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A combined ion exchange–nanofiltration process for water desalination: III. Pilot scale studies
DES-12363; No of Pages 6 Desalination xxx (2014) xxx–xxx
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A combined ion exchange–nanofiltration process for water desalination: III. Pilot scale studies Nidal Hilal a,b,⁎, Victor Kochkodan b, Hasan Al Abdulgader c, Stephen Mandale a, Saad A. Al-Jlil d a
Centre for Water Advanced Technologies and Environmental Research (CWATER), College of Engineering, Swansea University, Swansea SA2 8PP, United Kingdom Qatar Environment and Energy Research Institute (QEERI), Doha, Qatar College of Engineering, King Faisal University, Al Ahsa, Saudi Arabia d King Adbul Aziz City for Science and Technology, Saudi Arabia b c
H I G H L I G H T S • • • • •
A pilot ion-exchange –nanofiltration rig for water desalination has been built IX section of the hybrid system converts N 95% of chloride to sulphate ions 4040 NF membrane elements provides N 99% rejection of sulphate ions NF reject stream may be used for regeneration of the exhausted IX resins Feasibility of the hybrid IX-NF process of water desalination has been proved
a r t i c l e
i n f o
Article history: Received 16 September 2014 Received in revised form 24 November 2014 Accepted 28 November 2014 Available online xxxx Keywords: Desalination Ion exchange Nanofiltration Hybrid ion exchange–nanofiltration process of water desalination
a b s t r a c t This paper describes pilot scale studies on a novel combined ion-exchange (IX)–nanofiltration (NF) process for water desalination. Based on the preliminary results of small scale IX and NF studies, a pilot scale hybrid IX–NF system for water desalination has been designed and built for testing and evaluation. Optimization of the operability of the designed pilot system has been done and the obtained results have proved the feasibility of the hybrid IX–NF desalination process. The IX section of the pilot system filled with Purolite A500TLSO4 resin managed to treat feed waters at various salinities and composition and to convert N95% of chloride ions to sulphate ions. The IX treated salty water was further desalinated using NF90 and NF270 spiral wound 4040 industrial membrane modules. The NF membranes were able to maintain N 99% rejection of sulphate ions. It was found that NF reject stream, which is rich in sulphate content, may be used for regeneration of the exhausted IX resins and thus no external regeneration solution is required for the hybrid IX–NF process of water desalination. An incorporation of IX stage in the hybrid process of water desalination allows membrane desalination at lower operating pressures thus reducing the energy consumption. © 2014 Elsevier B.V. All rights reserved.
1. Introduction A sharp growth in the worlds' population coupled with urbanization and the depletion of fossil fuels has resulted in a rapidly increasing demand for fresh water and energy. Both water shortages and energy crises have plagued many communities around the world [1–3]. It has been reported that more than 1.2 billion people in the world lack access to clean and safe drinking water [1,4]. Although most of the planet is surrounded by oceans, only approximately 0.8% of the world's total water is considered potable water [5]. Problems with water are expected ⁎ Corresponding author at: Centre for Water Advanced Technologies and Environmental Research (CWATER), College of Engineering, Swansea University, Swansea SA2 8PP, United Kingdom. E-mail address: [email protected] (N. Hilal).
to grow worse in the coming decades. Therefore, a lot of efforts are focused on suitable methods to obtain freshwater by sea water or brackish water desalination. A variety of desalination technologies, both thermally-driven and membrane-based, have been increasingly employed to treat saltwater to enhance the limited fresh water supply. Among them reverse osmosis (RO) is regarded as one of the most attractive methods for water desalination. However, RO is still hampered by high energy consumption [1]. Typically energy inputs can account for 44% of the total water costs of a RO plant [3]. Therefore a search for new processes capable of producing fresh water by desalting brackish water and/or seawater at the lowest possible cost is of crucial importance. Among these approaches are, so called hybrid membrane processes. In the mid-90s, hybrid membrane processes were described as processes where “one or more membrane process is coupled with another unit process such as adsorption, ion exchange, coagulation, bioconversion,
http://dx.doi.org/10.1016/j.desal.2014.11.030 0011-9164/© 2014 Elsevier B.V. All rights reserved.
Please cite this article as: N. Hilal, et al., A combined ion exchange–nanofiltration process for water desalination: III. Pilot scale studies, Desalination (2014), http://dx.doi.org/10.1016/j.desal.2014.11.030
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catalysis, etc.” [6]. In recent years, the interest on hybrid processes has increased substantially and is derived by the need for overall process optimization and/or cost reduction. Recently it has been shown that the combination of ion exchange (IX) resins with pressure-driven membrane processes has a pronounced synergetic effect in the creation of efficient and cost-effective technologies for water treatment, in prevention of membrane scaling/fouling and reduction of desalination costs [7]. For example, IX treatment has been used to completely demineralize the RO permeate [8–10]. Among several conventional and nonconventional demineralisation technologies, a combined double stage RO and mixed bed IX treatment would lower the operation costs by up to 28% compared with a conventional high density RO membrane system [10]. Sarkar and SenGupta [11] used IX resins to reduce the osmotic pressure of NaCl solutions and to increase membrane flux following nanofiltration (NF) water treatment. Using IX process, monovalent chloride ions of salt water have been converted into divalent sulphate ions, which were efficiently rejected with NF membranes. Thus, potentially water desalination can be carried out by replacing the more energy-consuming RO membranes with more energy-efficient NF membranes coupled with IX. However these experiments were performed only on a small laboratory scale with model sodium chloride solutions and the authors acknowledged that further more detailed investigations are needed to confirm the overall viability of the approach used. This paper for the first time describes the performance of a hybrid IX–NF process to desalinate salty water on a pilot scale. The principle of this combined process is as follows: incoming salty water, which contains mainly sodium chloride, is passed through the IX column filled with an anion exchange resin in sulphate form, where the sulphate–chloride exchange takes place. Then, the produced feed containing mainly sodium sulphate is fed under pressure through a NF membrane. Because the osmotic pressure of the IX–treated salty water is reduced, high permeate flux at a relatively low operating pressure can be obtained. The reject stream from NF stage, which is rich in sulphate content, is fed through the exhausted anion-exchange resin in chloride form for its regeneration. Thus there is no need for an external regeneration solution to regenerate the exhausted IX resin in the hybrid IX–NF process because the NF retentante may be used for this purpose.
On the preliminary laboratory stage the experimental studies have been conducted to determine the main regularities and operating parameters, which govern both sulphate–chloride-exchange and NF separation of IX-treated salty water [12,13]. These data have been used for further scaling up and optimization of a pilot scale hybrid IX–NF system, which have been designed and built in parallel. The pilot scale trials have been conducted to prove the feasibility of the developed hybrid process for water desalination. 2. Design of a pilot IX–NF system for water desalination To progress the experimental work to pilot scale trials a pilot scale IX–NF system to desalinate saline water has been designed and built. Anion exchange resin in sulphate form is used to convert monovalent chloride ions in the feed to divalent sulphate ions. NF membrane is utilised to sufficiently remove divalent ions from IX-treated water. The system is designed so that it can run with different IX resins and 4040 spiral wound industrial NF modules. Fig. 1 shows the process and instrumentation diagram (P&ID) of the pilot IX–NF system. Four tanks are constructed to home IX feed, regenerant solution, NF permeate and rinsing water. Two IX vessels of 33 L capacity and of 8 inch diameter have been installed (IXR-1 and IXR-2) to insure continuous IX operation. With this arrangement it is possible to operate one IX column in conversion mode, while the other column is operated in regeneration mode. All pipe work in the IX section of the plant is constructed from polyvinylchloride material for excellent corrosion resistance. The IX treated water is delivered to the NF feed tank before being sent to the NF stage. The NF system can also be operated continuously with the NF feed tank operating as a break tank or decoupler between the two stages. NF reject is recycled back to the NF feed tank. Two pumps are used to operate NF stage, both of which are vertically mounted multistage centrifugal pumps. The first provides the filtration driving pressure and the second smaller pump the volumetric flow. This two pump approach allows for better control of operating parameters. Heat exchanger is installed to control NF feed temperature. Drain points are placed in a number of places inside the system to allow effective system drainage when necessary. Pipes in the NF section are constructed from stainless steel to withstand high pressure operation and resist corrosion from concentrated salt solutions. The NF section is fitted with
Fig. 1. P&ID diagram of the hybrid IX–NF pilot system.
Please cite this article as: N. Hilal, et al., A combined ion exchange–nanofiltration process for water desalination: III. Pilot scale studies, Desalination (2014), http://dx.doi.org/10.1016/j.desal.2014.11.030
N. Hilal et al. / Desalination xxx (2014) xxx–xxx
3
IX treated water or a model salt solution has been added in the NF feed tank. NF retentate has been recirculated at an operating pressure of 8.0–15.0 bar, cross flow rate of 20.0–30.0 L/min and temperature of 25 °C, while permeate samples have been periodically collected and analyzed for content of chloride and sulphate ions as was discussed before. The chloride and sulphate ion rejection (R) with the membranes is calculated using the following equation: R ¼ 1−C p =C f 100% where Cp and Cf are the ion concentrations in the permeate and in the feed solution, respectively. The membrane flux (J) was calculated by measuring the time needed to collect some volume of permeate using the following equation: J¼
Fig. 2. Photo of the developed hybrid IX–NF pilot system for water desalination.
high pressure membrane housing suitable for a typical 4040 size membrane element. IX and NF sections are connected electronically to one control cabinet mounted on the framework for the NF section. The control panel enables switching on/off and speed control of the pumps as well as displays pressure, temperature and flow rate reading. Photo of the constructed pilot IX–NF system for water desalination is shown in Fig. 2.
V At
where J represents flux (L/m2 h), V is volume of permeate (L), A is the effective membrane area (m2) and t is the time taken to collect the permeate (h). For comparison RO BW30 and SW30 (Dow Chemical, USA) flat sheet membranes have been also used in experiments with sea water. A Sterlitech HP 4750 membrane cell of dead-end mode with an effective membrane area of 14.6 cm2 was used for membrane testing. The basic properties of NF90, NF270, BW30 and SW30 membranes are presented in Table 2. 3. Results and discussion
2.1. Operating procedure
3.1. Chloride/sulphate exchange in IX vessels
Based on the data of laboratory-scale screening experiments [12], Purolite A500TLSO4 resin, whose characteristics are presented in Table 1, has been used for pilot-scale studies. 25 L of Purolite A500TLSO4 resin has been loaded carefully inside the IX vessel. After loading, the resin is washed with ultrapure water. Then, feed solution is prepared in the feed tank using RO water and appropriate salts. RO water is obtained from an RO unit in the lab with conductivity of 3.4 μS. Feed volume used was between 400 and 650 L to ensure complete exhaustion of the IX bed. Once the feed is prepared, it is delivered to IX vessel at an operating pressure of 0.2–0.8 bar. IX treated water is collected and periodically analysed for Cl− and SO−2 content using an 4 ion chromatography analyser (ICS-900, Thermo Scientific Dionex) to allocate the breakthrough point of chloride ions. Exhausted IX resin has been regenerated with 20.0 g/L Na2SO4 solution. After regeneration, the resin bed is washed with ultrapure water to remove any excess sulphate ions inside the bed. NF90 and NF270 4040 industrial membrane elements (Dow Chemical, USA) were used for the experiments with the developed pilot scale IX–NF system. The separation properties of these membranes with mono- and mixed NaCl/Na2SO4 solutions have been preliminarily studied on the laboratory scale [13].
Fig. 3 shows breakthrough curves of chloride and sulphate ions when 2.0 and 5.0 g/L NaCl solutions were treated with Purolite A500TLSO4 resin. Content of chloride and sulphate ions in IX-treated water is expressed as relative concentrations. Relative chloride concentration corresponds to outlet over inlet IX concentration. Relative sulphate concentration corresponds to outlet IX concentration over − maximum sulphate concentration in the IX treated water. SO2− 4 /Cl ion exchange occurs when NaCl solution passing through an IX resin in sulphate form in accordance with the following reaction:
Table 1 Physical and chemical characteristics of anion exchange Purolite A500TLSO4 resin.
Table 2 Properties of the thin film composite polyamide NF/RO membranes as specified by the manufacturer.
Physical form
Spherical beads
Matrix Functional groups Ionic form as shipped Total exchange capacity Moisture holding capacity Shipping weight Particle size Maximum reversible swelling Temperature limit, Cl− form
Polyacrylic cross-linked with divinylbenzene Quaternary ammonium SO4 1.15 eq/L (Cl− form) 53–58% (Cl− form) 665–695 g/L (approx) 425–850 μm Cl−–OH−: 15% 100 °C
(RN+)2SO2− + 2NaCl ⇔ 2(RN+)Cl− + SO2− + 2Na+, 4 4 where (RN+)2SO2− is a polymer matrix of IX resin in sulphate form. 4 The chloride exchange curve breaks after about 21 bed volumes (BVs) (525 L) and 11 BVs (275 L) when treated 2.0 g/L and 5.0 g/L NaCl solutions, respectively. It should be noted, that small (about 5%) but relatively constant quantity of chloride ions is present in the IXtreated water before a chloride breakthrough point is reached. This can be attributed to the imperfect flow distribution inside the IX vessel which could have caused channelling and small feed leakage toward the product stream.
Membrane
MgSO4 rejection, %
NF270 NF90 BW30 SW30
97.0a N97a
a b c
NaCl rejection, %
Flux, m3/m2 d
Molecular weight cut off, Daltons
85–95b 99.5b 99.4c
1.28a 0.97a/0.77b 1.08b 1.0c
~200–400 ~200–400 ~100
Test conditions: 2.0 g/L MgSO4, 15% recovery, operating pressure 4.8 bar, 25 °C. Test conditions: 2.0 g/L NaCl, 15% recovery, operating pressure 15.5 bar, 25 °C. Test conditions: 32.0 g/L NaCl, 15% recovery, operating pressure 55 bar, 25 °C.
Please cite this article as: N. Hilal, et al., A combined ion exchange–nanofiltration process for water desalination: III. Pilot scale studies, Desalination (2014), http://dx.doi.org/10.1016/j.desal.2014.11.030
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Fig. 3. Breakthrough curves of chloride and sulphate ions during IX treatment of (a) 2.0 g/L and (b) 5.0 g/L NaCl solutions using 25 L Purolite A500TLSO4 resin at 80.0 L/h average feed flow rate.
After each IX treatment, a 20.0 g/L Na2SO4 solution is prepared to regenerate the IX bed. Na2SO4 solution when passing through an exhausted IX resin in chloride form would transform it back to the sulphate form according to following reaction: + 2NaCl 2(RN+)Cl− + Na2SO4 ⇔ (RN+)2SO2− 4
through a fully saturated IX bed at 500 L/h average feed flow rate. Compared with regeneration at 105 L/h flow rate, the nearly five times increase in regenerant flow rate caused a substantial increase in the total regenerant volume needed for effective regeneration of the bed (Fig. 6). It is estimated that at least 18 BVs (450.0 L) of regenerant is needed to regenerate the bed at 500.0 L/h. This means that about 500% increases in flow rate (from 105 L/h to 500 L/h) would require additional 150 L of regenerant solution (50% volume increase) to regenerate a given IX bed.
where (RN+)Cl− is a polymer matrix of IX resin in chloride form. In the first and second regeneration cycles the solutions were delivered at 85.0 L/h and 105.0 L/h, respectively. The breakthrough curves of chloride and sulphate ions during both regeneration cycles are shown in Fig. 4. As seen in this figure, the resin bed is successfully regenerated with about 12 BVs of regenerant needed for complete regeneration. Regenerant flow rate appears to have an effect on the sharpness of chloride and sulphate curves and the total volume needed for effective regeneration. Slower flow rate entails sharper curves and smaller volume needed for regeneration. The effect of feed flow rate on IX resin performance has also been examined during treatment (production) runs. Fig. 5 shows breakthrough curves of chloride and sulphate ions in the IX product stream during treatment of 5.0 g/L NaCl solution at 80.0 L/h and 650.0 L/h feed flow rates. As seen in this figure, at the faster rate, the chloride breakthrough point is detected after around 7 BVs (175.0 L). This means that an eightfold increase in feed flow rate caused the chloride breakthrough point to retract form 11 to 7 BVs. In other words, there would be a 36.4% decline in the IX product volume if feed flow rate is increased from 80 L/h to 650 L/h. The sharpness of the breakthrough curves is also slightly affected by the increase in the flow rate. It is believed that at a high flow rate of 650 L/h the chloride/sulphate exchange might not be fast enough to achieve effective removal of chloride ions. Also, at this rate, chance of feed channelling is considerably increased and the degree of flow imperfection through the IX bed is enhanced. The effect of the regenerant flow rate on the efficiency of IX regeneration has also been studied. A 20.0 g/L Na2SO4 solution is passed
Changes in membrane flux as a function of permeate recovery during filtration of 5.0 g/L model Na2SO4 –NaCl solution (70%–30%) are illustrated in Fig. 7. With an increase in the permeate recovery the feed becomes more concentrated which will result in a higher feed osmotic pressure and a lower permeate flux. Sulphate and chloride rejection throughout the filtration time is shown in Fig. 8. As seen in this figure the rejection of sulphate ions is maintained at a high level of 99.9%– 99.8% throughout the filtration cycle. As a result, at 84% permeate recovery, the sulphate content in the retentante increased by 290%. Chloride rejection starts at 98.5% at the beginning of the filtration, then the rejection gradually declines and reaches 92.0% at 84.0% recovery. Chloride concentration in the retentate increased by 260% compared to its initial feed. This situation might be undesirable when NF retentate is intended to be used for regeneration of exhausted IX resin because it will probably lead to incomplete and ineffective regeneration of the IX bed. So, to use NF90 membrane in hybrid IX–NF system for water desalination the proper operation of IX stage is needed to provide complete sulphate– chloride exchange. NF270 membrane performance during filtration of 5.0 g/L Na2SO4– NaCl solution (95%–5%) is presented in Fig. 8. The increase in feed concentration with time causes the flux to fall gradually. When feed recovery has reached 86.0% the flux has declined by 59.0% (Fig. 8).
Fig. 4. Comparison in the regeneration performance between the first and second regeneration cycles using 20.0 g/L Na2SO4 solutions.
Fig. 5. Comparison in IX performance at slow (80.0 L/h) and fast (650.0 L/h) feed flow rates during IX treatment of 5.0 g/L NaCl solution using 25 L Purolite A500TLSO4 resin.
3.2. NF desalination of IX-treated water
Please cite this article as: N. Hilal, et al., A combined ion exchange–nanofiltration process for water desalination: III. Pilot scale studies, Desalination (2014), http://dx.doi.org/10.1016/j.desal.2014.11.030
100
0.8
60 Chloride (105 L/h)
0.6 Sulphate (105 L/h) 0.4
Chloride (500 L/h)
80
60
20 40 -20 0
20
-60
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Fig. 6. Comparison in the regeneration performance at slow (105 L/h) and fast (500 L/h) feed flow rates using 20.0 g/L Na2SO4 solutions.
Rejection of sulphate ions was practically stable at 99.7–99.8% level during NF cycle, while a large variation is observed in chloride rejection throughout the filtration period (Fig. 8). At the beginning of filtration, rejection of chloride ions has been maintained above 40%. A strong decline is recorded afterward and no rejection is noted at 53% feed recovery. After this point, negative chloride rejection is observed. Negative rejection of chloride ions as have been discussed previously is attributed to the Donnan distribution of salt between the solution and the membrane [14–17]. As almost all of the sulphate ions are rejected and with greater permeation flux of sodium ions at higher feed concentration, more chloride ions are drawn toward the permeate side in order to preserve the electroneutrality principle [18–21]. Toward the end of the filtration test and at 86% recovery, the membrane rejection of chloride ions fell down to − 87.0%. Analysis of NF retentate shows an increase in sulphate concentration by 406% and only 17% increase in the concentration of chloride ions. The possibility of “selective” separation of chloride and sulphate ions with NF270 membrane seems to be a positive attribute for the hybrid IX–NF system of water desalination when NF retentate is used to regenerate the exhausted IX resin. For that purpose, it is desirable to have less content of chloride ions in the NF retentate for effective regeneration of IX resin. As seen in Fig. 9 during combined IX–NF treatment of salty water with a total salt content of 8 g/L with Purolite A500TLSO4 resin in the sulphate form and following filtration of the treated water through the NF90-4040 industrial membrane module, the concentrations of sulphate and chloride ions in the permeate are quite low and they don't exceed 1 g/L total dissolved solid level, which is recommended by WHO for drinking water [22]. On the other hand, the practically complete rejection of sulphate ions with NF90-4040 membrane element would be beneficial for using of NF retentate for regeneration of an
20
Flux
0
Recovery (%)
20
Bed volume
100
Chloride
-100
0 0
40 Sulphate
Sulphate (500 L/h)
0.2
5
Flux (L/(m2h))
1
Rejecon (%)
Relavent concentraon
N. Hilal et al. / Desalination xxx (2014) xxx–xxx
Fig. 8. Rejection of sulphate/chloride ions and permeate flux during filtration of 5.0 g/L Na2SO4–NaCl solution (95%–5% w.w.) through NF270 membrane at an operating pressure of 12 bar.
− exhausted IX column after SO2− 4 /Cl exchange. It should be noted that in proper operation of the IX stage there ought to be low chloride content in the IX product water. Data on rejection of sulphate ions and membrane fluxes during filtration of real sea water treated with Purolite A500TLSO4 resin through different NF and RO membranes are shown in Fig. 10. It is seen that both NF90 and RO SW30 membranes have similar sulphate rejection but at the same operating conditions NF membrane offer much higher permeate flux (almost ten times higher) compared to RO membrane. Thus, because the osmotic pressure of the IX-treated salty water is reduced, high permeate flux at a relatively low operating pressure can be obtained with NF membranes during water desalination.
4. Conclusion The performance of the developed pilot hybrid IX–NF system for water desalination has been evaluated during treatment of feed solutions of various salinity and Na2SO4 to NaCl composition. It was shown that Purolite A500TLSO4 anion exchange resin in sulphate form managed to convert N 95% of chloride ions in the feed to sulphate ions. NF90 and NF270 spiral wound 4040 industrial membrane elements have been used for desalination of IX treated saline water. It was shown that the membrane rejection of sulphate ions was largely unaffected by concentration of the NF feed and in most of the tests performed, the rejection was maintained at values greater than 99%. In contrast, chloride rejection varied largely at different feed concentrations, operating pressures and the feed composition (Na2SO4 to NaCl ratio). It was found that that the NF reject stream, which is rich in sulphate content, may be used for the efficient regeneration of the exhausted IX resins and no external 20
100
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98
40
96
30 20
Sulphate Chloride
92
10
5
0
90
0 20
SO4
10
Flux 0
C, mg-eq/l
94
2 Flux (L/(m h))
Rejecon (%)
Cl 15
40
60
80
100
0
20
40
60
80
NF me, min
Recovery (%) Fig. 7. Rejection of sulphate/chloride ions and permeate flux during filtration of 5.0 g/L Na2SO4–NaCl solution (70%–30% w.w.) through NF90 membrane at an operating pressure of 12 bar. Recovery has been calculated as a percent ratio of permeate to feed volume.
Fig. 9. Concentrations of chloride and sulphate ions in permeate during filtration of IXtreated (Purolite A500TLSO4 resin, flow rate through the column is 5 L/min) 8.0 g/L NaCl solution through NF90-4040 membrane element at an operating pressure of 12 bar. The initial volume of IX-treated feed is 500 L. The final volume of NF retentante is 50 L.
Please cite this article as: N. Hilal, et al., A combined ion exchange–nanofiltration process for water desalination: III. Pilot scale studies, Desalination (2014), http://dx.doi.org/10.1016/j.desal.2014.11.030
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N. Hilal et al. / Desalination xxx (2014) xxx–xxx
Rejecon,%
100
and long term tests are needed to confirm the overall viability of the proposed approach for water desalination. Also, further studies are needed regarding the optimal concentration range of the feed water where the hybrid IX–NF process might be economically efficient for long term operability.
98
96
Acknowledgement
NF90
BW30
SW30 The authors would like to thank King Adbul Aziz City for Science and Technology for funding this work.
94 5
10
15
20
25
30
35
References
Operang pressure, bar a) 12 10
Flux, l/m2h
IX-treated sea water 8
Sea water
6 4 2 0 NF-90
BW-30
SW-30
b) Fig. 10. Rejection of sulphate ions (a) and membrane fluxes (b) with different NF and RO membranes during filtration of sea water (Swansea Bay) after treatment with Purolite A500TLSO4 resin. The total salt content in sea water is 32.0 g/L, the concentrations of sulphate and chloride ions are 2.43 and 17.46 g/L, respectively. Operating pressure is 20 bar.
regeneration solution is required for the hybrid IX–NF process of water desalination. It was shown that using the combined IX–NF treatment of salty water of 8.0 g/L total salt content with Purolite A500TLSO4 resin and NF90 membrane, high quality fresh water can be produced as the NF permeate. During desalination of IX-treated sea water it was found that at very similar values of sulphate rejection NF membranes offer much higher permeate flux compared to RO SW30 membrane at the same operating conditions. Thus, incorporation of IX stage in the hybrid process of water desalination creates a synergy that allows membrane desalination at lower operating pressures thus potentially reducing the energy consumption for desalination process. In general the pilot scale trials and the obtained results have proved the feasibility of the developed hybrid IX–NF process of water desalination. From an application viewpoint, the developed hybrid desalination process would be of particular interest in places where reductions in energy requirement and/or higher product water recovery are beneficial, for example people who are living in arid and coastal areas or around brackish water resources, and farmers. However, future large scale
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Please cite this article as: N. Hilal, et al., A combined ion exchange–nanofiltration process for water desalination: III. Pilot scale studies, Desalination (2014), http://dx.doi.org/10.1016/j.desal.2014.11.030
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A combined ion exchange–nanofiltration process for water desalination: III. Pilot scale studies Nidal Hilal a,b,⁎, Victor Kochkodan b, Hasan Al Abdulgader c, Stephen Mandale a, Saad A. Al-Jlil d a
Centre for Water Advanced Technologies and Environmental Research (CWATER), College of Engineering, Swansea University, Swansea SA2 8PP, United Kingdom Qatar Environment and Energy Research Institute (QEERI), Doha, Qatar College of Engineering, King Faisal University, Al Ahsa, Saudi Arabia d King Adbul Aziz City for Science and Technology, Saudi Arabia b c
H I G H L I G H T S • • • • •
A pilot ion-exchange –nanofiltration rig for water desalination has been built IX section of the hybrid system converts N 95% of chloride to sulphate ions 4040 NF membrane elements provides N 99% rejection of sulphate ions NF reject stream may be used for regeneration of the exhausted IX resins Feasibility of the hybrid IX-NF process of water desalination has been proved
a r t i c l e
i n f o
Article history: Received 16 September 2014 Received in revised form 24 November 2014 Accepted 28 November 2014 Available online xxxx Keywords: Desalination Ion exchange Nanofiltration Hybrid ion exchange–nanofiltration process of water desalination
a b s t r a c t This paper describes pilot scale studies on a novel combined ion-exchange (IX)–nanofiltration (NF) process for water desalination. Based on the preliminary results of small scale IX and NF studies, a pilot scale hybrid IX–NF system for water desalination has been designed and built for testing and evaluation. Optimization of the operability of the designed pilot system has been done and the obtained results have proved the feasibility of the hybrid IX–NF desalination process. The IX section of the pilot system filled with Purolite A500TLSO4 resin managed to treat feed waters at various salinities and composition and to convert N95% of chloride ions to sulphate ions. The IX treated salty water was further desalinated using NF90 and NF270 spiral wound 4040 industrial membrane modules. The NF membranes were able to maintain N 99% rejection of sulphate ions. It was found that NF reject stream, which is rich in sulphate content, may be used for regeneration of the exhausted IX resins and thus no external regeneration solution is required for the hybrid IX–NF process of water desalination. An incorporation of IX stage in the hybrid process of water desalination allows membrane desalination at lower operating pressures thus reducing the energy consumption. © 2014 Elsevier B.V. All rights reserved.
1. Introduction A sharp growth in the worlds' population coupled with urbanization and the depletion of fossil fuels has resulted in a rapidly increasing demand for fresh water and energy. Both water shortages and energy crises have plagued many communities around the world [1–3]. It has been reported that more than 1.2 billion people in the world lack access to clean and safe drinking water [1,4]. Although most of the planet is surrounded by oceans, only approximately 0.8% of the world's total water is considered potable water [5]. Problems with water are expected ⁎ Corresponding author at: Centre for Water Advanced Technologies and Environmental Research (CWATER), College of Engineering, Swansea University, Swansea SA2 8PP, United Kingdom. E-mail address: [email protected] (N. Hilal).
to grow worse in the coming decades. Therefore, a lot of efforts are focused on suitable methods to obtain freshwater by sea water or brackish water desalination. A variety of desalination technologies, both thermally-driven and membrane-based, have been increasingly employed to treat saltwater to enhance the limited fresh water supply. Among them reverse osmosis (RO) is regarded as one of the most attractive methods for water desalination. However, RO is still hampered by high energy consumption [1]. Typically energy inputs can account for 44% of the total water costs of a RO plant [3]. Therefore a search for new processes capable of producing fresh water by desalting brackish water and/or seawater at the lowest possible cost is of crucial importance. Among these approaches are, so called hybrid membrane processes. In the mid-90s, hybrid membrane processes were described as processes where “one or more membrane process is coupled with another unit process such as adsorption, ion exchange, coagulation, bioconversion,
http://dx.doi.org/10.1016/j.desal.2014.11.030 0011-9164/© 2014 Elsevier B.V. All rights reserved.
Please cite this article as: N. Hilal, et al., A combined ion exchange–nanofiltration process for water desalination: III. Pilot scale studies, Desalination (2014), http://dx.doi.org/10.1016/j.desal.2014.11.030
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catalysis, etc.” [6]. In recent years, the interest on hybrid processes has increased substantially and is derived by the need for overall process optimization and/or cost reduction. Recently it has been shown that the combination of ion exchange (IX) resins with pressure-driven membrane processes has a pronounced synergetic effect in the creation of efficient and cost-effective technologies for water treatment, in prevention of membrane scaling/fouling and reduction of desalination costs [7]. For example, IX treatment has been used to completely demineralize the RO permeate [8–10]. Among several conventional and nonconventional demineralisation technologies, a combined double stage RO and mixed bed IX treatment would lower the operation costs by up to 28% compared with a conventional high density RO membrane system [10]. Sarkar and SenGupta [11] used IX resins to reduce the osmotic pressure of NaCl solutions and to increase membrane flux following nanofiltration (NF) water treatment. Using IX process, monovalent chloride ions of salt water have been converted into divalent sulphate ions, which were efficiently rejected with NF membranes. Thus, potentially water desalination can be carried out by replacing the more energy-consuming RO membranes with more energy-efficient NF membranes coupled with IX. However these experiments were performed only on a small laboratory scale with model sodium chloride solutions and the authors acknowledged that further more detailed investigations are needed to confirm the overall viability of the approach used. This paper for the first time describes the performance of a hybrid IX–NF process to desalinate salty water on a pilot scale. The principle of this combined process is as follows: incoming salty water, which contains mainly sodium chloride, is passed through the IX column filled with an anion exchange resin in sulphate form, where the sulphate–chloride exchange takes place. Then, the produced feed containing mainly sodium sulphate is fed under pressure through a NF membrane. Because the osmotic pressure of the IX–treated salty water is reduced, high permeate flux at a relatively low operating pressure can be obtained. The reject stream from NF stage, which is rich in sulphate content, is fed through the exhausted anion-exchange resin in chloride form for its regeneration. Thus there is no need for an external regeneration solution to regenerate the exhausted IX resin in the hybrid IX–NF process because the NF retentante may be used for this purpose.
On the preliminary laboratory stage the experimental studies have been conducted to determine the main regularities and operating parameters, which govern both sulphate–chloride-exchange and NF separation of IX-treated salty water [12,13]. These data have been used for further scaling up and optimization of a pilot scale hybrid IX–NF system, which have been designed and built in parallel. The pilot scale trials have been conducted to prove the feasibility of the developed hybrid process for water desalination. 2. Design of a pilot IX–NF system for water desalination To progress the experimental work to pilot scale trials a pilot scale IX–NF system to desalinate saline water has been designed and built. Anion exchange resin in sulphate form is used to convert monovalent chloride ions in the feed to divalent sulphate ions. NF membrane is utilised to sufficiently remove divalent ions from IX-treated water. The system is designed so that it can run with different IX resins and 4040 spiral wound industrial NF modules. Fig. 1 shows the process and instrumentation diagram (P&ID) of the pilot IX–NF system. Four tanks are constructed to home IX feed, regenerant solution, NF permeate and rinsing water. Two IX vessels of 33 L capacity and of 8 inch diameter have been installed (IXR-1 and IXR-2) to insure continuous IX operation. With this arrangement it is possible to operate one IX column in conversion mode, while the other column is operated in regeneration mode. All pipe work in the IX section of the plant is constructed from polyvinylchloride material for excellent corrosion resistance. The IX treated water is delivered to the NF feed tank before being sent to the NF stage. The NF system can also be operated continuously with the NF feed tank operating as a break tank or decoupler between the two stages. NF reject is recycled back to the NF feed tank. Two pumps are used to operate NF stage, both of which are vertically mounted multistage centrifugal pumps. The first provides the filtration driving pressure and the second smaller pump the volumetric flow. This two pump approach allows for better control of operating parameters. Heat exchanger is installed to control NF feed temperature. Drain points are placed in a number of places inside the system to allow effective system drainage when necessary. Pipes in the NF section are constructed from stainless steel to withstand high pressure operation and resist corrosion from concentrated salt solutions. The NF section is fitted with
Fig. 1. P&ID diagram of the hybrid IX–NF pilot system.
Please cite this article as: N. Hilal, et al., A combined ion exchange–nanofiltration process for water desalination: III. Pilot scale studies, Desalination (2014), http://dx.doi.org/10.1016/j.desal.2014.11.030
N. Hilal et al. / Desalination xxx (2014) xxx–xxx
3
IX treated water or a model salt solution has been added in the NF feed tank. NF retentate has been recirculated at an operating pressure of 8.0–15.0 bar, cross flow rate of 20.0–30.0 L/min and temperature of 25 °C, while permeate samples have been periodically collected and analyzed for content of chloride and sulphate ions as was discussed before. The chloride and sulphate ion rejection (R) with the membranes is calculated using the following equation: R ¼ 1−C p =C f 100% where Cp and Cf are the ion concentrations in the permeate and in the feed solution, respectively. The membrane flux (J) was calculated by measuring the time needed to collect some volume of permeate using the following equation: J¼
Fig. 2. Photo of the developed hybrid IX–NF pilot system for water desalination.
high pressure membrane housing suitable for a typical 4040 size membrane element. IX and NF sections are connected electronically to one control cabinet mounted on the framework for the NF section. The control panel enables switching on/off and speed control of the pumps as well as displays pressure, temperature and flow rate reading. Photo of the constructed pilot IX–NF system for water desalination is shown in Fig. 2.
V At
where J represents flux (L/m2 h), V is volume of permeate (L), A is the effective membrane area (m2) and t is the time taken to collect the permeate (h). For comparison RO BW30 and SW30 (Dow Chemical, USA) flat sheet membranes have been also used in experiments with sea water. A Sterlitech HP 4750 membrane cell of dead-end mode with an effective membrane area of 14.6 cm2 was used for membrane testing. The basic properties of NF90, NF270, BW30 and SW30 membranes are presented in Table 2. 3. Results and discussion
2.1. Operating procedure
3.1. Chloride/sulphate exchange in IX vessels
Based on the data of laboratory-scale screening experiments [12], Purolite A500TLSO4 resin, whose characteristics are presented in Table 1, has been used for pilot-scale studies. 25 L of Purolite A500TLSO4 resin has been loaded carefully inside the IX vessel. After loading, the resin is washed with ultrapure water. Then, feed solution is prepared in the feed tank using RO water and appropriate salts. RO water is obtained from an RO unit in the lab with conductivity of 3.4 μS. Feed volume used was between 400 and 650 L to ensure complete exhaustion of the IX bed. Once the feed is prepared, it is delivered to IX vessel at an operating pressure of 0.2–0.8 bar. IX treated water is collected and periodically analysed for Cl− and SO−2 content using an 4 ion chromatography analyser (ICS-900, Thermo Scientific Dionex) to allocate the breakthrough point of chloride ions. Exhausted IX resin has been regenerated with 20.0 g/L Na2SO4 solution. After regeneration, the resin bed is washed with ultrapure water to remove any excess sulphate ions inside the bed. NF90 and NF270 4040 industrial membrane elements (Dow Chemical, USA) were used for the experiments with the developed pilot scale IX–NF system. The separation properties of these membranes with mono- and mixed NaCl/Na2SO4 solutions have been preliminarily studied on the laboratory scale [13].
Fig. 3 shows breakthrough curves of chloride and sulphate ions when 2.0 and 5.0 g/L NaCl solutions were treated with Purolite A500TLSO4 resin. Content of chloride and sulphate ions in IX-treated water is expressed as relative concentrations. Relative chloride concentration corresponds to outlet over inlet IX concentration. Relative sulphate concentration corresponds to outlet IX concentration over − maximum sulphate concentration in the IX treated water. SO2− 4 /Cl ion exchange occurs when NaCl solution passing through an IX resin in sulphate form in accordance with the following reaction:
Table 1 Physical and chemical characteristics of anion exchange Purolite A500TLSO4 resin.
Table 2 Properties of the thin film composite polyamide NF/RO membranes as specified by the manufacturer.
Physical form
Spherical beads
Matrix Functional groups Ionic form as shipped Total exchange capacity Moisture holding capacity Shipping weight Particle size Maximum reversible swelling Temperature limit, Cl− form
Polyacrylic cross-linked with divinylbenzene Quaternary ammonium SO4 1.15 eq/L (Cl− form) 53–58% (Cl− form) 665–695 g/L (approx) 425–850 μm Cl−–OH−: 15% 100 °C
(RN+)2SO2− + 2NaCl ⇔ 2(RN+)Cl− + SO2− + 2Na+, 4 4 where (RN+)2SO2− is a polymer matrix of IX resin in sulphate form. 4 The chloride exchange curve breaks after about 21 bed volumes (BVs) (525 L) and 11 BVs (275 L) when treated 2.0 g/L and 5.0 g/L NaCl solutions, respectively. It should be noted, that small (about 5%) but relatively constant quantity of chloride ions is present in the IXtreated water before a chloride breakthrough point is reached. This can be attributed to the imperfect flow distribution inside the IX vessel which could have caused channelling and small feed leakage toward the product stream.
Membrane
MgSO4 rejection, %
NF270 NF90 BW30 SW30
97.0a N97a
a b c
NaCl rejection, %
Flux, m3/m2 d
Molecular weight cut off, Daltons
85–95b 99.5b 99.4c
1.28a 0.97a/0.77b 1.08b 1.0c
~200–400 ~200–400 ~100
Test conditions: 2.0 g/L MgSO4, 15% recovery, operating pressure 4.8 bar, 25 °C. Test conditions: 2.0 g/L NaCl, 15% recovery, operating pressure 15.5 bar, 25 °C. Test conditions: 32.0 g/L NaCl, 15% recovery, operating pressure 55 bar, 25 °C.
Please cite this article as: N. Hilal, et al., A combined ion exchange–nanofiltration process for water desalination: III. Pilot scale studies, Desalination (2014), http://dx.doi.org/10.1016/j.desal.2014.11.030
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Fig. 3. Breakthrough curves of chloride and sulphate ions during IX treatment of (a) 2.0 g/L and (b) 5.0 g/L NaCl solutions using 25 L Purolite A500TLSO4 resin at 80.0 L/h average feed flow rate.
After each IX treatment, a 20.0 g/L Na2SO4 solution is prepared to regenerate the IX bed. Na2SO4 solution when passing through an exhausted IX resin in chloride form would transform it back to the sulphate form according to following reaction: + 2NaCl 2(RN+)Cl− + Na2SO4 ⇔ (RN+)2SO2− 4
through a fully saturated IX bed at 500 L/h average feed flow rate. Compared with regeneration at 105 L/h flow rate, the nearly five times increase in regenerant flow rate caused a substantial increase in the total regenerant volume needed for effective regeneration of the bed (Fig. 6). It is estimated that at least 18 BVs (450.0 L) of regenerant is needed to regenerate the bed at 500.0 L/h. This means that about 500% increases in flow rate (from 105 L/h to 500 L/h) would require additional 150 L of regenerant solution (50% volume increase) to regenerate a given IX bed.
where (RN+)Cl− is a polymer matrix of IX resin in chloride form. In the first and second regeneration cycles the solutions were delivered at 85.0 L/h and 105.0 L/h, respectively. The breakthrough curves of chloride and sulphate ions during both regeneration cycles are shown in Fig. 4. As seen in this figure, the resin bed is successfully regenerated with about 12 BVs of regenerant needed for complete regeneration. Regenerant flow rate appears to have an effect on the sharpness of chloride and sulphate curves and the total volume needed for effective regeneration. Slower flow rate entails sharper curves and smaller volume needed for regeneration. The effect of feed flow rate on IX resin performance has also been examined during treatment (production) runs. Fig. 5 shows breakthrough curves of chloride and sulphate ions in the IX product stream during treatment of 5.0 g/L NaCl solution at 80.0 L/h and 650.0 L/h feed flow rates. As seen in this figure, at the faster rate, the chloride breakthrough point is detected after around 7 BVs (175.0 L). This means that an eightfold increase in feed flow rate caused the chloride breakthrough point to retract form 11 to 7 BVs. In other words, there would be a 36.4% decline in the IX product volume if feed flow rate is increased from 80 L/h to 650 L/h. The sharpness of the breakthrough curves is also slightly affected by the increase in the flow rate. It is believed that at a high flow rate of 650 L/h the chloride/sulphate exchange might not be fast enough to achieve effective removal of chloride ions. Also, at this rate, chance of feed channelling is considerably increased and the degree of flow imperfection through the IX bed is enhanced. The effect of the regenerant flow rate on the efficiency of IX regeneration has also been studied. A 20.0 g/L Na2SO4 solution is passed
Changes in membrane flux as a function of permeate recovery during filtration of 5.0 g/L model Na2SO4 –NaCl solution (70%–30%) are illustrated in Fig. 7. With an increase in the permeate recovery the feed becomes more concentrated which will result in a higher feed osmotic pressure and a lower permeate flux. Sulphate and chloride rejection throughout the filtration time is shown in Fig. 8. As seen in this figure the rejection of sulphate ions is maintained at a high level of 99.9%– 99.8% throughout the filtration cycle. As a result, at 84% permeate recovery, the sulphate content in the retentante increased by 290%. Chloride rejection starts at 98.5% at the beginning of the filtration, then the rejection gradually declines and reaches 92.0% at 84.0% recovery. Chloride concentration in the retentate increased by 260% compared to its initial feed. This situation might be undesirable when NF retentate is intended to be used for regeneration of exhausted IX resin because it will probably lead to incomplete and ineffective regeneration of the IX bed. So, to use NF90 membrane in hybrid IX–NF system for water desalination the proper operation of IX stage is needed to provide complete sulphate– chloride exchange. NF270 membrane performance during filtration of 5.0 g/L Na2SO4– NaCl solution (95%–5%) is presented in Fig. 8. The increase in feed concentration with time causes the flux to fall gradually. When feed recovery has reached 86.0% the flux has declined by 59.0% (Fig. 8).
Fig. 4. Comparison in the regeneration performance between the first and second regeneration cycles using 20.0 g/L Na2SO4 solutions.
Fig. 5. Comparison in IX performance at slow (80.0 L/h) and fast (650.0 L/h) feed flow rates during IX treatment of 5.0 g/L NaCl solution using 25 L Purolite A500TLSO4 resin.
3.2. NF desalination of IX-treated water
Please cite this article as: N. Hilal, et al., A combined ion exchange–nanofiltration process for water desalination: III. Pilot scale studies, Desalination (2014), http://dx.doi.org/10.1016/j.desal.2014.11.030
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0.8
60 Chloride (105 L/h)
0.6 Sulphate (105 L/h) 0.4
Chloride (500 L/h)
80
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20 40 -20 0
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Fig. 6. Comparison in the regeneration performance at slow (105 L/h) and fast (500 L/h) feed flow rates using 20.0 g/L Na2SO4 solutions.
Rejection of sulphate ions was practically stable at 99.7–99.8% level during NF cycle, while a large variation is observed in chloride rejection throughout the filtration period (Fig. 8). At the beginning of filtration, rejection of chloride ions has been maintained above 40%. A strong decline is recorded afterward and no rejection is noted at 53% feed recovery. After this point, negative chloride rejection is observed. Negative rejection of chloride ions as have been discussed previously is attributed to the Donnan distribution of salt between the solution and the membrane [14–17]. As almost all of the sulphate ions are rejected and with greater permeation flux of sodium ions at higher feed concentration, more chloride ions are drawn toward the permeate side in order to preserve the electroneutrality principle [18–21]. Toward the end of the filtration test and at 86% recovery, the membrane rejection of chloride ions fell down to − 87.0%. Analysis of NF retentate shows an increase in sulphate concentration by 406% and only 17% increase in the concentration of chloride ions. The possibility of “selective” separation of chloride and sulphate ions with NF270 membrane seems to be a positive attribute for the hybrid IX–NF system of water desalination when NF retentate is used to regenerate the exhausted IX resin. For that purpose, it is desirable to have less content of chloride ions in the NF retentate for effective regeneration of IX resin. As seen in Fig. 9 during combined IX–NF treatment of salty water with a total salt content of 8 g/L with Purolite A500TLSO4 resin in the sulphate form and following filtration of the treated water through the NF90-4040 industrial membrane module, the concentrations of sulphate and chloride ions in the permeate are quite low and they don't exceed 1 g/L total dissolved solid level, which is recommended by WHO for drinking water [22]. On the other hand, the practically complete rejection of sulphate ions with NF90-4040 membrane element would be beneficial for using of NF retentate for regeneration of an
20
Flux
0
Recovery (%)
20
Bed volume
100
Chloride
-100
0 0
40 Sulphate
Sulphate (500 L/h)
0.2
5
Flux (L/(m2h))
1
Rejecon (%)
Relavent concentraon
N. Hilal et al. / Desalination xxx (2014) xxx–xxx
Fig. 8. Rejection of sulphate/chloride ions and permeate flux during filtration of 5.0 g/L Na2SO4–NaCl solution (95%–5% w.w.) through NF270 membrane at an operating pressure of 12 bar.
− exhausted IX column after SO2− 4 /Cl exchange. It should be noted that in proper operation of the IX stage there ought to be low chloride content in the IX product water. Data on rejection of sulphate ions and membrane fluxes during filtration of real sea water treated with Purolite A500TLSO4 resin through different NF and RO membranes are shown in Fig. 10. It is seen that both NF90 and RO SW30 membranes have similar sulphate rejection but at the same operating conditions NF membrane offer much higher permeate flux (almost ten times higher) compared to RO membrane. Thus, because the osmotic pressure of the IX-treated salty water is reduced, high permeate flux at a relatively low operating pressure can be obtained with NF membranes during water desalination.
4. Conclusion The performance of the developed pilot hybrid IX–NF system for water desalination has been evaluated during treatment of feed solutions of various salinity and Na2SO4 to NaCl composition. It was shown that Purolite A500TLSO4 anion exchange resin in sulphate form managed to convert N 95% of chloride ions in the feed to sulphate ions. NF90 and NF270 spiral wound 4040 industrial membrane elements have been used for desalination of IX treated saline water. It was shown that the membrane rejection of sulphate ions was largely unaffected by concentration of the NF feed and in most of the tests performed, the rejection was maintained at values greater than 99%. In contrast, chloride rejection varied largely at different feed concentrations, operating pressures and the feed composition (Na2SO4 to NaCl ratio). It was found that that the NF reject stream, which is rich in sulphate content, may be used for the efficient regeneration of the exhausted IX resins and no external 20
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30 20
Sulphate Chloride
92
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0
90
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SO4
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Flux 0
C, mg-eq/l
94
2 Flux (L/(m h))
Rejecon (%)
Cl 15
40
60
80
100
0
20
40
60
80
NF me, min
Recovery (%) Fig. 7. Rejection of sulphate/chloride ions and permeate flux during filtration of 5.0 g/L Na2SO4–NaCl solution (70%–30% w.w.) through NF90 membrane at an operating pressure of 12 bar. Recovery has been calculated as a percent ratio of permeate to feed volume.
Fig. 9. Concentrations of chloride and sulphate ions in permeate during filtration of IXtreated (Purolite A500TLSO4 resin, flow rate through the column is 5 L/min) 8.0 g/L NaCl solution through NF90-4040 membrane element at an operating pressure of 12 bar. The initial volume of IX-treated feed is 500 L. The final volume of NF retentante is 50 L.
Please cite this article as: N. Hilal, et al., A combined ion exchange–nanofiltration process for water desalination: III. Pilot scale studies, Desalination (2014), http://dx.doi.org/10.1016/j.desal.2014.11.030
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Rejecon,%
100
and long term tests are needed to confirm the overall viability of the proposed approach for water desalination. Also, further studies are needed regarding the optimal concentration range of the feed water where the hybrid IX–NF process might be economically efficient for long term operability.
98
96
Acknowledgement
NF90
BW30
SW30 The authors would like to thank King Adbul Aziz City for Science and Technology for funding this work.
94 5
10
15
20
25
30
35
References
Operang pressure, bar a) 12 10
Flux, l/m2h
IX-treated sea water 8
Sea water
6 4 2 0 NF-90
BW-30
SW-30
b) Fig. 10. Rejection of sulphate ions (a) and membrane fluxes (b) with different NF and RO membranes during filtration of sea water (Swansea Bay) after treatment with Purolite A500TLSO4 resin. The total salt content in sea water is 32.0 g/L, the concentrations of sulphate and chloride ions are 2.43 and 17.46 g/L, respectively. Operating pressure is 20 bar.
regeneration solution is required for the hybrid IX–NF process of water desalination. It was shown that using the combined IX–NF treatment of salty water of 8.0 g/L total salt content with Purolite A500TLSO4 resin and NF90 membrane, high quality fresh water can be produced as the NF permeate. During desalination of IX-treated sea water it was found that at very similar values of sulphate rejection NF membranes offer much higher permeate flux compared to RO SW30 membrane at the same operating conditions. Thus, incorporation of IX stage in the hybrid process of water desalination creates a synergy that allows membrane desalination at lower operating pressures thus potentially reducing the energy consumption for desalination process. In general the pilot scale trials and the obtained results have proved the feasibility of the developed hybrid IX–NF process of water desalination. From an application viewpoint, the developed hybrid desalination process would be of particular interest in places where reductions in energy requirement and/or higher product water recovery are beneficial, for example people who are living in arid and coastal areas or around brackish water resources, and farmers. However, future large scale
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Please cite this article as: N. Hilal, et al., A combined ion exchange–nanofiltration process for water desalination: III. Pilot scale studies, Desalination (2014), http://dx.doi.org/10.1016/j.desal.2014.11.030