary effect of the use of PX is the increasing of feed salinityto the desalination section due to the mixing of the concentrate coming from RO section with the portion of feedwater passing trough the PX device, on the contact layer between the two streams. The different software suppliedby the membrane manufactures does not take into account this phenomenon, making chemical dosing calculationon RO train inlet feed analysis that is different from the raw water analysis. An original model calculation to takeinto account these different seawater conditions has been developed based on the classical carbonate systemequilibrium allowing production of a complete chemical and mass balance of the entire system including thechemicals dosing rate based on raw water characteristics. The model is not affected by the membrane characteristicsand can be utilized for an easy and quick basic design of the plant.

Keywords: Reverse osmosis; Pressure exchanger; Seawater salinity; Carbonate system; Mass balance

1. Introduction

The focal point of a reverse osmosis desalina-tion plant is the seawater pre-treatment. The pre-

treatment operations can be divided in two mainfamilies:• Physical pre-treatments• Chemical pre-treatments

The first family includes mainly the filtration

290 G. Migliorini and E. Luzzo / Desalination 165 (2004) 289–298

operations that are carried out trough sand filtersfollowed by cartridge filters to control the particlesmaximum size. The second family includes thetreatment of the feed like coagulants and poly-electrolyte injection, disinfection, scale reduction,de-chlorination before entering the membranetrains. The calculation programs available fromthe membrane manufacturers are focused on thecalculation of the membrane performances andin general, starting from the seawater analysis andfrom the temperature an pH conditions, with thelimiting parameter of the system flux , define thefeed pressure, the permeate characteristics andaccording to the pre treated feed pH, the aciddosing rate.

The recent introduction of energy recoverysystem in RO, where the concentrate is in contactwith a part of the seawater feed to the plant, hasmodified the design approach to RO plant sizing.The salinity of the feed to the RO train in factincreases with respect to the raw seawater at theplant battery limits and, as a consequence, whilethe membrane performances have to be calculatedunder the increased salinity conditions, thechemical pretreatment has to be calculated on theraw water original conditions.

An original calculation model to take intoaccount these phenomena has been developed toproduce the complete mass balance of the system.In the following sections an overview of thechemical treatment system is made and thecalculation program is described.

2. Feed water chemical treatments

2.1. Chlorination

As regards the chlorine dosing rate and thechlorination management it is possible use twodifferent modes of operation — the continuousdosing system and the shock dosing system orboth the systems. The dosing rate for continuoustreatment is in general between 1 up to 3 ppm,whereas the shock can reach 5 ppm; also thefrequency for the shock dosing can be different

for different plant and can change from 4 times aday to once a day.

The recent trends in RO design suggest toadopt a procedure of random shock dosing. It isimportant to define what is the chlorine residualon feed water because before feeding the water tothe membranes it is necessary to provide thedechlorination of the stream to avoid troubles tothe membranes.

2.2. Coagulation

2.2.1. Premises

Flocculation and coagulation agents are addedto water to create nuclei onto which colloidal andsuspended material in the water can adsorb, thuscreating floc of larger dimensions and mass whichcan be removed by sedimentation followed bysand filtration.

The design of coagulation process involvesselection of proper coagulant chemicals and theirdosage that can be determined experimentally foreach raw water source; the coagulants can be oftwo different types:• Polyelectrolytes• Iron or aluminium salts

In general the most common coagulant utilizedis the ferric chloride but in some installations it ispossible to use both the types to improve the sandfilters performances.

2.2.2. Aqueous chemistry of iron salts

Coagulants reactions are carried out by theaddition of a coagulant, usually a metal salt towater. Commonly used coagulants are ferric sul-phate (Fe2(SO4)3), ferric chloride (FeCl3), andalum (aluminium sulphate, Al2(SO4)3×14 H2O).

For the RO feedwater treatment the most com-mon coagulant used is the ferric and the relevantreaction is:

3 3 2 3

2 2

2FeCl 3Ca(HCO ) 2Fe(OH)+ 3CaCl 6CO

+ ⇔ ++ (1)

G. Migliorini and E. Luzzo / Desalination 165 (2004) 289–298 291

Because of the consumption of alkalinity, CO2is produced during coagulation. The actual amountof coagulant required for destabilization of col-loids may depend, not only on the reaction stoi-chiometry, but also on other operational conditionssuch as ionic species, pH value, temperature, typeand properties of particles, mixing energy inputand the content of metal ions in the coagulant.Normally the dosing rate usually utilized is in therange between 0.3 and 3 ppm.

According to the stoichiometric reaction it ispossible to calculate the relevant CO2 released andthe residual bicarbonate content after the injection.

32 FeCl6 44CO ppm

2 162.5X ×= ⋅

×(2)

The corresponding bicarbonates reaction is:

3

23 FeCl

3 122HCO ppm2 162.5

X− ×= ⋅×

(3)

2.3. Scaling control

2.3.1. Carbonic acid equilibria

The seawater, under normal conditions, isusually supersaturated with calcium carbonate.The pH of most natural waters is generallyassumed to be controlled by the carbonic acidsystem.

The applicable equilibrium reactions are:+

2 2 2 3 3CO H O (H CO ) H HCO−+ ↔ ↔ + (4)

+ 23 3HCO H CO− −→ + (5)

The CO2/HCO3–/CO3

2– equilibrium in seawatercan be calculated starting from alkalinity and pHvalues trough the calculation of the followingequations:

+[H ] [OH ]wk −= ⋅ (6)

kw is the ionisation product of water =4471.336.0486 0.017053 ( 273)

27310w

wT

T− − ⋅ +

+=

k1 is the first dissociation constant =3404.7114.8435 0.032786 ( 273)

27310w

wT

T− − ⋅ +

+=

k2 is the second dissociation constant =2909.396.498 0.02379 ( 273)

27310w

wT

T− − ⋅ +

+=

The following relations have to be taken intoaccount:

3total alkalinity as CaCO[Alk] g-eq/l50 1000

=×

(7)

[H+] = hydrogen ion concentration, g-ion/l = 10–pH

(8)

33

ppm [HCO ][HCO ] g-ion/l61 1000

−− =

×(9)

22 33

ppm [CO ][CO ] g-ion/l

60 1000

−− =

×(10)

22

ppm [CO ][CO ] g-ion/l44 1000

=×

(11)

The following ionic balance can be alsowritten:

+ 23 3[Alk] [H ] [HCO ] 2 [CO ] [OH ]− − −+ = + + (12)

And using the ionisation and dissociation con-stants defined above, the following final relationsare obtained:

++

23

2+

[Alk] [H ][H ][HCO ] g-ion/l21

[H ]

wk

k−

+ −=

+(13)

2 23 3+[CO ] [HCO ] g-ion/l

[H ]k− −= (14)

+

2 31

[H ][CO ] [HCO ] g-ion/lk

−= (15)


For a defined pH and alkalinity is possible todetermine the bicarbonate content that contributeto the CO2 formation.

2.3.2. Limiting salt calculation

Scaling control is essential in RO/NF mem-brane filtration. The amount of antiscalant or acidaddition is determined by the limiting salt. Adiffusion controlled membrane process willnaturally concentrate salt on the feed side of themembrane. If excessive water is passed throughthe membrane, this concentration process willcontinue until salt precipitates and scaling occursreducing membrane productivity and, con-sequently the system recovery. The limiting saltcan be determined from the solubility products ofpotential limiting salts and the actual feed-streamwater quality and from the ionic strength. Calciumcarbonate scaling is commonly controlled bysulphuric acid addition; however, sulphate saltsare often the limiting salt.

General equations for the solubility productsand ionic strength approximations are given asfollows:

p qn mA B nA mB+ −⇔ + (16)

( ) ( )n mp qspK A B+ −= (17)

n mp q

spA BK a bx x

+ −⎛ ⎞ ⎛ ⎞= ⎜ ⎟ ⎜ ⎟⎝ ⎠ ⎝ ⎠

(18)

where x is the fraction remaining, Ksp — solubilityproduct, a — fraction of cation retained, b —fraction of anion retained.

212 i iC Zµ = ∑ (19)

where µ is ionic strength,C = mol/l, Z is the ioncharge.

( ) 2log 0.521

Z pµ

γ = − = γ+ µ (20)

where γ is the activity coefficient.Once the required pH has been determined for

calcium carbonate scaling, the required acid dosecan be calculated. The calculation can be appliedin any case to calculate the relationship betweenpH and acid dosing rate.

2.3.3. Acid dosing calculation

The following calculation has the scope todefine the relationship between the pH and theacid dosing.

We can start from the equation:

+3

12

[H ] [HCO ][CO ]

k−⋅= (21)

that we can also write evidencing the activity coef-ficient like:

+3

12

[H ] [ HCO ][CO ]

k−⋅ γ= (22)

where k1 is the first dissociation constant as de-scribed before. pγ1 is already calculated in limitingsalt calculation.

2.4. Dechlorination

2.4.1. Premises

Before entering in the membrane trains the feedwater residual chlorine content has to be removedto avoid damages to the membrane. The stoichio-metric weight ratios of the most common sulphitecompounds needed per mg/l of residual chlorineare given in Table 1.

2.5. Scale inhibitors

Scale inhibitors (antiscalants) can be used tocontrol carbonate scaling, sulphate scaling and


Table 1Dechlorination compounds needs

Dechlorination compound Quantity, mg/(mg/l) residual Name Formula Molecular weight Stoichiometric amount Range in use Sulphur dioxide SO2 64.09 0.903 1.0–1.2 Sodium sulphite Na2SO3 126.04 1.775 1.8–2.0 Sodium bisulphite NaHSO3 104.06 1.465 1.5–1.7 Sodium metabisulphite Na2S2O5 190.10 1.338 1.4–1.6

calcium fluoride scaling. Scale inhibitors have a“threshold effect”, which means that minor amountadsorb specifically to the surface of microcrystalsthereby preventing further growth and precipi-tation of the crystals.

The dosage rates are in general defined by theantiscalant manufactures. In RO plants operatingon seawater with TDS in the range of 35,000 ppm,scaling is not such a problem as in brackish waterplants, because the recovery of seawater plants islimited by the osmotic pressure of the concentratestream to 30–45%. However, for safety reasons,for recovery ratio greater than 35%, is necessaryuse an antiscale inhibitor.

As final comment we underline that thecalcium presented in the raw water may form aprecipitate with the antiscalant at high antiscalantconcentrations.

3. Permeate posttreatment

3.1. Premises

The primary post treatment unit operations arethe disinfection and alkalinity recovery. Soluteremoval eliminates carbonate alkalinity, but alldissolved gases including carbon dioxide andhydrogen sulphide pass through the membranes.The sequence of unit operations assumed here isdisinfection followed by alkalinity recovery.

3.2. Disinfection

If chlorine is added to the process stream beforeaeration, stabilization occurs during aeration. The

chlorine converts some alkalinity that passesthrough the membranes to carbon dioxide. ThepH following chlorination can be determinedusing pK1 for carbonate system and the alpha forOCl–; this equation is applicable only when HCO3

–

is present. Once HCO3– is neutralized during

chlorination, pH can be determined by summingthe protons from the HCl added past the point ofneutralization to the protons at neutralization.

2 2Cl H O HOCl HCl+ ⇒ + (23)

The saturation pK for this last reaction is equalto 7.4

( )( )

Cl2

2 3

Cl2

3 OClH CO

2 3 OCl

HCO 1 CpH pK log

H CO 1 CT

T

−

−

−⎡ ⎤− + α ⋅⎢ ⎥= +⎢ ⎥+ + α ⋅⎣ ⎦

(24)

Chlorine addition to water will produce equalmoles of hypochlorous acid and hydrochloric acid.The hypochlorous acid will partially ionize tohypochlorite ions and protons; the hydrochloricacid will completely ionize, producing protons andchloride ions. One mole of protons will be pro-duced for every mole of hydrochloric acid andevery mole of hypochlorite ion produced. Con-sequently, the complete proton production duringchlorination would be cancelled by the additionof OH– as shown here.

3.3. Alkalinity recovery

If acid addition is used for scaling control, allthe alkalinity in the raw water will be destroyed


but not lost. The membrane is a closed systemand the carbon dioxide will remain under pressureuntil exposed to an open system. Alkalinityrecovery needs to be considered during scalingcontrol and depends on how much carbon dioxideand bicarbonate is in the raw water.

Normally finished waters with 1–3 meq/l ofbicarbonate alkalinity are considered highly de-sirable for corrosion control. Since carbon dioxidewill pass unhindered through the membrane, thedesired amount of alkalinity can be recovered inthe permeate by acidifying the desired amount,passing it trough the membrane, and adding thedesired amount of base to convert the carbondioxide back to its original bicarbonate form.

In the case the base used is Ca(OH)2 thereaction can be resumed as:

2+2 2 2 3

2

2CO H O Ca(OH) Ca 2HCO H O

−+ + ⇒ ++ (25)

Considering to leave on the permeate a CO2residual content of 0.5 ppm (CO2res) we obtain thedosing rate of lime to convert the alkalinity inbicarbonate form.

( )2 2res2

CO COCa(OH) 74

88−

= ⋅ (26)

The quantity of bicarbonates produced is equalto:

( )2 2res3

CO COHCO 61

44− −

= ⋅ (27)

The quantity of Ca2+ added is:

( )2 2res2+ CO COCa 40

88−

= ⋅ (28)

4. Energy recovery system balance

The pressure exchanger (PX) device transfersthe energy from the concentrate stream directlyto the feed stream using a cylindrical rotor withlongitudinal ducts. A virtual liquid piston moves

back and forth inside each duct creating a barrierzone that inhibits mixing between the concentratereject and new seawater streams. At 1500 rpm onerevolution is completed every 1/25 s. Due to thisshort cycle time membrane feed water concentra-tion typically increase of 2–3%.

The consequence of this concentration in-creasing is that the reverse osmosis section has tobe designed to treat water with a different salinityof the original raw water.

The chemicals dosing rates and in particularthe acid addition have to be calculated on theoriginal water, whereas the reverse osmosis mem-brane performances have to be evaluated on themodified salinity figures.

The calculation programs that the membranemanufacturers supply to the designers does nottake into account this fact because are focused onthe membrane performances calculation startingfrom the feed analysis. The PX circuit is shownin Fig. 1.

The results of the balance around the systemfor a train of 1 MIGD are shown in Table 2.

As evidenced in the sheet the feed watersalinity at the membranes inlet is increased of2.3% with respect to the raw water. Moreover alsothe carbonic species equilibrium are modifiedchanging the membrane inlet conditions.

Fig. 1. PX circuit.

PE

Booster Pump

PX

MEMBRANES

AB

C

D

E F

G

H


Table 2Results of the balance around the system for a train of 1 MIGD

A B C D E F G H Flow

GPM 2087.0 1213.2 873.8 1213.3 2087.0 834.8 1252.2 1252.2 m3/h 474 276 198 276 474 190 284 284 m3/d 11375 6612 4762 6612 11375 4549.9 6825 6825

Pressure psi 14.5 14.5 914.96 871.45 914.96 0 885.96 0 bar 1.0 1.0 63.1 60.1 63.1 0 61.1 0

Quality sea sea sea sea sea perm brine brine TDS 42,000 42,000 42,000 43,663 42,967 236 71,454 68,732

5. Calculation model

The model developed has the aim to make thecomplete balance calculation of a reverse osmosisdesalination plant adopting the energy recoverydevice.

In particular for each stream the followingparameters are calculated:• Ionic salt balance• Carbonic species equilibrium• pH• Saturation pH• Scale indexes

The main inputs to the calculation workbookare resumed in Table 3.

The raw water analysis input is correctedtaking into account the PX salt losses to make theexact feeding conditions at the membranes inlet(Table 4).

The chemicals dosing rate for the feed waterare calculated starting from the raw water analysisand for each step of the treatment the completecarbonic species equilibrium is calculated. Thecalculation workbook includes:• Data input worksheet• Calculation worksheets divided in three

sections:– Seawater pre treatment– PX balance– Permeate post treatment

Table 3RO desalination plant summary

General information Plant RO plant Seawater temperature, °C 27 Permeate production, m3/h 189.58 Number of units 3 Recovery, % 40 Average system flux, l/m2h 14 Membrane DP, bar 2

Design parameters Raw water pH 8.1 Treated water pH 7.5 Residual CO2 in permeate, ppm 0.05 Salinity increase in respect to nominal conditions, %

0

Salinity increase through ERS, % 3.96 Fouling factor 1

Circuit pressure data Concentrate residual pressure 1 Sand filters pressure drop 2 Cartridge filters pressure drop 0.5 Line pressure drop 0.1

• Mass balance

The PX balance is reported in Appendix 1,whereas Appendix 2 shows the complete massbalance of the plant for a single pass solution.

The model can quickly simulate the behaviourof the complete RO system in different runningconditions starting from a first design approach


Table 4Seawater analysis

made by means of common membrane manu-facturers’ software to define the main systemcharacteristics.

All the software available does not include thepossibility to calculate the feed watercharacteristics variation during the different ROpre-treatment operation before the membrane traininlet and constrain the designer to make boringcalculation to define the true feed to the membranesection.

The main feature of the new calculation systemis that the water quality in terms of pH, TDS andcarbonic species during the various phases of theprocess can be followed and the PX effect on thesystem performances is taken into account,carrying out a new approach to the RO plantdesign software.

Seawater analysis Seawater analysis design Seawater RO inlet ppm ions ppm CaCO3 ppm ions ppm CaCO3 ppm ions ppm CaCO3 Calcium (++) 500.60 1251.50 500.60 1,251.50 512.49 1,281.24 Magnesium (++) 1,773.90 7,294.00 1,773.90 7,294.00 1,816.05 7,467.30 Sodium (++) 12,573.10 27,336.39 12,573.10 27,336.39 12,871.84 27,985.90 Potassium (+) 454.70 581.52 454.70 581.52 465.50 595.33 Total cations (+) 15,302.30 36,463.41 15,302.30 36,463.41 25,665.68 37,329.78 Chloride (–) 23,561.00 33,224.75 23,561.00 33,224.75 24,123.91 34,018.54 Bromides (–) 0.00 0.00 0.00 0.00 0.00 0.00 Sulfate (– –) 3,024.20 3,148.24 3,024.20 3,148.24 3,101.25 3,228.45 Phosphate (– –) 0.00 0.00 0.00 0.00 0.00 0.00 Fluorides 1.40 3.68 1.40 3.68 1.43 3.77 Nitrates 0.00 0.00 0.00 0.00 0.00 0.00 Carbonate 0.51 0.85 0.51 0.85 0.16 0.26 Bicarbonate 110.70 90.71 110.70 90.71 108.74 89.10 Total anions (–) 26,697.81 36,468.24 26,697.81 36,468.24 27,335.48 37,349.12 M (T) alkalinity 91.56 91.56 89.36 P alkalinity 0.43 0.43 0.13 Carbonate hardness 91.56 91.56 89.36 Non-carbonate hardness 8,453.93 8,453.93 8,659.18 Total hardness 8,545.50 8,545.50 8,748.54 TDS 42,000.11 42,000.11 43,001.36

BibliographyHigh Temperature Scale Inhibitors for Seawater Dis-

tillation. Watson Desalination Consultants, October1979.

H.E. Homig, Seawater and Seawater Distillation, VulkanVerlag, 1978

J.A. Medina San Juan, Desalacion de agues salobres y demar — osmosis inverse. Ediciones Mundi, Prensa,2000.

Water Quality and Treatment. American Water WorksAssociation, 5th ed., McGraw Hill, 1999.

Water Treatment Plant. American Water Works Asso-ciation, 3rd ed., McGraw Hill, 1998.

Water Treatment — Membrane Processes. AmericanWater Works Association Research Foundation,Lyonnaise des Eaux, Water Research Commissionof South Africa, McGraw Hill, 1996.


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71 2

9542

.28

4015

2.03

17

3.56

23

9.32

492

36.1

1 66

918.

96 4

7286

.36

6426

8.97

M (T

) alk

alin

ity

11

6.27

116.

27

11

6.27

12

0.88

118.

95

1.

19

19

8.02

190.

18P

alka

linity

0.09

0.09

0.09

0.

09

0.

09

0.

00

0.

15

0.

15C

arbo

nate

ha

rdne

ss

11

6.27

116.

27

11

6.27

12

0.88

118.

95

1.

19

19

8.02

190.

18

Non

-car

bona

te

hard

ness

7501

.48

75

01.4

8

7501

.48

77

98.5

1

7674

.15

12

.61

12

788.

91

12

282.

47

Tota

l har

dnes

s

7617

.73

76

17.7

3

7617

.73

79

19.3

9

7793

.10

13

.80

12

986.

93

12

472.

65TD

S, m

g/l

4572

0.47

4572

0.47

4572

0.47

4753

1.00

46

773.

03

286.

09

7795

3.40

7486

6.45

pH

7.

20

7.

20

7.

20

7.22

7.21

5.21

7.43

7.41

Satu

ratio

n pH

6.50

6.50

6.50

6.

46

6.

48

11

.17

6.

06

6.

09La

ngel

ier i

ndex

0.70

0.70

0.70

0.

75

0.

73

–5

.96

1.

38

1.

32St

iff a

nd D

avis

in

dex

–0

.59

–0

.59

–0

.59

–0

.55

–0

.57

–5

.78

0.

05

–0

.01

Ryz

ner i

ndex

5.79

5.79

5.79

5.

71

5.

75

17

.13

4.

68

4.

76C

a/M

g

0.35

0.35

0.35

0.

21

0.

21

0.

21

0.

21

0.

212 4

3

c(C

l)

2c(S

O)

HC

O

−−

+

mol

e/m

ole

337.

03

33

7.03

337.

03

33

7.03

33

7.03

19

6.45

33

7.42

337.

42


App

endi

x 2

Rev

erse

osm

osis

pla

nt 3

MG

D. M

ass

bala

nce.

DW

G.Se

awat

er te

mpe

ratu

re 3

0°C

.

Seawater reverse osmosis plant using the pressure ... · X × =⋅ × (2) The corresponding...

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Transcript of Seawater reverse osmosis plant using the pressure ... · X × =⋅ × (2) The corresponding...