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Haemodialysis water production by double reverse osmosis
Desalination 267 (2011) 88–92
Contents lists available at ScienceDirect
Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Haemodialysis water production by double reverse osmosis E. Pislor a,b,⁎, M. Alignan a,b, P.-Y. Pontalier a,b, S. Grangé c a b c
Université de Toulouse, INP, LCA (Laboratoire de Chimie AgroIndustrielle), ENSIACET, 4 Allées Emile MONSO, F-31029 Toulouse, France INRA, LCA (Laboratoire de Chimie AgroIndustrielle), F-31029 Toulouse, France Société C2R, 8 Route Belberaud, 31450 Pompertuzat, France
a r t i c l e
i n f o
Article history: Received 6 May 2010 Received in revised form 9 September 2010 Accepted 9 September 2010 Available online 16 October 2010 Keywords: Haemodialysis water Water treatment Double reverse osmosis
a b s t r a c t A filtration unit composed of two reverse osmosis membranes in series was evaluated for haemodialysis water production. For this purpose, hard and soft water were formulated based on European drinking water directives. The study showed that the double reverse osmosis process is efficient to produce haemodialysis water with the quality defined by the pharmacopeia requirements from synthetic soft water and hard water. In both cases, nitrate and sodium ions diffuse through the membrane limiting the water treatment. This phenomenon is correlated to the electrostatic interactions, the Donnan effect and the ion diffusivity, which govern the membrane selectivity. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Since the early 1960s, haemodialysis (HD) has been increasingly used for the treatment of acute and end-stage renal failure [1,2]. According to technologic advances in dialyser membranes, dialysis machines and vascular access, HD is now a routine procedure. Nonetheless, water may contain impurities, bacteria, endotoxins, metal, mud, sediments or chemicals. Those elements could be transferred to the bloodstream of the patient by diffusion through the dialyser membranes causing disease and injury [3]. As the dialysis uses large amount of water (300 L/week), even scarce amount of contaminants can be dangerous. Fortunately, the advances in dialysis practice have been paralleled to continuous improvement in water treatment technology [4]. Natural water contains some undesirable substances which can be removed by various processes such as deionisation, clarification and filtration. Drinking water treatment systems are composed of several devices applied in series. The choice of these devices and their combination depends on the quality of the water supply or the amount of water required. Usually, systems to produce HD water combine an iron remover, a water softener, a deioniser, an activated charcoal filter, a particle filter and a reverse osmosis. Actually, no treatment system for standard dialysis water production exists. As modalities are to be adapted to the needs of the studied unit [4], the conception time of those units is long.
⁎ Corresponding author. Université de Toulouse, INP, LCA (Laboratoire de Chimie AgroIndustrielle), ENSIACET, 4 Allées Emile MONSO, F-31029 Toulouse, France. Tel.: +33 5 34 32 35 55; fax: +33 5 34 32 35 98. E-mail address: [email protected] (E. Pislor). 0011-9164/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2010.09.010
The production of drinking water from sea water is widely used in many countries. As those treatment systems are similar to HD water treatment units, they could inspire development of new HD procedures. Reverse osmosis membrane technology has allowed a rapid expansion of desalination industry to respond to water needs [5,6]. In such plants, deionised water is produced from two successive reverse osmosis steps. Sea water membranes are used in the first step and brackish water membranes in the second [7]. Despite the high salt content of the sea water, the double reverse osmosis systems produce high quality deionised drinking water. The double reverse osmosis could be integrated to the dialysis water treatment system, in order to limit the pre-treatment sequence, shunting the deioniser and/or the softener step for example. This process could also facilitate the conception of HD units. The aims of this work are to describe how the double reverse osmosis performances reach the chemical requirement of HD water, and to demonstrate why this technology is applicable to any drinking water supply in Europe. 2. Material and methods 2.1. Analysis The anion analyses were carried out with the Dionex ICS-2000 ion exchange chromatography system constituted of a KOH EluGen® EGC II KOH cartridge, an anionic suppressor ASRS-ULTRA II 4 mm, a column IonPac ICE-AS18 250 × 4 mm and a pre column IonPac ICEAS18 50 × 4 mm. The analyses were done under isocratic mode of NaOH at 32 mM, with a flow rate of 1.0 mL min−1, at a column temperature of 20 °C and a suppressor current of 80 mA.
E. Pislor et al. / Desalination 267 (2011) 88–92
89
Table 2 Composition of the feed solutions, compared to the European drinking water standard. Ions
Drinking water
Hard water
Soft water
40 0.3 100 546 77 0.6 50 200 0.03 0.725
508 9 100 700 639 20 15 32 0.5 0.300
OJEC 1998
Fig. 1. Double reverse osmosis installation purchased by C2R Company.
All cations, except aluminium, were identified with a Dionex ICS-3000 ion exchange chromatography system, which includes a methane sulfonic acid cartridge (MSA), a cationic suppressor CSRS-ULTRA II 4 mm, a column IonPac CS 12A 250×4 mm and a pre column IonPac CG 12A 50×4 mm. Mobile phase solution consisted of 20 mM methane sulfonic acid. The flow rate of the system was maintained at 1.0 mL min−1, with a column temperature of 20 °C and a suppressor current of 59 mA. The software Chromeleon version 6.6, from Dionex, was used for data acquisition. Before sample analysis, a calibration curve of each ion was done using five standard solutions. 25 μL of every standard and diluted sample was injected twice and successively. To avoid column saturation, water was also injected twice after each sample analysis. Aluminium analyses were carried out by Bio-Pôle laboratory (Labège, France) by optical emission spectroscopy with high frequency plasma induction ICP-OES, according to the NF EN ISO 11885 norm. 2.2. Apparatus The installation purchased by C2R Company was composed of an 80 L feed tank, a heat exchanger, a gear pump and two membranes in series. The formulated water was introduced in the feed tank and flowed to the first membrane, called the first stage. The pressure of feed was maintained to 40 bar whatever the formulated water. The retentate was recycled to the feed tank, while the permeate was flowed to the second membrane, named the second stage. On the second step, the permeate and retentate were both recycled to the feed tank (Fig. 1). For the both feed solution, the pressure of the permeate was 2 bar. The water was maintained at a constant temperature of 15 °C. The first cartridge contained a TM 810 membrane (Toray), while the second contained a TR-70 4021 membrane (Toray). The characteristics of each membrane are described in (Table 1). During filtration, flow rate, pressure and conductometric measurements were done on-line twice a day by means of manometers, area flowmeters (Blue-White Industries) and conductometers (8225r serie, Bürkert). The measurement points were on the feed pipe and on the retentate and permeate pipes for first and second stages. Twice a day, two samples were taken from the permeate pipes of the first and second levels. Every five days, the apparatus was stopped for cleaning.
Chloride (Cl−) (mg/L) Fluoride (F−) (mg/L) Nitrate (NO− 3 ) (mg/L) Sulphate (SO2− 4 ) (mg/L) Sodium (Na+) (mg/L) Potassium (K+) (mg/L) Magnesium (Mg2+) (mg/L) Calcium (Ca2+) (mg/L) Aluminium (Al3+) (mg/L) Resistivity (kΩ cm)
50 1.5 50 250 175 12 50 100 0.2 0.750
The cleaning procedure has been adapted to the double reverse osmosis unit. First of all, the pilot was drained and 12 L of Dialox™ (solution of peracetic acid at 3%v) was introduced in the feed side of the second stage with a volumetric pump (Aldos Eicher Gmbh, type Primus). The solution, supplied between the first and second membranes by the permeate pipe, was left for 30 min to clean both membranes by diffusion. Then the solution was filtered during 30 min to assure a dynamic cleaning. After filtration, Dialox™ was rinsed out with deionised water. When the permeate resistivity of the second stage reached 999 kΩ cm, the highest value of the conductometer, the rinsing procedure was stopped. 2.3. Chemicals and solutions HD water is normally produced from drinking water but in this work, in order to control the composition of water supply, the filtration was done with synthetic water. The European Directive 98/ 83/EC sets the highest limit of anion and cation contents in drinking water. According to the geographic area, variations may occur in the ion composition such as nitrate or calcium. For that reason two types of feed, hard and soft water, were prepared (Table 2). The solutions were prepared with AlCl3, CaCl2, Ca(NO3)2, NaHCO3, KF, CaSO4, MgSO4 and NaCl from Aldrich. Those salts were diluted in milliQ water. 3. Results and discussion The composition of the dialysate is defined by a monograph, regularly revised. The values for the main chemical compounds of the third edition are proposed in Table 3. 3.1. Hard water The filtration of hard water was carried out during fifteen days. Only one cleaning procedure was done after ten days. So the filtration occurred in two parts. HD water was produced with a constant transmembrane pressure of 17 bar at the first stage while the pressure at the second stage could vary from 15 to 18 bar. According to the slight increase in permeate flux (Fig. 2) and the transmembrane
Table 3 Ion composition of dialysed water (extract of the third European monograph edition (1997)).
Table 1 Membrane characteristics. Characteristics
TM 810
TR-70 4021
Size Filtering area (m2) Composition NaCl rejection rate (%) Permeability (L/h m2) Pressure (bar) pH limit Charge
4″ × 40″ 6.8 Polyamide aromatic 99.1 2.7 55 2–11 Negative
4″ × 21″ 3.5 Polyamide aromatic 98 4.3 15 3–11 Negative
Composition (mg/L)
Standard or highest limit
F− Cl− NO− 3 SO2− 4 + Na K+ Mg2+ Ca2+ Al3+
0.2 50 2 50 50 2 2 2 0.01
90
E. Pislor et al. / Desalination 267 (2011) 88–92
Fig. 2. Evolution of the two stages of permeate flux during hard water filtration.
Fig. 3. Evolution of the two stages of permeate resistivity during hard water filtration.
pressure, the permeability of the two membranes remained constant at 1.1 L/h m² bar for the first stage and 1.5 L/h m² bar for the second. It appears that the membrane fouling was not significant. During the first five days, the flux increased for both membranes while the transmembrane pressure was almost constant. The flux enhancement could be correlated to the progressive penetration of the ions into the membranes, swelling the latter. As the membranes were new, another hypothesis could be the liberation of some blocked
pores. After a long term filtration, the permeate flux remained stable. After cleaning the flux becomes higher but rapidly comes back to the previous value, probably due to membrane ion fouling. Filtration performance could be evaluated by measurement of the permeate resistivity. This measure was done on the first and second stages. During the first five days, the resistivity of both permeates decreased. In order to determine if the decreasing salt was constant or tended to a limit value, the experiments were conducted during ten days without cleaning (Fig. 3). As the selectivity of membranes depends on ion diffusivity and electrostatic repulsion, the resistivities reached a limit value. At the beginning of the filtration, ions could be fixed on the membrane causing the membrane charge screening. Then electrostatic repulsion effect is limited and ions may diffuse through the membranes. So the membrane selectivity is due to the difference in ion diffusivity. According to Fig. 3, permeate resistivity of the first membrane decreased from 300 to 200 kΩ cm and reached a constant value. So the selectivity of this membrane is partly affected by the membrane charge screening but a large part of its selectivity depends on ion diffusion. Thus, the first filtration stage induces the reduction of ionic concentration in the permeate. On the contrary, the permeate resistivity of the second membrane declined from 900 to 400 kΩ cm before stabilising. The membrane charge screening effect seems to have an important impact on the permeate resistivity. So the selectivity of this membrane depends more on electrostatic effects than on ion diffusion. This hypothesis is confirmed because the second membrane receives the highest water permeability, probably because the first membrane is tighter. After cleaning, the selectivity was partly recovered for the two membranes. As the first part, the permeate resistivity of the second membrane decreased more than the first one due to the membrane charge screening effect. The limit value of resistivity was reached for the permeate of the first stage contrary to the second one because the filtration time was too short. In order to eliminate salts fixed on the membrane, the cleaning procedure was done using peracetic acid [8]. After ten days of filtration, the cleaning led to the recovery of the initial permeate resistivity of the first stage. In the second stage, as the permeate resistivity of 999 kΩ cm was difficult to obtain, the rising procedure was stopped when the value reached 800 kΩ cm. Even if during the first ten days the water produced with respect to the pharmacopeia requirement, the disinfection had to be done every week to recover filtration performances. The high rejection rate of the first membrane can be assumed because in the permeate the ion concentrations were all below the analytical detection value, except for sodium and nitrate (Table 4). According to the
Table 4 First stage permeate composition.
Before cleaning
After cleaning
Time (min)
F− (mg/L)
Cl− (mg/L)
NO− 3 (mg/L)
SO2− 4 (mg/L)
Na+ (mg/L)
K+ (mg/L)
Mg2+ (mg/L)
Ca2+ (mg/L)
Al3+ (mg/L)
20 3400 6870 9742 25 4895
n.d n.d n.d n.d n.d n.d
b 0.5 b 0.5 b 0.5 b 0.5 b 0.5 b 0.5
0.66 0.60 0.59 0.57 0.66 0.59
b 0.5 b 0.5 b 0.5 b 0.5 0.56 b 0.5
0.59 0.39 b0.45 0.50 0.54 0.40
b 0.5 n.d b 0.5 b 0.5 b 0.5 n.d
b 0.25 b 0.25 b 0.25 b 0.25 b 0.25 b 0.25
b0.5 b0.5 b0.5 b0.5 b0.5 b0.5
b0.02 b0.02 b0.02 b0.02 b0.02 b0.02
Time (min)
F− (mg/L)
Cl− (mg/L)
NO− 3 (mg/L)
SO2− (mg/L) 4
Na+ (mg/L)
K+ (mg/L)
Mg2+ (mg/L)
Ca2+ (mg/L)
Al3+ (mg/L)
20 3400 6870 9742 25 4895
n.d n.d n.d n.d n.d n.d
n.d n.d n.d n.d b 0.5 n.d
n.d n.d n.d n.d n.d n.d
n.d n.d n.d n.d b 0.5 n.d
b 0.2 b 0.2 b 0.2 b 0.2 b 0.2 b 0.2
n.d n.d n.d n.d n.d n.d
n.d n.d n.d n.d n.d n.d
n.d n.d n.d n.d n.d n.d
b0.02 b0.02 b0.02 b0.02 b0.02 b0.02
n.d: not detected.
Table 5 Second stage permeate composition.
Before cleaning
After cleaning n.d: not detected.
E. Pislor et al. / Desalination 267 (2011) 88–92
91
exclusion of the membrane, the Donnan effect could favor the diffusion of anions depending on the diffusion of the counterions. In this study, the nitrate transfer favors the sodium permeability by Donnan effect. The second permeate composition showed that the double reverse osmosis treatment produced HD water according to the pharmacopeia requirement (Table 5). Even if the first step of filtration is sufficient to produce high quality water, the second step must be maintained to prevent system failure.
3.2. Soft water
Fig. 4. Permeate flux evolution of the two stages during the filtration of the soft water.
Fig. 5. Evolution of the permeate resistivity during the soft water filtration.
stability of sodium and nitrate concentrations all along the filtration, the membrane charge screen seems to have no impact on the ion rejection rate. This affirmation is confirmed by the results of resistivity, which showed that the first membrane strongly depends on ion diffusion. Authors report that the transfer of monovalent anions through the membrane seems to be easier than divalent anions [9]. In the case of monovalent anions, the rejection of chloride appears higher than of nitrate, probably because of better hydration of this ion in aqueous solution [10]. According to solution pH, membrane charge or dielectric
Soft water was filtered after cleaning the membranes with Dialox. The first stage transmembrane pressure was maintained at 17 bar during ten days while the second stage varied from 13 to 15 bar. One cleaning was done at midterm, delimiting the first and second parts of the experimentation. The graphic of the permeate flux to time showed us an evolution. Hence, similarly to hard water, there is no long term fouling during filtration of soft water (Fig. 4) and the permeability of the two membranes remained constant at 0.95 L/h m² bar at the first stage and 1.40 L/h m² bar for the second. The flux increased slightly at the beginning of the first stage filtration while it was almost constant for the second stage. After one week of filtration, the cleaning seemed to be efficient. The permeability was slightly lower than the initial value for the first membrane and the same for the second step. The permeate resistivity of the first membrane was slightly above 100 kΩ cm. It remained constant during the whole filtration time (Fig. 5). This value was lower than the resistivity obtained during hard water filtration, probably because the ion retention is less efficient. At the beginning, the second membrane allowed the production of HD water with a resistivity higher than 900 kΩ cm, but after one week of decreasing, it reached to 450 kΩ cm. The cleaning brings back the initial resistivity of the permeate, but this effect was lower for the first membrane because the electrostatic interaction with ions is lower for this membrane. After the cleaning, the resistivity of the second membrane permeate is higher than 999 kΩ cm at the beginning, but regularly decreased during the filtration. So in order to keep the filtration efficient and ensure the quality of HD water produced, the cleaning is required every week. The monovalent cation concentrations were higher than in the case of hard water filtration (Table 6). Thus the high sodium chloride concentration into soft water seems to facilitate the diffusion of sodium and nitrate through the membrane. As the diffusion of counterions is high, the diffusion of anions was facilitated by Donnan effect [11]. Furthermore, after more than two days filtration, the ion concentration in the permeate
Table 6 First stage permeate composition.
Before cleaning After cleaning
Time (min)
F− (mg/L)
Cl− (mg/L)
NO− 3 (mg/L)
SO2− (mg/L) 4
Na+ (mg/L)
K+ (mg/L)
Mg2+ (mg/L)
Ca2+ (mg/L)
Al3+ (mg/L)
35 6230 46 3890
n.d n.d n.d n.d
1.23 0.92 1.10 0.95
1.18 0.80 0.98 0.87
0.64 b0.5 0.62 b0.5
2.00 1.51 1.79 1.54
b0.5 b0.5 b0.5 b0.5
n.d n.d b0.25 b0.25
b 0.5 n.d b 0.5 b 0.5
b 0.02 b 0.02 b 0.02 b 0.02
Time (min)
F− (mg/L)
Cl− (mg/L)
NO− 3 (mg/L)
SO2− (mg/L) 4
Na+ (mg/L)
K+ (mg/L)
Mg2+ (mg/L)
Ca2+ (mg/L)
Al3+ (mg/L)
35 6230 46 3890
n.d n.d n.d n.d
b 0.5 b 0.5 b 0.5 b 0.5
b 0.5 b 0.5 n.d b 0.5
b 0.5 b 0.5 b 0.5 b 0.5
b 0.2 b 0.2 b 0.2 b 0.2
n.d n.d n.d n.d
n.d n.d n.d n.d
n.d n.d n.d n.d
b 0.02 b 0.02 b 0.02 b 0.02
n.d: not detected.
Table 7 Second stage permeate composition.
Before cleaning After cleaning n.d: not detected.
92
E. Pislor et al. / Desalination 267 (2011) 88–92
might be influenced by the progressive shielding of membrane charge, but the rejection rates of divalent cations remained high during ten days. In the case of HD water production from soft water, one step filtration is insufficient because the monovalent ion concentration is over the pharmacopeia requirements. Therefore the second stage is necessary to produce good quality water (Table 7). At this step, the rejection rate of the divalent cations is very high, which could explain why the detection of magnesium and calcium was not possible. Nevertheless, the monovalent cations, mainly sodium, go through the membrane by Donnan effect because of the high anion concentration in the permeate. In all cases, the concentrations remained low, below the quantification level. 4. Conclusion The double reverse osmosis system, combining sea water and brackish water membranes, has been tested for the **haemodialysate production from hard and soft water. Since some ions could reach concentrations higher than those define by the European settlement, hard and soft water were representative of drinking water. Nevertheless, the double osmosis system allowed the production of HD water, as defined in the pharmacopeia requirement, with a simple pre-treatment which combines activated carbon and filtration unit. This pre-treatment was not described in the present study because of the use of synthetic water. But, even with this step, the double reverse osmosis system is still interesting to reduce the cost and the complexity of HD unit. The results showed that the second membrane is necessary to produce ultrapure water when the water supply contains high
monovalent anion content. Charge screening and Donnan effect play a key role in the efficiency of this system by changing the permeability of the counterions.
References [1] H. Lonsdale, The growth of membrane technology, J. Membr. Sci. 10 (1982) 81–181. [2] D. Stamatialis, B. Papenburg, M. Gironès, S. Saiful, S. Bettahalli, S. Schmitmeier, M. Wessling, Medical applications of membranes: drug delivery, artificial organs and tissue engineering, J. Membr. Sci. 308 (2008) 1–34. [3] M.S. Favero, Good clean water — how dialysis centers treat the vital fluid, Renalife 2 (1987) 14–16. [4] [4]M. Tong, W. Wang, T.H. Kwan, L. Chan and T.C. Au, Water treatment for hemodialysis, 3 (1) (2001) 7–14. [5] H. Mehdizadeh, Membrane desalination plants from an energy–exergy viewpoint, Desalination 191 (2005) 200–209. [6] I.C. Karagiannis, P.G. Soldatos, Water desalination cost literature: review and assessment, Desalination 223 (2008) 448–456. [7] Y.M. Kim, S.J. Kim, Y.S. Kim, S. Lee, I.S. Kim, J.H. Kim, Overview of systems engineering approaches for a large-scale seawater desalination plant with a reverse osmosis network, Desalination 238 (2009) 312–332. [8] P. Aimar, Filtration membranaire (OI, NF, UF) Mise en oeuvre et performances, Traité Génie des procédés des Techniques de l'Ingénieur, vol. W 4 110, 2006, pp. 1–15. [9] K. Linde, A.S. Jönsson, Nanofiltration of salts solutions and landfill leachate, Desalination 103 (1995) 223–232. [10] L. Paugam, C.K. Diawara, J.P. Schlumpf, P. Jaouen, F. Quéméneur, Transfer of monovalent anions and nitrates especially through nanofiltration membranes in brackish water conditions, Sep. Purif. Technol. 40 (2004) 237–242. [11] S. Choi, Z. Yun, S. Hong, K. Ahn, The effect of co-existing ions and surface characteristics of nanomembranes on the removal of nitrate and fluoride, Desalination 133 (2001) 53–64.
Contents lists available at ScienceDirect
Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Haemodialysis water production by double reverse osmosis E. Pislor a,b,⁎, M. Alignan a,b, P.-Y. Pontalier a,b, S. Grangé c a b c
Université de Toulouse, INP, LCA (Laboratoire de Chimie AgroIndustrielle), ENSIACET, 4 Allées Emile MONSO, F-31029 Toulouse, France INRA, LCA (Laboratoire de Chimie AgroIndustrielle), F-31029 Toulouse, France Société C2R, 8 Route Belberaud, 31450 Pompertuzat, France
a r t i c l e
i n f o
Article history: Received 6 May 2010 Received in revised form 9 September 2010 Accepted 9 September 2010 Available online 16 October 2010 Keywords: Haemodialysis water Water treatment Double reverse osmosis
a b s t r a c t A filtration unit composed of two reverse osmosis membranes in series was evaluated for haemodialysis water production. For this purpose, hard and soft water were formulated based on European drinking water directives. The study showed that the double reverse osmosis process is efficient to produce haemodialysis water with the quality defined by the pharmacopeia requirements from synthetic soft water and hard water. In both cases, nitrate and sodium ions diffuse through the membrane limiting the water treatment. This phenomenon is correlated to the electrostatic interactions, the Donnan effect and the ion diffusivity, which govern the membrane selectivity. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Since the early 1960s, haemodialysis (HD) has been increasingly used for the treatment of acute and end-stage renal failure [1,2]. According to technologic advances in dialyser membranes, dialysis machines and vascular access, HD is now a routine procedure. Nonetheless, water may contain impurities, bacteria, endotoxins, metal, mud, sediments or chemicals. Those elements could be transferred to the bloodstream of the patient by diffusion through the dialyser membranes causing disease and injury [3]. As the dialysis uses large amount of water (300 L/week), even scarce amount of contaminants can be dangerous. Fortunately, the advances in dialysis practice have been paralleled to continuous improvement in water treatment technology [4]. Natural water contains some undesirable substances which can be removed by various processes such as deionisation, clarification and filtration. Drinking water treatment systems are composed of several devices applied in series. The choice of these devices and their combination depends on the quality of the water supply or the amount of water required. Usually, systems to produce HD water combine an iron remover, a water softener, a deioniser, an activated charcoal filter, a particle filter and a reverse osmosis. Actually, no treatment system for standard dialysis water production exists. As modalities are to be adapted to the needs of the studied unit [4], the conception time of those units is long.
⁎ Corresponding author. Université de Toulouse, INP, LCA (Laboratoire de Chimie AgroIndustrielle), ENSIACET, 4 Allées Emile MONSO, F-31029 Toulouse, France. Tel.: +33 5 34 32 35 55; fax: +33 5 34 32 35 98. E-mail address: [email protected] (E. Pislor). 0011-9164/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2010.09.010
The production of drinking water from sea water is widely used in many countries. As those treatment systems are similar to HD water treatment units, they could inspire development of new HD procedures. Reverse osmosis membrane technology has allowed a rapid expansion of desalination industry to respond to water needs [5,6]. In such plants, deionised water is produced from two successive reverse osmosis steps. Sea water membranes are used in the first step and brackish water membranes in the second [7]. Despite the high salt content of the sea water, the double reverse osmosis systems produce high quality deionised drinking water. The double reverse osmosis could be integrated to the dialysis water treatment system, in order to limit the pre-treatment sequence, shunting the deioniser and/or the softener step for example. This process could also facilitate the conception of HD units. The aims of this work are to describe how the double reverse osmosis performances reach the chemical requirement of HD water, and to demonstrate why this technology is applicable to any drinking water supply in Europe. 2. Material and methods 2.1. Analysis The anion analyses were carried out with the Dionex ICS-2000 ion exchange chromatography system constituted of a KOH EluGen® EGC II KOH cartridge, an anionic suppressor ASRS-ULTRA II 4 mm, a column IonPac ICE-AS18 250 × 4 mm and a pre column IonPac ICEAS18 50 × 4 mm. The analyses were done under isocratic mode of NaOH at 32 mM, with a flow rate of 1.0 mL min−1, at a column temperature of 20 °C and a suppressor current of 80 mA.
E. Pislor et al. / Desalination 267 (2011) 88–92
89
Table 2 Composition of the feed solutions, compared to the European drinking water standard. Ions
Drinking water
Hard water
Soft water
40 0.3 100 546 77 0.6 50 200 0.03 0.725
508 9 100 700 639 20 15 32 0.5 0.300
OJEC 1998
Fig. 1. Double reverse osmosis installation purchased by C2R Company.
All cations, except aluminium, were identified with a Dionex ICS-3000 ion exchange chromatography system, which includes a methane sulfonic acid cartridge (MSA), a cationic suppressor CSRS-ULTRA II 4 mm, a column IonPac CS 12A 250×4 mm and a pre column IonPac CG 12A 50×4 mm. Mobile phase solution consisted of 20 mM methane sulfonic acid. The flow rate of the system was maintained at 1.0 mL min−1, with a column temperature of 20 °C and a suppressor current of 59 mA. The software Chromeleon version 6.6, from Dionex, was used for data acquisition. Before sample analysis, a calibration curve of each ion was done using five standard solutions. 25 μL of every standard and diluted sample was injected twice and successively. To avoid column saturation, water was also injected twice after each sample analysis. Aluminium analyses were carried out by Bio-Pôle laboratory (Labège, France) by optical emission spectroscopy with high frequency plasma induction ICP-OES, according to the NF EN ISO 11885 norm. 2.2. Apparatus The installation purchased by C2R Company was composed of an 80 L feed tank, a heat exchanger, a gear pump and two membranes in series. The formulated water was introduced in the feed tank and flowed to the first membrane, called the first stage. The pressure of feed was maintained to 40 bar whatever the formulated water. The retentate was recycled to the feed tank, while the permeate was flowed to the second membrane, named the second stage. On the second step, the permeate and retentate were both recycled to the feed tank (Fig. 1). For the both feed solution, the pressure of the permeate was 2 bar. The water was maintained at a constant temperature of 15 °C. The first cartridge contained a TM 810 membrane (Toray), while the second contained a TR-70 4021 membrane (Toray). The characteristics of each membrane are described in (Table 1). During filtration, flow rate, pressure and conductometric measurements were done on-line twice a day by means of manometers, area flowmeters (Blue-White Industries) and conductometers (8225r serie, Bürkert). The measurement points were on the feed pipe and on the retentate and permeate pipes for first and second stages. Twice a day, two samples were taken from the permeate pipes of the first and second levels. Every five days, the apparatus was stopped for cleaning.
Chloride (Cl−) (mg/L) Fluoride (F−) (mg/L) Nitrate (NO− 3 ) (mg/L) Sulphate (SO2− 4 ) (mg/L) Sodium (Na+) (mg/L) Potassium (K+) (mg/L) Magnesium (Mg2+) (mg/L) Calcium (Ca2+) (mg/L) Aluminium (Al3+) (mg/L) Resistivity (kΩ cm)
50 1.5 50 250 175 12 50 100 0.2 0.750
The cleaning procedure has been adapted to the double reverse osmosis unit. First of all, the pilot was drained and 12 L of Dialox™ (solution of peracetic acid at 3%v) was introduced in the feed side of the second stage with a volumetric pump (Aldos Eicher Gmbh, type Primus). The solution, supplied between the first and second membranes by the permeate pipe, was left for 30 min to clean both membranes by diffusion. Then the solution was filtered during 30 min to assure a dynamic cleaning. After filtration, Dialox™ was rinsed out with deionised water. When the permeate resistivity of the second stage reached 999 kΩ cm, the highest value of the conductometer, the rinsing procedure was stopped. 2.3. Chemicals and solutions HD water is normally produced from drinking water but in this work, in order to control the composition of water supply, the filtration was done with synthetic water. The European Directive 98/ 83/EC sets the highest limit of anion and cation contents in drinking water. According to the geographic area, variations may occur in the ion composition such as nitrate or calcium. For that reason two types of feed, hard and soft water, were prepared (Table 2). The solutions were prepared with AlCl3, CaCl2, Ca(NO3)2, NaHCO3, KF, CaSO4, MgSO4 and NaCl from Aldrich. Those salts were diluted in milliQ water. 3. Results and discussion The composition of the dialysate is defined by a monograph, regularly revised. The values for the main chemical compounds of the third edition are proposed in Table 3. 3.1. Hard water The filtration of hard water was carried out during fifteen days. Only one cleaning procedure was done after ten days. So the filtration occurred in two parts. HD water was produced with a constant transmembrane pressure of 17 bar at the first stage while the pressure at the second stage could vary from 15 to 18 bar. According to the slight increase in permeate flux (Fig. 2) and the transmembrane
Table 3 Ion composition of dialysed water (extract of the third European monograph edition (1997)).
Table 1 Membrane characteristics. Characteristics
TM 810
TR-70 4021
Size Filtering area (m2) Composition NaCl rejection rate (%) Permeability (L/h m2) Pressure (bar) pH limit Charge
4″ × 40″ 6.8 Polyamide aromatic 99.1 2.7 55 2–11 Negative
4″ × 21″ 3.5 Polyamide aromatic 98 4.3 15 3–11 Negative
Composition (mg/L)
Standard or highest limit
F− Cl− NO− 3 SO2− 4 + Na K+ Mg2+ Ca2+ Al3+
0.2 50 2 50 50 2 2 2 0.01
90
E. Pislor et al. / Desalination 267 (2011) 88–92
Fig. 2. Evolution of the two stages of permeate flux during hard water filtration.
Fig. 3. Evolution of the two stages of permeate resistivity during hard water filtration.
pressure, the permeability of the two membranes remained constant at 1.1 L/h m² bar for the first stage and 1.5 L/h m² bar for the second. It appears that the membrane fouling was not significant. During the first five days, the flux increased for both membranes while the transmembrane pressure was almost constant. The flux enhancement could be correlated to the progressive penetration of the ions into the membranes, swelling the latter. As the membranes were new, another hypothesis could be the liberation of some blocked
pores. After a long term filtration, the permeate flux remained stable. After cleaning the flux becomes higher but rapidly comes back to the previous value, probably due to membrane ion fouling. Filtration performance could be evaluated by measurement of the permeate resistivity. This measure was done on the first and second stages. During the first five days, the resistivity of both permeates decreased. In order to determine if the decreasing salt was constant or tended to a limit value, the experiments were conducted during ten days without cleaning (Fig. 3). As the selectivity of membranes depends on ion diffusivity and electrostatic repulsion, the resistivities reached a limit value. At the beginning of the filtration, ions could be fixed on the membrane causing the membrane charge screening. Then electrostatic repulsion effect is limited and ions may diffuse through the membranes. So the membrane selectivity is due to the difference in ion diffusivity. According to Fig. 3, permeate resistivity of the first membrane decreased from 300 to 200 kΩ cm and reached a constant value. So the selectivity of this membrane is partly affected by the membrane charge screening but a large part of its selectivity depends on ion diffusion. Thus, the first filtration stage induces the reduction of ionic concentration in the permeate. On the contrary, the permeate resistivity of the second membrane declined from 900 to 400 kΩ cm before stabilising. The membrane charge screening effect seems to have an important impact on the permeate resistivity. So the selectivity of this membrane depends more on electrostatic effects than on ion diffusion. This hypothesis is confirmed because the second membrane receives the highest water permeability, probably because the first membrane is tighter. After cleaning, the selectivity was partly recovered for the two membranes. As the first part, the permeate resistivity of the second membrane decreased more than the first one due to the membrane charge screening effect. The limit value of resistivity was reached for the permeate of the first stage contrary to the second one because the filtration time was too short. In order to eliminate salts fixed on the membrane, the cleaning procedure was done using peracetic acid [8]. After ten days of filtration, the cleaning led to the recovery of the initial permeate resistivity of the first stage. In the second stage, as the permeate resistivity of 999 kΩ cm was difficult to obtain, the rising procedure was stopped when the value reached 800 kΩ cm. Even if during the first ten days the water produced with respect to the pharmacopeia requirement, the disinfection had to be done every week to recover filtration performances. The high rejection rate of the first membrane can be assumed because in the permeate the ion concentrations were all below the analytical detection value, except for sodium and nitrate (Table 4). According to the
Table 4 First stage permeate composition.
Before cleaning
After cleaning
Time (min)
F− (mg/L)
Cl− (mg/L)
NO− 3 (mg/L)
SO2− 4 (mg/L)
Na+ (mg/L)
K+ (mg/L)
Mg2+ (mg/L)
Ca2+ (mg/L)
Al3+ (mg/L)
20 3400 6870 9742 25 4895
n.d n.d n.d n.d n.d n.d
b 0.5 b 0.5 b 0.5 b 0.5 b 0.5 b 0.5
0.66 0.60 0.59 0.57 0.66 0.59
b 0.5 b 0.5 b 0.5 b 0.5 0.56 b 0.5
0.59 0.39 b0.45 0.50 0.54 0.40
b 0.5 n.d b 0.5 b 0.5 b 0.5 n.d
b 0.25 b 0.25 b 0.25 b 0.25 b 0.25 b 0.25
b0.5 b0.5 b0.5 b0.5 b0.5 b0.5
b0.02 b0.02 b0.02 b0.02 b0.02 b0.02
Time (min)
F− (mg/L)
Cl− (mg/L)
NO− 3 (mg/L)
SO2− (mg/L) 4
Na+ (mg/L)
K+ (mg/L)
Mg2+ (mg/L)
Ca2+ (mg/L)
Al3+ (mg/L)
20 3400 6870 9742 25 4895
n.d n.d n.d n.d n.d n.d
n.d n.d n.d n.d b 0.5 n.d
n.d n.d n.d n.d n.d n.d
n.d n.d n.d n.d b 0.5 n.d
b 0.2 b 0.2 b 0.2 b 0.2 b 0.2 b 0.2
n.d n.d n.d n.d n.d n.d
n.d n.d n.d n.d n.d n.d
n.d n.d n.d n.d n.d n.d
b0.02 b0.02 b0.02 b0.02 b0.02 b0.02
n.d: not detected.
Table 5 Second stage permeate composition.
Before cleaning
After cleaning n.d: not detected.
E. Pislor et al. / Desalination 267 (2011) 88–92
91
exclusion of the membrane, the Donnan effect could favor the diffusion of anions depending on the diffusion of the counterions. In this study, the nitrate transfer favors the sodium permeability by Donnan effect. The second permeate composition showed that the double reverse osmosis treatment produced HD water according to the pharmacopeia requirement (Table 5). Even if the first step of filtration is sufficient to produce high quality water, the second step must be maintained to prevent system failure.
3.2. Soft water
Fig. 4. Permeate flux evolution of the two stages during the filtration of the soft water.
Fig. 5. Evolution of the permeate resistivity during the soft water filtration.
stability of sodium and nitrate concentrations all along the filtration, the membrane charge screen seems to have no impact on the ion rejection rate. This affirmation is confirmed by the results of resistivity, which showed that the first membrane strongly depends on ion diffusion. Authors report that the transfer of monovalent anions through the membrane seems to be easier than divalent anions [9]. In the case of monovalent anions, the rejection of chloride appears higher than of nitrate, probably because of better hydration of this ion in aqueous solution [10]. According to solution pH, membrane charge or dielectric
Soft water was filtered after cleaning the membranes with Dialox. The first stage transmembrane pressure was maintained at 17 bar during ten days while the second stage varied from 13 to 15 bar. One cleaning was done at midterm, delimiting the first and second parts of the experimentation. The graphic of the permeate flux to time showed us an evolution. Hence, similarly to hard water, there is no long term fouling during filtration of soft water (Fig. 4) and the permeability of the two membranes remained constant at 0.95 L/h m² bar at the first stage and 1.40 L/h m² bar for the second. The flux increased slightly at the beginning of the first stage filtration while it was almost constant for the second stage. After one week of filtration, the cleaning seemed to be efficient. The permeability was slightly lower than the initial value for the first membrane and the same for the second step. The permeate resistivity of the first membrane was slightly above 100 kΩ cm. It remained constant during the whole filtration time (Fig. 5). This value was lower than the resistivity obtained during hard water filtration, probably because the ion retention is less efficient. At the beginning, the second membrane allowed the production of HD water with a resistivity higher than 900 kΩ cm, but after one week of decreasing, it reached to 450 kΩ cm. The cleaning brings back the initial resistivity of the permeate, but this effect was lower for the first membrane because the electrostatic interaction with ions is lower for this membrane. After the cleaning, the resistivity of the second membrane permeate is higher than 999 kΩ cm at the beginning, but regularly decreased during the filtration. So in order to keep the filtration efficient and ensure the quality of HD water produced, the cleaning is required every week. The monovalent cation concentrations were higher than in the case of hard water filtration (Table 6). Thus the high sodium chloride concentration into soft water seems to facilitate the diffusion of sodium and nitrate through the membrane. As the diffusion of counterions is high, the diffusion of anions was facilitated by Donnan effect [11]. Furthermore, after more than two days filtration, the ion concentration in the permeate
Table 6 First stage permeate composition.
Before cleaning After cleaning
Time (min)
F− (mg/L)
Cl− (mg/L)
NO− 3 (mg/L)
SO2− (mg/L) 4
Na+ (mg/L)
K+ (mg/L)
Mg2+ (mg/L)
Ca2+ (mg/L)
Al3+ (mg/L)
35 6230 46 3890
n.d n.d n.d n.d
1.23 0.92 1.10 0.95
1.18 0.80 0.98 0.87
0.64 b0.5 0.62 b0.5
2.00 1.51 1.79 1.54
b0.5 b0.5 b0.5 b0.5
n.d n.d b0.25 b0.25
b 0.5 n.d b 0.5 b 0.5
b 0.02 b 0.02 b 0.02 b 0.02
Time (min)
F− (mg/L)
Cl− (mg/L)
NO− 3 (mg/L)
SO2− (mg/L) 4
Na+ (mg/L)
K+ (mg/L)
Mg2+ (mg/L)
Ca2+ (mg/L)
Al3+ (mg/L)
35 6230 46 3890
n.d n.d n.d n.d
b 0.5 b 0.5 b 0.5 b 0.5
b 0.5 b 0.5 n.d b 0.5
b 0.5 b 0.5 b 0.5 b 0.5
b 0.2 b 0.2 b 0.2 b 0.2
n.d n.d n.d n.d
n.d n.d n.d n.d
n.d n.d n.d n.d
b 0.02 b 0.02 b 0.02 b 0.02
n.d: not detected.
Table 7 Second stage permeate composition.
Before cleaning After cleaning n.d: not detected.
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might be influenced by the progressive shielding of membrane charge, but the rejection rates of divalent cations remained high during ten days. In the case of HD water production from soft water, one step filtration is insufficient because the monovalent ion concentration is over the pharmacopeia requirements. Therefore the second stage is necessary to produce good quality water (Table 7). At this step, the rejection rate of the divalent cations is very high, which could explain why the detection of magnesium and calcium was not possible. Nevertheless, the monovalent cations, mainly sodium, go through the membrane by Donnan effect because of the high anion concentration in the permeate. In all cases, the concentrations remained low, below the quantification level. 4. Conclusion The double reverse osmosis system, combining sea water and brackish water membranes, has been tested for the **haemodialysate production from hard and soft water. Since some ions could reach concentrations higher than those define by the European settlement, hard and soft water were representative of drinking water. Nevertheless, the double osmosis system allowed the production of HD water, as defined in the pharmacopeia requirement, with a simple pre-treatment which combines activated carbon and filtration unit. This pre-treatment was not described in the present study because of the use of synthetic water. But, even with this step, the double reverse osmosis system is still interesting to reduce the cost and the complexity of HD unit. The results showed that the second membrane is necessary to produce ultrapure water when the water supply contains high
monovalent anion content. Charge screening and Donnan effect play a key role in the efficiency of this system by changing the permeability of the counterions.
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