Characterization of nanofiltration membranes

for scale prevention in seawater reverse

osmosisL. Llenas*, G. Ribera*, A. Yaroshchuk**.***, X. Martínez-Lladó*, M. Rovira*.***, J. De Pablo*.***

* CTM Centre Tecnològic; Av. Bases de Manresa 1, 08242 Manresa, Spain

(E-mail: laia.llenas@ctm.com.es, xavier.martinez@ctm.com.es)

** Institució Catalana de Recerca i Estudis Avançats (ICREA)

***Department of Chemical Engineering, Polytechnic University of Catalonia; Av. Diagonal 647, 08028

Barcelona,Spain

One of the major problems in seawater reverse osmosis (SWRO) and a limiting

factor for its proper operation is membrane scaling (Schäfer et al. 2006). Hassan et

al (Hassan et al. 1998), proposed the use of nanofiltration (NF) as a pretreatment of

desalination. Feeding of the NF permeate to desalination plants, not only made

possible their operation with less chemicals and raising significantly their permeate

and distillate recovery ratios, but also allowed for lowering their energy consumption.

Several studies have shown that the rejection of scale-forming ions is not the same

for various membranes (Hassan et al. 2000). The aim of this work is to study the

productivity and the selectivity of eleven different nanofiltration membranes to

prevent fouling in SWRO caused by inorganic compounds presents in seawater. All

membranes have been characterized in terms of surface roughness by Atomic

Force Microscopy.

INTRODUCTION

All the experiments were carried out in a laboratory scale

cross-flow test cell in total recirculation mode. See Fig. 1:

• The rejection of divalent ions is good in most of membranes tested and that is

very important for the prevention of scaling.

• Roughness have been measured with AFM. Membranes with a higher roughness

present a high conductivity rejection and vice versa. Colloidal fouling in NF

membranes is strongly correlated with membrane surface roughness. Membranes

with high surface roughness are more prone to fouling.

• The most suitable NF membranes as pretreatment for scaling prevention are:

NF270 (Dow Chemical), K-SR2 (Koch) and NF99HF (Alfa Laval).

METHODOLOGY ION REJECTION RESULTS AND MODEL FITTING

Atomic Force Microscopy (AFM) has been used to characterize

nanofiltration membranes tested in terms of surface roughness.

Analytical methods for the analysis of ion rejections: Ionic

Chromatography (Dionex ICS-2100); Total Carbon Analyzer.

Shimadzu TOC-5050A. was used to analyze inorganic carbon

and Inductively Coupled Plasma Mass Spectrometry (ICP-MS).

Agilent 9500cx

Fig.1. Flow sheet of experimental system used

ATOMIC FORCE MICROSCOPY RESULTS

Table 1. Surface roughness of membranes at a scan size of 5 x

5 µm2 compared with permeate flow and conductivity rejection

NF270

NF200

NF

NF90

ESNA 1-

LF2

K-SR2

K-SR3

K-TFCS

NF99

NF99HF

CONCLUSIONS

Fig.6. AFM images for ten membranes characterized

30

40

50

60

70

80

90

100

0 50 100 150

% S

ulp

ha

te r

ejec

tio

n

Permeate flow (l·h-1·m-2)

NF270

NF200

NF

ESNA 1-LF2

K-TFCS

K-SR2

K-SR3

ALNF99

NF99HF

NF90

0

10

20

30

40

50

60

70

80

90

100

0 50 100 150

% C

alc

ium

rej

ecti

on

Permeate flow (l·h-1·m-2)

NF270

NF200

NF

ESNA 1-LF2

K-TFCS

K-SR2

K-SR3

ALNF99

NF99HF

NF90

0

10

20

30

40

50

60

70

80

90

100

0 50 100 150

%

Ma

gn

esiu

m r

ejec

tion

Permeate flow (l·h-1·m-2)

NF270

NF200

NF

ESNA 1-LF2

K-TFCS

K-SR2

K-SR3

ALNF99

NF99HF

NF90

0

10

20

30

40

50

60

70

80

90

100

0 50 100 150

%

Bic

arb

on

ate

rej

ecti

on

Permeate flow (l·h-1·m-2)

NF270

NF200

NF

ESNA 1-LF2

K-TFCS

K-SR2

K-SR3

ALNF99

NF99HF

NF90

Fig.2. Calcium rejection and SK fit Fig.3. Sulphate rejection and SK fit

Fig.4. Magnesium rejection and SK fit Fig.5. Bicarbonate rejection and SK fit

Figures 2-5 show the scaling forming ions rejections of different NF membranes tested, as well

as the fitting with the Spiegler Kedem model.

Membrane Rms (nm) Permeate flow (lmh) % RejectionNF90 103.3 10.7 60.8

K-TFCS 50.17 26.3 52.8

ESNA 1-LF2 49.07 137.9 22.1

NF99HF 12.29 104.3 28.6

NF99 11.87 50.0 31.3

NF 7.43 58.0 29.6

NF200 7.39 59.5 23.2

NF270 5.35 108.8 20.5

K-SR3 1.78 49.0 32.8

K-SR2 0.76 127.0 15.7

• Hassan A.M, et al (1998) A new approach to membrane and thermal seawater

desalination processes using nanofiltration membranes (Part 1). Desalination 118, 35-

51

• Hassan A.M, et al (2000) A demonstration plant based on the new NF-SWRO

process, Desalination 131, 157-171

• Schäfer A.I, Fane A.G, Waite T.D (2006) Nanofiltration principles and applications.

Elsevier publications

• Spiegler K.S., Kedem O.; Thermodynamics of hyperfiltration (reverse osmosis):

criteria for efficient membranes (1966) Desalination 1, 311-326

REFERENCES

For the parameterization of our experimental data we used the Spiegler – Kedem

model (Spiegler and Kedem. 1966). Within its scope the solute rejection. R. is

related to the trans-membrane volume flow in this way:

The fitting parameters obtained with the Spiegler Kedem model are apparent ones,

in particular, because they depend on the cross-flow velocity. However, if this is not

varied (as it was the case in the present study) the apparent reflection coefficients

and solute permeabilities can be used for the quantitative comparison of

performance of various membranes as well as for the interpolation of rejection data

between the available experimental points.

This study was financially supported by Sociedad General de Aguas de Barcelona (AGBAR) within the scope of CENIT project “Desarrollos tecnológicos hacia un ciclo del agua

urbano auto-sostenible (SOSTAQUA)”.