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The reclamation of brine generated from desalination process by bipolar membrane electrodialysis
Journal of Membrane Science 452 (2014) 54–61
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The reclamation of brine generated from desalination process by bipolar membrane electrodialysis Meng Wang a,b,n, Kai-kai Wang a,b, Yu-xiang Jia a,b, Qing-chun Ren c a
Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, China College of Chemistry and Chemical Engineering, Ocean University of China, Qingdao 266100, China c Beijing Unisplendour Empyreal Environmental Engineering Technology Co. Ltd., Beijing 100083, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 29 June 2013 Received in revised form 22 September 2013 Accepted 12 October 2013 Available online 19 October 2013
Some techno-economical analyses and environmental impact evaluation have proved that bipolar membrane electrodialysis (BMED) is feasible for the reclamation of industrial saline water. However, to date, the technology cannot be put into practice due to some unsolved application-oriented problems, such as the requirements of BMED for feed solution, the availability of the produced acid and base, relatively high salt concentration of the effluent, and relatively low desalinating efficiency and capacity. In this work, a novel hybrid process, which coupled conventional electrodialysis installed with monovalent selective cation-exchange membranes with BMED running in a constant-voltage mode, was designed to reclaim brine generated from surface water desalination by the ion-exchange process. Subsequently, the response surface methodology was employed to establish the empirical models for understanding the influences of some initial operating conditions on BMED performance. Finally, the BMED-based reclamation scheme was confirmed again by a continuous BMED experiment on real solution. Specially, the effects of product concentration on current efficiency and energy consumption were investigated. In this case, an acceptable current efficiency and energy consumption were obtained on the basis of the conventional membranes and spacers when the product concentration was set as 0.9 M, which is adequate for the regeneration of ion-exchange resins. & 2013 Elsevier B.V. All rights reserved.
Keywords: Bipolar membrane electrodialysis Industrial saline water Reclamation Response surface methodology Monovalent selective ion-exchange membrane
1. Introduction To meet the growing needs of industrial production and daily life of people, the production capacity of freshwater by desalination of surface water, brackish groundwater, and seawater is increasing at a rapid pace. For example, the global water production by desalination is projected to exceed 38 billion m3 per year in 2016, twice the rate that in 2008 [1]. However, every coin has two sides, i.e., managing the high-salinity side-stream generated during the desalination process has become a thorny problem people must face. In addition, it could be noticed that many highly saline effluents are also formed in relevant industrial scenarios, such as the organic synthesis industry, the food processing industry, the surface treatment industry, and the hydrometallurgy industry. For example, many fine chemicals and pharmaceutical products were synthesized by multi-step processes, such as condensation reaction, substitution reaction, and the neutralization reaction of acid and basic solutions, which will lead to the formation of salts [2]. Undoubtedly, whether from environmental protection or from
n Corresponding author at: College of Chemistry and Chemical Engineering, Ocean University of China, Qingdao 266100, China. Tel./fax: þ 86 532 6678 6513. E-mail addresses: [email protected], [email protected] (M. Wang).
0376-7388/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2013.10.029
resource reuse points of views, an effective scheme for the reclamation of industrial saline water is urgently needed. In view of the synchronous achievement of desalination of feed solution and the production of the corresponding acid and base, bipolar membrane electrodialysis (BMED) has been regarded as a potential technology for the reclamation of industrial saline water. Besides the concentrate from the desalination processes of surface water [3], seawater [4], and even wastewater [5], the processing waste stream, such as the production of glutamate [6], glyphosate [7], brominated butyl rubber [8], and the extraction of the relevant metal [9], was also treated tentatively by means of BMED. Although some techno-economic analyses based on the abovementioned works have confirmed the feasibility of the BMEDbased reclamation scheme for industrial saline water many times, almost all of the works still remain at the stage of laboratory test or pilot demonstration at present. It cannot be denied that, for the BMED-based reclamation scheme for industrial saline water, environmental protection significance seems more prominent than its economic significance due to some well-known facts, such as relatively high cost and short life span of the bipolar membrane, and severe operational conditions of BMED. In a word, the above non-commercialization phenomena indeed show some application-oriented problems still remain unsolved. First, the BMED process has serious requirements for water quality of feed solution, especially its contents of divalent cations,
M. Wang et al. / Journal of Membrane Science 452 (2014) 54–61
Table 1 Water quality of the saline effluent investigated in this study. Parameters
Units
Values
Na þ Ca2 þ Mg2 þ SO42 Cl pH UV254 Conductivity
mmol/L mmol/L mmol/L mmol/L mmol/L / / mS/cm
158.5 22.9 5.8 2.9 222.5 7.45 0.756 20.4
55
produced by BMED are enough to be used for regeneration of ionexchange resins, generally 3–4 wt%. Therefore, an almost zerodischarged desalination technology can be expected after the coupling of the BMED-based reclamation scenario. To understand the influences of some initial operating conditions on process performances, such as current efficiency (CE) and energy consumption (EC), the well-known response surface methodology (RSM) was used to design experiments and establish empirical models. Finally, based on a continuous BMED experiment on real solution, the BMED-based reclamation scheme was confirmed again and effects of product concentration on CE and EC were also investigated.
2. Experimental section silica, and organic matter. For example, Mavrov et al. [10] coupled lime-alkaline precipitation enhanced by coagulation and flocculation with ion exchange to remove any hardness components and decrease the organic and silica contents. And then, the feed solution was filtered after being acidified to pH 2.0 for the deep removal of silica; Tran et al. [11] employed a pellet reactor for reducing the scaling potential of the feed solution for BMED by adding sodium carbonate to precipitate calcium on granular seed material. Obviously, the introduction of specified reactors and extra consumption of Na2CO3, NaOH, HCl, etc. will not only increase investment and operation cost but also produce some harmful byproducts – CaCO3, Ca(OH)2, and Mg(OH)2. Second, according to the reported literatures on the BMED-based reclamation scheme for industrial saline water, we noticed that almost all the works were run at the constant-current mode, which was usually utilized in BMED. Hence, some concomitant problems will appear. On the one hand, if a relatively high current density was employed, the relatively high content of salt still remained in the effluent from BMED. Obviously, it may not meet the requirements for direct discharge or reuse. For example, a BMED was run at 60– 130 mA/cm2 of current density for NaCl splitting in wastewater of amino acid processing into reusable diluted caustic and HCl. The experimental results showed that saline water with a relatively high concentration, as much as about 1 wt%, has to be directly discharged [12]. Obviously, this will result in not only a possible environmental pollution but also a resource waste because a serious pretreatment for the saline water has been performed before its being fed into BMED. On the other hand, if a relatively low current density were exerted, the BMED will be not given to full play. For example, Badruzzaman et al. operated a BMED only at 10 mA/cm2 of current density to reclaim RO concentrate generated from wastewater desalination [5]. Their results showed that the salt concentration can go down from 9 mS/cm to less than 2 mS/ cm and only about 0.2 M acid and base can be obtained. In addition, it is worth noting that the BMED process can only produce the acid and base with relatively low concentrations, for example, generally 1–2 N for strong acid and base. Obviously, the limited applications of the dilute HCl and NaOH also impeded the BMED-based reclamation process from being put into practice. In this study, a hybrid process coupling BMED with conventional ED was attempted to reclaim brine generated from surface water desalination by ion exchange (IE). First, an electrodialysis (ED) process, composed of monovalent selective ion-exchange membranes, was employed to pre-treat the saline effluent, including concentrating and preliminarily reducing the hardness contents at the same time. After the deep removal of hardness by subsequent IE, BMED was carried out in a constant-voltage mode to reclaim the concentrated saline effluent. It can be easily understood that the saline water after being desalinated by ED can be directly returned to the ion-exchange tower for freshwater production. On the other hand, the concentrations of acid and base
2.1. Water quality The Yangtse River's water was desalinated by IE to supply production water at a certain chemical plant located in Shanghai. In this study, a water sample was taken from its neutralization tank in which the waste acid and base produced during the regeneration of ion-exchange resins were discharged. Some typical water quality parameters are included in Table 1. 2.2. Experimental equipment The BMED experiments were carried out by a laboratory-scale plant (10 cm 20 cm), which was designed and assembled in our lab. The membrane stack consisted of two electrode compartments in which titanium electrodes coated with ruthenium oxide were inserted and there were eight repeating units in which monopolar membranes (DF120, Shan-dong Tian-wei Membrane Technology Co., Ltd., China) and bipolar membranes (He-bei Guang-ya Co. Ltd., China) were arranged in a three-compartment mode. In the BMED stack, the membranes were divided by 1 mmthick viton gaskets and spacers. At the beginning of each condition experiment, all the solution volumes, including acid, base, and salt solution, were set at 3 L. During the whole BMED process, a desired fixed electrical voltage was supplied by a variable power source (0–64V, 0–20A, TPR series, Longwei instruments Co. LTD., H.K.). Four plastic magnetic pumps (MP-6R, Shanghai Magnetic Pump Manufacture Co. Ltd., China) were employed to drive flows in the stack with flow rates ranging from 10 to 90 L/h, which were denoted by some specified flow meters (LZBF DN10, Tian-jin Wuhuan Instrument Factory, China). To make the BMED reclamation scheme run effectively, two auxiliary steps, including a conventional ED process and an IE process, were coupled in this study. The objectives of the former lay in the preliminary hardness removal and concentration for the industrial brine at the same time, and that of the latter was deep removal for hardness content in the concentrated brine to meet the requirements of the BMED process. Basically, the construction and operation of the conventional electrodialyzer were similar to the above-mentioned bipolar membrane electrodialyzer. A noteworthy point was that the ED stack (10 cm 20 cm) was composed of eight pairs of monovalent selective cation exchange membranes (CEMs) (CSO, SELEMION) and conventional anion exchange membranes (AEMs) (DF120, Shan-dong Tian-wei Membrane Technology Co., Ltd., China). In addition, a laboratory-scale ion-exchange column (diameter: 5 cm) into which a kind of macro-porous strong acidic cation resins (D001-form, Shanghai Hui-zhi Resin Factory, China) were packaged was used to further remove metal contaminants. During the IE process, a plastic magnetic pump was used to drive the concentrated brine with the flow rates ranging from 20 to 40 m/h, which were denoted by
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a specified flow meter according to the requirements of the resin manufacturer. 2.3. Experimental design As a combination of mathematical and statistical techniques, RSM can be used to optimize processes, predict processes, and evaluate the relative significance of relevant affecting factors [13]. Therefore, the well-known RSM was employed to establish the regression models to understand the influences of some independent factors and their interactions on BMED performance and to predict BMED performance. After a comprehensive consideration of the BMED-based reclamation process, the Central Composite Design (CCD) for response surface modeling was carried out using three independent initial variables, namely the concentration of saline effluent pre-concentrated by ED, the initial concentration of HCl/NaOH at the beginning of every BMED cycle, and the exerted voltage. An axial spacing (71.682) and a range of high ( þ1) level and low ( 1) level were adopted based on our preliminary experiments in the CCD. The detailed descriptions on variable factors with the coded and actual values are given in Table 2. For the CCD with three factors, each at five levels, six axial points and six replicates at the center points, a total of 20 runs of experiments were required. During the experimental work, a randomized experimental sequence was adopted to minimize the effects of the uncontrolled factors. The CCD matrix and experimental data analysis were performed using Design Expert software version 8.0.7.1 (STAT-EASE Inc., Minneapolis, USA). A second-order polynomial function expressed by Eq. (1) can be formed to demonstrate the mathematical relation between the factors and the response. Finally, some evaluations, including the statistical significance of the model coefficients, the accuracy, and general ability of the regressed models were carried out by the analysis of variance (ANOVA): Y ¼ A0 þ ∑Ai X i þ ∑Aii X 2ii þ ∑Aij X i X j
ð1Þ
where Y is the estimated response variable, including Current efficiency and Energy consumption; X i and X j (i ¼1–3; j¼ 1–3) represent the coded independent variables; A0 is the intercept; Ai represents the linear effect of X i ; Aij represents the quadratic effect of X i ; Aij represents the interaction between X i and X j . 2.4. Determination of CE and EC In view of the pronounced influence of proton leakage, the current efficiency η of the whole BMED process was estimated conservatively in terms of the ratio of the theoretically required current for producing a certain amount of acid to the practically supplied current. It can be described by Eq. (2) as follows: η¼
nF∑ðC facidt C iacid ÞV Rt N 0 Idt
ð2Þ
Here, C facid and C iacid are the final and initial concentrations of HCl for twice continuously samplings, respectively; V is the corresponding
volume of HCl solution; n is the ion absolute valence; I is the corresponding current; F is the Faraday constant; and N is the number of cell triplets. Likewise, the energy consumption E of the whole BMED process was also calculated according to Eq. (3) on the basis of acid production: Rt 0 UI dt E¼ ð3Þ ∑ðC facidt C iacid ÞV Here, C facid and C iacid are the final and initial concentrations of HCl for twice continuously sampling, respectively; V is the corresponding volume of HCl solution; U is the exerted voltage across the cell stack; I is the corresponding current; and M is the molar weight of hydrochloric acid (36.5 g/mol). 2.5. Analysis Samples were taken every 10 min. Acid and base concentrations were determined by acid-base titration. The volume variations of the remaining fluid in the reservoirs were recorded in real time according to the change of the liquid level of tanks. Some key ions, such as Ca2 þ , Mg2 þ , SO42 , and Cl , were analyzed by an automatic potentiometric titration instrument (ZD-2, Leici). Sodium ion analyzer (DWS-51, Leici) was employed to measure the concentration of Na þ . UV254, denoting the content of organic matter in the water sample, was measured by a spectrophotometer (UV–1600PC, Mapada) at a wavelength of 254 nm using a 1 cm quartz cell after being diluted twice. In addition, conductivity meter (DDSJ-308A, Lei-ci) and pH meter (pHS-3G, Lei-ci) were also used, which were all daily calibrated.
3. Results and discussions. 3.1. The proposal for the reclamation of brine generated from surface water desalination by IE. Compared with some typical desalination technologies, for example, the well-known Reverse Osmosis (RO) process, IE embraces some outstanding advantages, such as high recovery ratio of water (larger than 90%), low power consumption, low investment and maintenance cost, and so on. Accordingly, IE is usually considered as the first choice for water desalination when the salt content of raw water is not very high (e.g. total dissolved salt (TDS) is smaller than 1200 mg/L.) However, some inherent disadvantages of IE technology, for example, the consumption of large quantity of HCl and NaOH for resin regeneration and the production of substantial waste acid, waste base, and saline water, need to be ameliorated. In this study, a hybrid process coupling a conventional ED, which was installed with monovalent selective CEMs with a BMED, was put forward for the reclamation of industrial saline water generated during the IE desalination process. A simplified flow diagram of the reclamation system is demonstrated in Fig. 1.
Table 2 Codes and levels of the experimental design. Factor
The exerted voltage (V) The initial concentration of saline water (M) The initial concentration of acid/base (M) (mol/L)
Code
X1 X2 X3
Coded level 1.682
1
0
1
1.682
15 0.2 0
17.03 0.26 0.02
20 0.35 0.05
22.97 0.44 0.08
25 0.5 0.1
The relation between the coded values ðX i Þ and actual values (xi ) were X 1 ¼ ðx1 20Þ=2:97, X 2 ¼ ðx2 0:35Þ=0:09, X 3 ¼ ðx3 0:05Þ=0:03.
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57
Produced acid and base for the regeneration of ion-exchange-resin
Softened salty water
Pellet reactor
Low-salinity water
waste acid
Surface water
IE1
waste base
Desalinated water
Neutralization tank
Concentrated brine
Concentrated brine
brine
ED
waste acid
IE2
BMED
Reclaimed water for further concentration
Fig. 1. A simplified flow diagram of the reclamation system. IE1. IE process used for surface water desalination; IE2. IE process used for hardness removal.
According to this scheme, the saline water after being desalinated by ED to a certain extent (e.g. the TDS is smaller than 1200 mg/L) will be directly returned to the ion-exchange tower for subsequent freshwater production. Obviously, for the whole desalination process, this will not only increase water recovery ratio but also take full advantage of the pretreatment of surface water for IE desalination. On the other hand, it was worth noting that the concentration of the acids and bases produced by BMED can easily meet the requirements for the regeneration of ion-exchange resins. This showed that the reclamation scheme will no longer make a zero-discharged process theoretically possible, but posses a significant potential for practical application. Here, it can be noticed that the introduction of a conventional ED can play some important roles from the following several aspects. Above all, the pre-concentration of the industrial brine by ED can make the post BMED run at a relatively high current density. Hence, the treating capacity of BMED can be significantly improved. Second, the employment of monovalent selective CEM can preliminarily reduce the contents of divalent cations in the concentrated brine. In general, only a few divalent anions (e.g. SO42 ) in hydrochloric acid cannot deteriorate the regeneration effect of ion-exchange resin. Therefore, the monovalent selective AEMs were not necessary here. Moreover, this will also be beneficial to the conventional ED process for avoiding the possible formation of precipitates (e.g. CaSO4) in the dilute compartments and for reducing membrane investment cost. Finally, in this scheme, the saline effluent reclaimed partly by BMED will be directly returned to the former ED process for the next concentration. Without question, the reuse of low-salinity effluent from BMED will make the subsequent pretreatment of the saline water for BMED more simple and environmental friendly. For example, some additional hydrochloric acid was generally required for silica removal during a pretreatment process for BMED [10]. However, in this scheme, the proton leakage taking place during the BMED can be effectively taken advantage of to solve the above problem. In addition, it has to be pointed out that for the surface water containing relatively high contents of divalent cations, some extra methods, for example, the pellet reactor, should be considered for hardness removal before reusing the partly desalinated saline water by ED [11]. Furthermore, spurred by the operating mode widely adopted in conventional ED, a BMED with a constant-voltage mode was attempted in this work to take full advantage of the saline water pre-concentrated by ED and make the BMED run at the relatively high current density at the same time. 3.2. The effects of some initial operating parameters on BMED performance 3.2.1. Model fitting In order to put the whole reclamation process into practical applications, it is very necessary to understand the influences of some important process parameters, including some initial operating conditions and the selection of endpoint for a BMED cycle, on the core
Table 3 Experimental values of the central composite design. Runs Coded levels
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Experimental results
X1
X2
X3
Current efficiency (%)
Energy consumption, (kWh/kg)
0 0 0 1 0 1 1.682 0 0 1 1 1 1.682 1 1 0 1 0 0 0
0 0 1.682 1 0 1 0 1.682 0 1 1 1 0 1 1 0 1 0 0 0
0 1.682 0 1 0 1 0 0 1.682 1 1 1 0 1 1 0 1 0 0 0
69.41 76.44 65.63 64.40 69.51 66.87 61.58 75.36 67.45 80.48 74.06 60.47 73.48 69.66 73.78 69.11 74.28 69.31 69.38 69.08
4.2527 4.3395 4.4891 5.2452 4.2315 3.7337 5.9625 3.8976 4.3547 3.1022 3.3708 5.5856 2.998 4.8486 4.9428 4.2402 3.3608 4.1402 4.2893 4.3193
technology. For the former, some key factors, such as exerted voltage, the concentration of saline effluent pre-concentrated by ED, and the initial concentration of HCl/NaOH at the beginning of every BMED cycle, and their interactions, should be investigated. The levels of the above-mentioned factors and the effects of their interactions on BMED performance were determined through the CCD of RSM. The final experimental design matrix and the obtained output responses are shown in Table 3. Moreover, the five-level three-factorial CCD of RSM yielded the empirical models that described the mathematical relationship between the BMED performance, including CE and EC, and the test variables in coded units as follows: For CE : Y CE ¼ 69:28 3:47X 1 3:54X 2 2:71X 3 0:65X 1 X 2 þ 0:72X 1 X 3 0:1X 2 X 3 0:53X 21 þ0:52X 22 þ 1:03X 23 ð4Þ For EC : Y EC ¼ 4:25 þ 0:88X 1 þ 0:19X 2 þ0:067X 3 þ0:052X 1 X 2 0:049X 1 X 3 þ 0:067X 2 X 3 þ0:068X 21 0:034X 22 þ 0:021X 23
ð5Þ
Above all, the statistical significance of the model coefficients should be determined by ANOVA combined with the application of F-test to ensure a good model. The checks for the above equations by F-test showed that both the regressed models had high values of F (F¼101.83 for the CE model and F¼ 140.41 for the EC model) and very low probability values (both are smaller than 0.0001). These indicated that both models were highly significant. Furthermore, the goodness of fit of the model was also evaluated by inspecting the determination coefficient (R2 ), the adjusted
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Fig. 2. Plots of predicted vs. actual values.
determination coefficient (R2adj ), and the coefficient of variation (C:V:%), which indicate the fraction of the variation of the response explained by the model, the fraction of variation of the response explained by the model adjusted for degrees of freedom, and the extent of variability in relation to mean of the population, respectively. The obtained values of R2 (R2 ¼0.9892 for CE and R2 ¼0.9921 for EC) showed that more than 98% of the data deviation can be explained by the two empirical models. However, a large value of R2 does not necessarily imply a good regression model as R2 always increases when adding any variable, significant or not significant, to the model [14]. Hence, R2adj was used to correct the R2 for the sample size and for the number of terms. In this case, the large values of R2adj (R2adj ¼0.9795 for CE and R2adj ¼0.9851 for EC) disclosed a high significance of the empirical models. Moreover, these also showed that all the significant terms were included in the empirical models. In addition, the relatively lower values of C:V:% (C:V:% ¼1.02 for CE and C:V:% ¼2.21 for EC) also indicated a better precision and reliability of the experiments. As can be shown visually by Fig. 2, the predicted data of the responses from the empirical models agreed well with the observed ones in the range of the operating variables. In short, all statistical estimators showed that the developed models were validated from the statistical point of view and can disclose the contributions of the factors to the work performance of the BMEDbased reclamation process.
3.2.2. The effects of the variables The significance of the coefficients in both models was further checked by the F test and is demonstrated in Tables 4 and 5, respectively. As is known, the smaller the p value, the greater the significance of the corresponding coefficient. For the response of CE, the corresponding p values indicated that the linear variables,
including X 1 , X 2 , and X 3 , and the quadratic term of X 3 had highly significant effects on the current efficiency. Next in importance to the above terms, the quadratic terms of X 1 and X 2 and the interactive terms of X 1 and X 2 , X 1 , and X 3 also significantly affected the current efficiency. As for the interactions between X 2 and X 3 , a very minor influence on current efficiency was observed since the p value was larger than 0.05. For the response of EC, the corresponding p values suggested that all the linear variables had significant effects on energy consumption. In contrast, the influence of X 3 was much weaker. However, with the exception of the quadratic term of X 1 , all the interactive terms and quadratic terms only possessed very weak effects on energy consumption since their corresponding p values were larger than 0.05. The parameter estimates can also suggest the effects of the factors on the responses. A positive coefficient indicates a synergistic effect, while a negative coefficient indicates an antagonistic effect. As can be seen clearly from Tables 4 and 5, with the exception of the coefficients of the X 3 quadratic term, the coefficients of the factors of one model had opposite signs in the other. For example, the increase of the exerted voltage seemed to significantly deteriorate work performance, such as the increase of energy consumption and the decrease of current efficiency, which were not expected. However, some actual conditions, for example, the expectation for high desalinating capacity and efficiency of the saline water and some inherent disadvantages of bipolar membranes, such as relatively high price and limited life span, usually require the BMED to work under relatively high current density. Accordingly, the relatively high initial concentration of saline water was also expected, though it is also a significant factor for decreasing current efficiency and increasing energy consumption. However, the effect of the initial concentration of acid/base seemed to be very interesting in this work. Generally speaking, the addition of a small amount of acid/base into acid/base compartments at the beginning of BMED is favored to increase solution conductivity and accordingly decrease energy consumption. In this case, just as the effect of initial concentration of saline water on current efficiency, the increase of the initial concentration of acid/base also significantly decreased current efficiency, which is shown in Figs. 3 and 4, respectively. On the other hand, compared with the effects of the exerted voltage and the initial concentration of saline water on energy consumption, influence of the initial concentration of acid/base was much weaker. This should be ascribed to the adoption of constant-voltage mode in this BMED process. That is, when the initial concentration in acid/ base compartments was low, the current density was also low and accordingly energy consumption did not significantly increase. A similar result was also observed in Wu's work, in which the initial concentration of acid/base compartments in the optimized scheme for salts recovery from 1,3-propanediol fermentation broth by BMED was recommended as 0 M [15]. Obviously, this should be favored to increase the production capacity of acid and base and to simplify the technological operation. 3.3. The continuous BMED experiment on the real solution pretreated by ED installed with monovalent selective CEMs and IE In order to further confirm the feasibility of the reclamation process, a continuous BMED experiment on a real solution was conducted. Above all, the real solution was pretreated by electrodialysis installed with monovalent selective CEMs and IE in turn. Experimental results showed that the contents of Ca and Mg in the saline water can be reduced easily by less than 1 ppm. This indicated that the requirements of BMED for hardness content were met [16]. Simultaneously, the conductivity of the saline water was increased from about 20 mS/cm to about 40 mS/cm. It is reasonable to think that the salt concentration was also
M. Wang et al. / Journal of Membrane Science 452 (2014) 54–61
59
Table 4 The significance of the coefficients in the regression model for CE. Sources
A0
A1
A2
A3
A1 A2
A1 A3
A2 A3
A11
A22
A33
Coefficient estimate F-value p-Value
69.28
3.47 322.22 o 0.0001
3.54 335.00 o0.0001
2.71 196.39 o 0.0001
0.65 6.57 0.0282
0.72 8.18 0.0169
0.1 0.16 0.7005
0.068 7.84 0.0188
0.52 7.67 0.0198
1.03 30.18 0.0003
Table 5 The significance of the coefficients in the regression model for EC. Sources
A0
A1
A2
A3
A1 A2
A1 A3
A2 A3
A11
A22
A33
Coefficient Estimate F-value p-Value
4.25
0.88 1180.35 o 0.0001
0.19 57.42 o 0.0001
0.067 6.79 0.0262
0.052 2.43 0.1503
0.049 2.17 0.1713
0.067 4.04 0.0723
0.068 7.35 0.0219
0.034 1.82 0.2071
0.021 0.68 0.4275
Fig. 3. Three-dimensional surface plot illustrating effects of the interactions between voltage and initial concentration of saline water on current efficiency.
Fig. 4. Three-dimensional surface plot illustrating effects of the interactions between voltage and initial concentration of acid/base on current efficiency.
doubled because the pH remained almost constant during the process. Along with the processing of BMED, the salt content of the concentrated saline water decreased gradually. Finally, the saline water that was desalinated partly will return directly to the ED step for the next concentration process. In this case, when the concentration of saline water was reduced to about 0.05 M, it was replaced by other “new” concentrated saline water for a “new” BMED cycle. Obviously, a preliminary hardness removal process can be carried out simply and in an environmental-friendly manner by ED, which was installed with monovalent selective CEMs with the synchronous achievement of concentrated saline
water. In addition, the reuse of saline water discharged from the BMED process can also effectively avoid some additional complex pretreatment operations for meeting the requirements of BMED for water quality, including silica removal, organic matter removal, and so on. In addition, it can be noticed that the concentration of the acid and base increased along with the BMED process. Sometimes, when the acid and base concentrations met the requirements for regeneration of ion-exchange resins (e.g. 3–4 wt%), the acid and base products were stored in tanks and equivalent water was supplemented in the acid and base compartments.
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M. Wang et al. / Journal of Membrane Science 452 (2014) 54–61
50
1.6
45
1.4
35 1.0
30
0.8
25 20
0.6
15
Conductivity, ms/cm
Acid concentration, M
40 1.2
case, an acceptable current efficiency and energy consumption can be obtained on the basis of some conventional ion-exchange membranes and spacers when the product concentration was set at 0.9 M. It can be noticed that the 0.9 M acid and base are enough for the regeneration of ion-exchange resins. Obviously, the coupling of BMED-based reclamation process with the conventional desalination process by IE will make the surface water desalination practically zero-discharged.
0.4 10 0.2
4. Conclusions
5
0.0
0 0
50
100
150
200
250
300
350
time, min
Fig. 5. The evolution of the concentration in acid compartments and the conductivity in saline water compartments during the BMED-based reclamation process. 65
8.5
Current efficiency, %
7.5 55 7.0 50
6.5
45
6.0
40
5.5
Energy consumption, Kwh/Kg
8.0
60
5.0 35 4.5 0.5
0.7
0.9
1.1
1.3
Acid concentration, M
Fig. 6. The current efficiency and energy consumption of a BMED process corresponding to the different product concentrations.
Fig. 5 demonstrates the gradual desalination process in every BMED cycle and the conversion between the two paths of saline water when one reached the endpoint of desalination. Along with the continuous BMED process, the product concentration increased gradually. It can be noticed that the relation between the product concentration and operating time is nonlinear, which is significantly different from the typical relation displayed by BMED running in a constant-current mode. That is, during the BMED process running in the constant-voltage mode, the current density decreased with the decrease of salt contents in saline water. Obviously, both higher treating ability at relatively high current density and lower salt content of saline water for further treatment can be simultaneously achieved when the BMED in a constant-voltage mode was adopted. On the other hand, the concentration that the product can reach should also be paid more attention. Generally speaking, a higher concentration of recovered acid and base will be favored for practical applications. Fig. 6 shows the corresponding current efficiency and energy consumption during a BMED process under different product concentrations. It can be seen clearly that the work performance deteriorated remarkably along with the increase of product concentration. Obviously, these should be attributed to the leakage of H þ and OH through ion-exchange membranes and current leakage through acid and base solution when their concentration increased remarkably. Undoubtedly, some improvements in BMED equipment, for example, the adoption of AEMs – which can block proton leakage – and spacers – which can prevent current leakage – were needed urgently. In this
With respect to the dilemma that some unsolved applicationoriented problems impeded the BMED technology to be put into practice for the reclamation of industrial brine, a novel hybrid BMED-based scheme was put forward in this study. That is, the industrial brine produced during surface water desalination process by IE was fed into ED installed with monovalent selective CEMs for the synchronous achievement of concentrated saline water and preliminary hardness removal. Besides the improvements of the subsequent BMED performance due to the concentration of the saline water, the recovery ratio of water for the whole surface desalination process can be increased because of the reuse of the low-salinity water from the ED process. After being further softened by IE, the concentrated saline water was desalinated to produce acid and base by a BMED in a constant-voltage mode. Therein, the saline water having been partly desalinated will be directly returned to ED for the next concentration, with the synchronous achievement of desalination of industrial brine. This can effectively simplify some relevant pretreatment operations for BMED, especially, the removal of divalent cations, silica, and organic matter. Moreover, the RSM analyses showed that some initial factors, including the exerted voltage, the concentration of the saline water pretreated by ED, the initial concentration of acid/base, and their interactions significantly influenced the current efficiency and energy consumption of the BMED process. When compared with that of the BMED process with a constantcurrent mode, it was worth noting that the optimized initial concentration of acid and base was recommended as 0 M in the BMED with a constant-voltage mode. Finally, a continuous BMED experiment on a real solution confirmed the feasibility of the reclamation scheme. The experimental results indicated that the required product concentration remarkably affected the work performance of BMED. In this case, an acceptable current efficiency and energy consumption can be obtained when the product concentration was set as 0.9 M. Obviously, the product concentration of 0.9 M is adequate for the regeneration of ion-exchange resins. Of course, if the BMED equipment can be improved, for example, the use of protonblock AEMs and specified spacers for preventing current leakage, better work performance can be expected.
Acknowledgments This research was supported in part by the National Science Foundation of China (No. 21276245) and Natural Science Foundation of Shandong Province (ZR2011EMQ004). This is MCTL Contribution no. 29. References [1] Q. Schiermeier, Water: purification with a pinch of salt, Nature 452 (2008) 260–261.
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[2] Q.C. Ren, M. Wang, Y.X. Jia, P.F. Zhou, Current patents on the innovative applications of electrodialysis technology in the treatment and reclamation of industrial waste effluent, Recent Pat. Chem. Eng. 6 (2013) 57–67. [3] V. Mavrov, H. Chmiel, B. Heitele, F. RÖgener, Desalination of surface water to industrial water with lower impact on the environment. Part 4: treatment of effluents from water desalination stages for reuse and balance of the new technological concept for water desalination, Desalination 24 (1999) 205–216. [4] R. Ibáñez, A. Pérez-González, P. Gómez, A.M. Urtiaga, I. Ortiz, Acid and base recovery from softened reverse osmosis (RO) brines. Experimental assessment using model concentrates, Desalination 309 (2013) 165–170. [5] M. Badruzzaman, J. Oppenheimer, S. Adham, M. Kumar, Innovative beneficial reuse of reverse osmosis concentrate using bipolar membrane electrodialysis and electrochlorination processes, J. Membr. Sci. 326 (2009) 392–399. [6] Q. Wang, P.B. Yang, W. Cong, Cation-exchange membrane fouling and cleaning in bipolar membraneelectrodialysis of industrial glutamate production wastewater, Sep. Purif. Technol. 79 (2011) 103–113. [7] X.X. Wang, M. Wang, Y.X. Jia, T.T. Yao, The feasible study on the reclamation of the glyphosate neutralization liquor by bipolar membrane electrodialysis, Desalination 300 (2012) 58–63. [8] Y.X. Wei, Y.M. Wang, X. Zhang, T.W. Xu, Treatment of simulated brominated butyl rubber wastewater by bipolar membrane electrodialysis, Sep. Purif. Technol. 80 (2011) 196–201. [9] K. Ghyselbrecht, M. Huygebaert, B. Van der Bruggen, R. Ballet, B. Meesschaert, L. Pinoy, Desalination of an industrial saline water with conventional and bipolar membrane electrodialysis, Desalination 318 (2013) 9–18.
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[10] V. Mavrov, H. Chmiel, B. Heitele, F. RÖgener, Desalination of surface water to industrial water with lower impact on the environment. Part 2: improved feed water pretreatment, Desalination 110 (1997) 65–74. [11] A.T.K. Tran, N. Jullok, B. Meesschaert, L. Pinoy, B. Van der Bruggen, Pellet reactor pre-treatment: a feasible method to reduce scaling in bipolar membrane electrodialysis, J. Colloid Interface Sci. 401 (2013) 107–115. [12] G. Pourcelly, C. Gavach, Electrodialysis water splitting – applications of electrodialysis with bipolar membranes (EDBM), in: A.J.B. Kemperman (Ed.), Handbook on Bipolar Membrane Technology, Twente University Press, Enschede, 2000, p. 45. [13] D.C. Montgomery, Response Surface Methods and other Approaches to Process Optimization Design and Analysis of Experiments, 5th edition, John Wiley & Sons, Inc., New York (2001) 2001; 427. [14] Verónica García, Junkal Landaburu-Aguirre, Eva Pongrácz, Paavo Perämäki, Riitta L. Keiski, Dehydration of water/dichloromethane/n-butanol mixtures by pervaporation; optimization and modelling by response surface methodology, J. Membr. Sci. 338 (2009) 111–118. [15] R.C. Wu, Y.Z. Xu, Y.Q. Song, J. Luo, D.H. Liu, A novel strategy for salts recovery from 1,3-propanediol fermentation broth by bipolar membrane electrodialysis, Sep. Purif. Technol. 83 (2011) 9–14. [16] G. Pourcelly, C. Gavach, Electrodialysis water splitting – applications of electrodialysis with bipolar membranes (EDBM), in: A.J.B. Kemperman (Ed.), Handbook on Bipolar Membrane Technology, Twente University Press, Enschede, 2000, p. 24.
Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
The reclamation of brine generated from desalination process by bipolar membrane electrodialysis Meng Wang a,b,n, Kai-kai Wang a,b, Yu-xiang Jia a,b, Qing-chun Ren c a
Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, China College of Chemistry and Chemical Engineering, Ocean University of China, Qingdao 266100, China c Beijing Unisplendour Empyreal Environmental Engineering Technology Co. Ltd., Beijing 100083, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 29 June 2013 Received in revised form 22 September 2013 Accepted 12 October 2013 Available online 19 October 2013
Some techno-economical analyses and environmental impact evaluation have proved that bipolar membrane electrodialysis (BMED) is feasible for the reclamation of industrial saline water. However, to date, the technology cannot be put into practice due to some unsolved application-oriented problems, such as the requirements of BMED for feed solution, the availability of the produced acid and base, relatively high salt concentration of the effluent, and relatively low desalinating efficiency and capacity. In this work, a novel hybrid process, which coupled conventional electrodialysis installed with monovalent selective cation-exchange membranes with BMED running in a constant-voltage mode, was designed to reclaim brine generated from surface water desalination by the ion-exchange process. Subsequently, the response surface methodology was employed to establish the empirical models for understanding the influences of some initial operating conditions on BMED performance. Finally, the BMED-based reclamation scheme was confirmed again by a continuous BMED experiment on real solution. Specially, the effects of product concentration on current efficiency and energy consumption were investigated. In this case, an acceptable current efficiency and energy consumption were obtained on the basis of the conventional membranes and spacers when the product concentration was set as 0.9 M, which is adequate for the regeneration of ion-exchange resins. & 2013 Elsevier B.V. All rights reserved.
Keywords: Bipolar membrane electrodialysis Industrial saline water Reclamation Response surface methodology Monovalent selective ion-exchange membrane
1. Introduction To meet the growing needs of industrial production and daily life of people, the production capacity of freshwater by desalination of surface water, brackish groundwater, and seawater is increasing at a rapid pace. For example, the global water production by desalination is projected to exceed 38 billion m3 per year in 2016, twice the rate that in 2008 [1]. However, every coin has two sides, i.e., managing the high-salinity side-stream generated during the desalination process has become a thorny problem people must face. In addition, it could be noticed that many highly saline effluents are also formed in relevant industrial scenarios, such as the organic synthesis industry, the food processing industry, the surface treatment industry, and the hydrometallurgy industry. For example, many fine chemicals and pharmaceutical products were synthesized by multi-step processes, such as condensation reaction, substitution reaction, and the neutralization reaction of acid and basic solutions, which will lead to the formation of salts [2]. Undoubtedly, whether from environmental protection or from
n Corresponding author at: College of Chemistry and Chemical Engineering, Ocean University of China, Qingdao 266100, China. Tel./fax: þ 86 532 6678 6513. E-mail addresses: [email protected], [email protected] (M. Wang).
0376-7388/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2013.10.029
resource reuse points of views, an effective scheme for the reclamation of industrial saline water is urgently needed. In view of the synchronous achievement of desalination of feed solution and the production of the corresponding acid and base, bipolar membrane electrodialysis (BMED) has been regarded as a potential technology for the reclamation of industrial saline water. Besides the concentrate from the desalination processes of surface water [3], seawater [4], and even wastewater [5], the processing waste stream, such as the production of glutamate [6], glyphosate [7], brominated butyl rubber [8], and the extraction of the relevant metal [9], was also treated tentatively by means of BMED. Although some techno-economic analyses based on the abovementioned works have confirmed the feasibility of the BMEDbased reclamation scheme for industrial saline water many times, almost all of the works still remain at the stage of laboratory test or pilot demonstration at present. It cannot be denied that, for the BMED-based reclamation scheme for industrial saline water, environmental protection significance seems more prominent than its economic significance due to some well-known facts, such as relatively high cost and short life span of the bipolar membrane, and severe operational conditions of BMED. In a word, the above non-commercialization phenomena indeed show some application-oriented problems still remain unsolved. First, the BMED process has serious requirements for water quality of feed solution, especially its contents of divalent cations,
M. Wang et al. / Journal of Membrane Science 452 (2014) 54–61
Table 1 Water quality of the saline effluent investigated in this study. Parameters
Units
Values
Na þ Ca2 þ Mg2 þ SO42 Cl pH UV254 Conductivity
mmol/L mmol/L mmol/L mmol/L mmol/L / / mS/cm
158.5 22.9 5.8 2.9 222.5 7.45 0.756 20.4
55
produced by BMED are enough to be used for regeneration of ionexchange resins, generally 3–4 wt%. Therefore, an almost zerodischarged desalination technology can be expected after the coupling of the BMED-based reclamation scenario. To understand the influences of some initial operating conditions on process performances, such as current efficiency (CE) and energy consumption (EC), the well-known response surface methodology (RSM) was used to design experiments and establish empirical models. Finally, based on a continuous BMED experiment on real solution, the BMED-based reclamation scheme was confirmed again and effects of product concentration on CE and EC were also investigated.
2. Experimental section silica, and organic matter. For example, Mavrov et al. [10] coupled lime-alkaline precipitation enhanced by coagulation and flocculation with ion exchange to remove any hardness components and decrease the organic and silica contents. And then, the feed solution was filtered after being acidified to pH 2.0 for the deep removal of silica; Tran et al. [11] employed a pellet reactor for reducing the scaling potential of the feed solution for BMED by adding sodium carbonate to precipitate calcium on granular seed material. Obviously, the introduction of specified reactors and extra consumption of Na2CO3, NaOH, HCl, etc. will not only increase investment and operation cost but also produce some harmful byproducts – CaCO3, Ca(OH)2, and Mg(OH)2. Second, according to the reported literatures on the BMED-based reclamation scheme for industrial saline water, we noticed that almost all the works were run at the constant-current mode, which was usually utilized in BMED. Hence, some concomitant problems will appear. On the one hand, if a relatively high current density was employed, the relatively high content of salt still remained in the effluent from BMED. Obviously, it may not meet the requirements for direct discharge or reuse. For example, a BMED was run at 60– 130 mA/cm2 of current density for NaCl splitting in wastewater of amino acid processing into reusable diluted caustic and HCl. The experimental results showed that saline water with a relatively high concentration, as much as about 1 wt%, has to be directly discharged [12]. Obviously, this will result in not only a possible environmental pollution but also a resource waste because a serious pretreatment for the saline water has been performed before its being fed into BMED. On the other hand, if a relatively low current density were exerted, the BMED will be not given to full play. For example, Badruzzaman et al. operated a BMED only at 10 mA/cm2 of current density to reclaim RO concentrate generated from wastewater desalination [5]. Their results showed that the salt concentration can go down from 9 mS/cm to less than 2 mS/ cm and only about 0.2 M acid and base can be obtained. In addition, it is worth noting that the BMED process can only produce the acid and base with relatively low concentrations, for example, generally 1–2 N for strong acid and base. Obviously, the limited applications of the dilute HCl and NaOH also impeded the BMED-based reclamation process from being put into practice. In this study, a hybrid process coupling BMED with conventional ED was attempted to reclaim brine generated from surface water desalination by ion exchange (IE). First, an electrodialysis (ED) process, composed of monovalent selective ion-exchange membranes, was employed to pre-treat the saline effluent, including concentrating and preliminarily reducing the hardness contents at the same time. After the deep removal of hardness by subsequent IE, BMED was carried out in a constant-voltage mode to reclaim the concentrated saline effluent. It can be easily understood that the saline water after being desalinated by ED can be directly returned to the ion-exchange tower for freshwater production. On the other hand, the concentrations of acid and base
2.1. Water quality The Yangtse River's water was desalinated by IE to supply production water at a certain chemical plant located in Shanghai. In this study, a water sample was taken from its neutralization tank in which the waste acid and base produced during the regeneration of ion-exchange resins were discharged. Some typical water quality parameters are included in Table 1. 2.2. Experimental equipment The BMED experiments were carried out by a laboratory-scale plant (10 cm 20 cm), which was designed and assembled in our lab. The membrane stack consisted of two electrode compartments in which titanium electrodes coated with ruthenium oxide were inserted and there were eight repeating units in which monopolar membranes (DF120, Shan-dong Tian-wei Membrane Technology Co., Ltd., China) and bipolar membranes (He-bei Guang-ya Co. Ltd., China) were arranged in a three-compartment mode. In the BMED stack, the membranes were divided by 1 mmthick viton gaskets and spacers. At the beginning of each condition experiment, all the solution volumes, including acid, base, and salt solution, were set at 3 L. During the whole BMED process, a desired fixed electrical voltage was supplied by a variable power source (0–64V, 0–20A, TPR series, Longwei instruments Co. LTD., H.K.). Four plastic magnetic pumps (MP-6R, Shanghai Magnetic Pump Manufacture Co. Ltd., China) were employed to drive flows in the stack with flow rates ranging from 10 to 90 L/h, which were denoted by some specified flow meters (LZBF DN10, Tian-jin Wuhuan Instrument Factory, China). To make the BMED reclamation scheme run effectively, two auxiliary steps, including a conventional ED process and an IE process, were coupled in this study. The objectives of the former lay in the preliminary hardness removal and concentration for the industrial brine at the same time, and that of the latter was deep removal for hardness content in the concentrated brine to meet the requirements of the BMED process. Basically, the construction and operation of the conventional electrodialyzer were similar to the above-mentioned bipolar membrane electrodialyzer. A noteworthy point was that the ED stack (10 cm 20 cm) was composed of eight pairs of monovalent selective cation exchange membranes (CEMs) (CSO, SELEMION) and conventional anion exchange membranes (AEMs) (DF120, Shan-dong Tian-wei Membrane Technology Co., Ltd., China). In addition, a laboratory-scale ion-exchange column (diameter: 5 cm) into which a kind of macro-porous strong acidic cation resins (D001-form, Shanghai Hui-zhi Resin Factory, China) were packaged was used to further remove metal contaminants. During the IE process, a plastic magnetic pump was used to drive the concentrated brine with the flow rates ranging from 20 to 40 m/h, which were denoted by
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M. Wang et al. / Journal of Membrane Science 452 (2014) 54–61
a specified flow meter according to the requirements of the resin manufacturer. 2.3. Experimental design As a combination of mathematical and statistical techniques, RSM can be used to optimize processes, predict processes, and evaluate the relative significance of relevant affecting factors [13]. Therefore, the well-known RSM was employed to establish the regression models to understand the influences of some independent factors and their interactions on BMED performance and to predict BMED performance. After a comprehensive consideration of the BMED-based reclamation process, the Central Composite Design (CCD) for response surface modeling was carried out using three independent initial variables, namely the concentration of saline effluent pre-concentrated by ED, the initial concentration of HCl/NaOH at the beginning of every BMED cycle, and the exerted voltage. An axial spacing (71.682) and a range of high ( þ1) level and low ( 1) level were adopted based on our preliminary experiments in the CCD. The detailed descriptions on variable factors with the coded and actual values are given in Table 2. For the CCD with three factors, each at five levels, six axial points and six replicates at the center points, a total of 20 runs of experiments were required. During the experimental work, a randomized experimental sequence was adopted to minimize the effects of the uncontrolled factors. The CCD matrix and experimental data analysis were performed using Design Expert software version 8.0.7.1 (STAT-EASE Inc., Minneapolis, USA). A second-order polynomial function expressed by Eq. (1) can be formed to demonstrate the mathematical relation between the factors and the response. Finally, some evaluations, including the statistical significance of the model coefficients, the accuracy, and general ability of the regressed models were carried out by the analysis of variance (ANOVA): Y ¼ A0 þ ∑Ai X i þ ∑Aii X 2ii þ ∑Aij X i X j
ð1Þ
where Y is the estimated response variable, including Current efficiency and Energy consumption; X i and X j (i ¼1–3; j¼ 1–3) represent the coded independent variables; A0 is the intercept; Ai represents the linear effect of X i ; Aij represents the quadratic effect of X i ; Aij represents the interaction between X i and X j . 2.4. Determination of CE and EC In view of the pronounced influence of proton leakage, the current efficiency η of the whole BMED process was estimated conservatively in terms of the ratio of the theoretically required current for producing a certain amount of acid to the practically supplied current. It can be described by Eq. (2) as follows: η¼
nF∑ðC facidt C iacid ÞV Rt N 0 Idt
ð2Þ
Here, C facid and C iacid are the final and initial concentrations of HCl for twice continuously samplings, respectively; V is the corresponding
volume of HCl solution; n is the ion absolute valence; I is the corresponding current; F is the Faraday constant; and N is the number of cell triplets. Likewise, the energy consumption E of the whole BMED process was also calculated according to Eq. (3) on the basis of acid production: Rt 0 UI dt E¼ ð3Þ ∑ðC facidt C iacid ÞV Here, C facid and C iacid are the final and initial concentrations of HCl for twice continuously sampling, respectively; V is the corresponding volume of HCl solution; U is the exerted voltage across the cell stack; I is the corresponding current; and M is the molar weight of hydrochloric acid (36.5 g/mol). 2.5. Analysis Samples were taken every 10 min. Acid and base concentrations were determined by acid-base titration. The volume variations of the remaining fluid in the reservoirs were recorded in real time according to the change of the liquid level of tanks. Some key ions, such as Ca2 þ , Mg2 þ , SO42 , and Cl , were analyzed by an automatic potentiometric titration instrument (ZD-2, Leici). Sodium ion analyzer (DWS-51, Leici) was employed to measure the concentration of Na þ . UV254, denoting the content of organic matter in the water sample, was measured by a spectrophotometer (UV–1600PC, Mapada) at a wavelength of 254 nm using a 1 cm quartz cell after being diluted twice. In addition, conductivity meter (DDSJ-308A, Lei-ci) and pH meter (pHS-3G, Lei-ci) were also used, which were all daily calibrated.
3. Results and discussions. 3.1. The proposal for the reclamation of brine generated from surface water desalination by IE. Compared with some typical desalination technologies, for example, the well-known Reverse Osmosis (RO) process, IE embraces some outstanding advantages, such as high recovery ratio of water (larger than 90%), low power consumption, low investment and maintenance cost, and so on. Accordingly, IE is usually considered as the first choice for water desalination when the salt content of raw water is not very high (e.g. total dissolved salt (TDS) is smaller than 1200 mg/L.) However, some inherent disadvantages of IE technology, for example, the consumption of large quantity of HCl and NaOH for resin regeneration and the production of substantial waste acid, waste base, and saline water, need to be ameliorated. In this study, a hybrid process coupling a conventional ED, which was installed with monovalent selective CEMs with a BMED, was put forward for the reclamation of industrial saline water generated during the IE desalination process. A simplified flow diagram of the reclamation system is demonstrated in Fig. 1.
Table 2 Codes and levels of the experimental design. Factor
The exerted voltage (V) The initial concentration of saline water (M) The initial concentration of acid/base (M) (mol/L)
Code
X1 X2 X3
Coded level 1.682
1
0
1
1.682
15 0.2 0
17.03 0.26 0.02
20 0.35 0.05
22.97 0.44 0.08
25 0.5 0.1
The relation between the coded values ðX i Þ and actual values (xi ) were X 1 ¼ ðx1 20Þ=2:97, X 2 ¼ ðx2 0:35Þ=0:09, X 3 ¼ ðx3 0:05Þ=0:03.
M. Wang et al. / Journal of Membrane Science 452 (2014) 54–61
57
Produced acid and base for the regeneration of ion-exchange-resin
Softened salty water
Pellet reactor
Low-salinity water
waste acid
Surface water
IE1
waste base
Desalinated water
Neutralization tank
Concentrated brine
Concentrated brine
brine
ED
waste acid
IE2
BMED
Reclaimed water for further concentration
Fig. 1. A simplified flow diagram of the reclamation system. IE1. IE process used for surface water desalination; IE2. IE process used for hardness removal.
According to this scheme, the saline water after being desalinated by ED to a certain extent (e.g. the TDS is smaller than 1200 mg/L) will be directly returned to the ion-exchange tower for subsequent freshwater production. Obviously, for the whole desalination process, this will not only increase water recovery ratio but also take full advantage of the pretreatment of surface water for IE desalination. On the other hand, it was worth noting that the concentration of the acids and bases produced by BMED can easily meet the requirements for the regeneration of ion-exchange resins. This showed that the reclamation scheme will no longer make a zero-discharged process theoretically possible, but posses a significant potential for practical application. Here, it can be noticed that the introduction of a conventional ED can play some important roles from the following several aspects. Above all, the pre-concentration of the industrial brine by ED can make the post BMED run at a relatively high current density. Hence, the treating capacity of BMED can be significantly improved. Second, the employment of monovalent selective CEM can preliminarily reduce the contents of divalent cations in the concentrated brine. In general, only a few divalent anions (e.g. SO42 ) in hydrochloric acid cannot deteriorate the regeneration effect of ion-exchange resin. Therefore, the monovalent selective AEMs were not necessary here. Moreover, this will also be beneficial to the conventional ED process for avoiding the possible formation of precipitates (e.g. CaSO4) in the dilute compartments and for reducing membrane investment cost. Finally, in this scheme, the saline effluent reclaimed partly by BMED will be directly returned to the former ED process for the next concentration. Without question, the reuse of low-salinity effluent from BMED will make the subsequent pretreatment of the saline water for BMED more simple and environmental friendly. For example, some additional hydrochloric acid was generally required for silica removal during a pretreatment process for BMED [10]. However, in this scheme, the proton leakage taking place during the BMED can be effectively taken advantage of to solve the above problem. In addition, it has to be pointed out that for the surface water containing relatively high contents of divalent cations, some extra methods, for example, the pellet reactor, should be considered for hardness removal before reusing the partly desalinated saline water by ED [11]. Furthermore, spurred by the operating mode widely adopted in conventional ED, a BMED with a constant-voltage mode was attempted in this work to take full advantage of the saline water pre-concentrated by ED and make the BMED run at the relatively high current density at the same time. 3.2. The effects of some initial operating parameters on BMED performance 3.2.1. Model fitting In order to put the whole reclamation process into practical applications, it is very necessary to understand the influences of some important process parameters, including some initial operating conditions and the selection of endpoint for a BMED cycle, on the core
Table 3 Experimental values of the central composite design. Runs Coded levels
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Experimental results
X1
X2
X3
Current efficiency (%)
Energy consumption, (kWh/kg)
0 0 0 1 0 1 1.682 0 0 1 1 1 1.682 1 1 0 1 0 0 0
0 0 1.682 1 0 1 0 1.682 0 1 1 1 0 1 1 0 1 0 0 0
0 1.682 0 1 0 1 0 0 1.682 1 1 1 0 1 1 0 1 0 0 0
69.41 76.44 65.63 64.40 69.51 66.87 61.58 75.36 67.45 80.48 74.06 60.47 73.48 69.66 73.78 69.11 74.28 69.31 69.38 69.08
4.2527 4.3395 4.4891 5.2452 4.2315 3.7337 5.9625 3.8976 4.3547 3.1022 3.3708 5.5856 2.998 4.8486 4.9428 4.2402 3.3608 4.1402 4.2893 4.3193
technology. For the former, some key factors, such as exerted voltage, the concentration of saline effluent pre-concentrated by ED, and the initial concentration of HCl/NaOH at the beginning of every BMED cycle, and their interactions, should be investigated. The levels of the above-mentioned factors and the effects of their interactions on BMED performance were determined through the CCD of RSM. The final experimental design matrix and the obtained output responses are shown in Table 3. Moreover, the five-level three-factorial CCD of RSM yielded the empirical models that described the mathematical relationship between the BMED performance, including CE and EC, and the test variables in coded units as follows: For CE : Y CE ¼ 69:28 3:47X 1 3:54X 2 2:71X 3 0:65X 1 X 2 þ 0:72X 1 X 3 0:1X 2 X 3 0:53X 21 þ0:52X 22 þ 1:03X 23 ð4Þ For EC : Y EC ¼ 4:25 þ 0:88X 1 þ 0:19X 2 þ0:067X 3 þ0:052X 1 X 2 0:049X 1 X 3 þ 0:067X 2 X 3 þ0:068X 21 0:034X 22 þ 0:021X 23
ð5Þ
Above all, the statistical significance of the model coefficients should be determined by ANOVA combined with the application of F-test to ensure a good model. The checks for the above equations by F-test showed that both the regressed models had high values of F (F¼101.83 for the CE model and F¼ 140.41 for the EC model) and very low probability values (both are smaller than 0.0001). These indicated that both models were highly significant. Furthermore, the goodness of fit of the model was also evaluated by inspecting the determination coefficient (R2 ), the adjusted
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M. Wang et al. / Journal of Membrane Science 452 (2014) 54–61
Fig. 2. Plots of predicted vs. actual values.
determination coefficient (R2adj ), and the coefficient of variation (C:V:%), which indicate the fraction of the variation of the response explained by the model, the fraction of variation of the response explained by the model adjusted for degrees of freedom, and the extent of variability in relation to mean of the population, respectively. The obtained values of R2 (R2 ¼0.9892 for CE and R2 ¼0.9921 for EC) showed that more than 98% of the data deviation can be explained by the two empirical models. However, a large value of R2 does not necessarily imply a good regression model as R2 always increases when adding any variable, significant or not significant, to the model [14]. Hence, R2adj was used to correct the R2 for the sample size and for the number of terms. In this case, the large values of R2adj (R2adj ¼0.9795 for CE and R2adj ¼0.9851 for EC) disclosed a high significance of the empirical models. Moreover, these also showed that all the significant terms were included in the empirical models. In addition, the relatively lower values of C:V:% (C:V:% ¼1.02 for CE and C:V:% ¼2.21 for EC) also indicated a better precision and reliability of the experiments. As can be shown visually by Fig. 2, the predicted data of the responses from the empirical models agreed well with the observed ones in the range of the operating variables. In short, all statistical estimators showed that the developed models were validated from the statistical point of view and can disclose the contributions of the factors to the work performance of the BMEDbased reclamation process.
3.2.2. The effects of the variables The significance of the coefficients in both models was further checked by the F test and is demonstrated in Tables 4 and 5, respectively. As is known, the smaller the p value, the greater the significance of the corresponding coefficient. For the response of CE, the corresponding p values indicated that the linear variables,
including X 1 , X 2 , and X 3 , and the quadratic term of X 3 had highly significant effects on the current efficiency. Next in importance to the above terms, the quadratic terms of X 1 and X 2 and the interactive terms of X 1 and X 2 , X 1 , and X 3 also significantly affected the current efficiency. As for the interactions between X 2 and X 3 , a very minor influence on current efficiency was observed since the p value was larger than 0.05. For the response of EC, the corresponding p values suggested that all the linear variables had significant effects on energy consumption. In contrast, the influence of X 3 was much weaker. However, with the exception of the quadratic term of X 1 , all the interactive terms and quadratic terms only possessed very weak effects on energy consumption since their corresponding p values were larger than 0.05. The parameter estimates can also suggest the effects of the factors on the responses. A positive coefficient indicates a synergistic effect, while a negative coefficient indicates an antagonistic effect. As can be seen clearly from Tables 4 and 5, with the exception of the coefficients of the X 3 quadratic term, the coefficients of the factors of one model had opposite signs in the other. For example, the increase of the exerted voltage seemed to significantly deteriorate work performance, such as the increase of energy consumption and the decrease of current efficiency, which were not expected. However, some actual conditions, for example, the expectation for high desalinating capacity and efficiency of the saline water and some inherent disadvantages of bipolar membranes, such as relatively high price and limited life span, usually require the BMED to work under relatively high current density. Accordingly, the relatively high initial concentration of saline water was also expected, though it is also a significant factor for decreasing current efficiency and increasing energy consumption. However, the effect of the initial concentration of acid/base seemed to be very interesting in this work. Generally speaking, the addition of a small amount of acid/base into acid/base compartments at the beginning of BMED is favored to increase solution conductivity and accordingly decrease energy consumption. In this case, just as the effect of initial concentration of saline water on current efficiency, the increase of the initial concentration of acid/base also significantly decreased current efficiency, which is shown in Figs. 3 and 4, respectively. On the other hand, compared with the effects of the exerted voltage and the initial concentration of saline water on energy consumption, influence of the initial concentration of acid/base was much weaker. This should be ascribed to the adoption of constant-voltage mode in this BMED process. That is, when the initial concentration in acid/ base compartments was low, the current density was also low and accordingly energy consumption did not significantly increase. A similar result was also observed in Wu's work, in which the initial concentration of acid/base compartments in the optimized scheme for salts recovery from 1,3-propanediol fermentation broth by BMED was recommended as 0 M [15]. Obviously, this should be favored to increase the production capacity of acid and base and to simplify the technological operation. 3.3. The continuous BMED experiment on the real solution pretreated by ED installed with monovalent selective CEMs and IE In order to further confirm the feasibility of the reclamation process, a continuous BMED experiment on a real solution was conducted. Above all, the real solution was pretreated by electrodialysis installed with monovalent selective CEMs and IE in turn. Experimental results showed that the contents of Ca and Mg in the saline water can be reduced easily by less than 1 ppm. This indicated that the requirements of BMED for hardness content were met [16]. Simultaneously, the conductivity of the saline water was increased from about 20 mS/cm to about 40 mS/cm. It is reasonable to think that the salt concentration was also
M. Wang et al. / Journal of Membrane Science 452 (2014) 54–61
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Table 4 The significance of the coefficients in the regression model for CE. Sources
A0
A1
A2
A3
A1 A2
A1 A3
A2 A3
A11
A22
A33
Coefficient estimate F-value p-Value
69.28
3.47 322.22 o 0.0001
3.54 335.00 o0.0001
2.71 196.39 o 0.0001
0.65 6.57 0.0282
0.72 8.18 0.0169
0.1 0.16 0.7005
0.068 7.84 0.0188
0.52 7.67 0.0198
1.03 30.18 0.0003
Table 5 The significance of the coefficients in the regression model for EC. Sources
A0
A1
A2
A3
A1 A2
A1 A3
A2 A3
A11
A22
A33
Coefficient Estimate F-value p-Value
4.25
0.88 1180.35 o 0.0001
0.19 57.42 o 0.0001
0.067 6.79 0.0262
0.052 2.43 0.1503
0.049 2.17 0.1713
0.067 4.04 0.0723
0.068 7.35 0.0219
0.034 1.82 0.2071
0.021 0.68 0.4275
Fig. 3. Three-dimensional surface plot illustrating effects of the interactions between voltage and initial concentration of saline water on current efficiency.
Fig. 4. Three-dimensional surface plot illustrating effects of the interactions between voltage and initial concentration of acid/base on current efficiency.
doubled because the pH remained almost constant during the process. Along with the processing of BMED, the salt content of the concentrated saline water decreased gradually. Finally, the saline water that was desalinated partly will return directly to the ED step for the next concentration process. In this case, when the concentration of saline water was reduced to about 0.05 M, it was replaced by other “new” concentrated saline water for a “new” BMED cycle. Obviously, a preliminary hardness removal process can be carried out simply and in an environmental-friendly manner by ED, which was installed with monovalent selective CEMs with the synchronous achievement of concentrated saline
water. In addition, the reuse of saline water discharged from the BMED process can also effectively avoid some additional complex pretreatment operations for meeting the requirements of BMED for water quality, including silica removal, organic matter removal, and so on. In addition, it can be noticed that the concentration of the acid and base increased along with the BMED process. Sometimes, when the acid and base concentrations met the requirements for regeneration of ion-exchange resins (e.g. 3–4 wt%), the acid and base products were stored in tanks and equivalent water was supplemented in the acid and base compartments.
60
M. Wang et al. / Journal of Membrane Science 452 (2014) 54–61
50
1.6
45
1.4
35 1.0
30
0.8
25 20
0.6
15
Conductivity, ms/cm
Acid concentration, M
40 1.2
case, an acceptable current efficiency and energy consumption can be obtained on the basis of some conventional ion-exchange membranes and spacers when the product concentration was set at 0.9 M. It can be noticed that the 0.9 M acid and base are enough for the regeneration of ion-exchange resins. Obviously, the coupling of BMED-based reclamation process with the conventional desalination process by IE will make the surface water desalination practically zero-discharged.
0.4 10 0.2
4. Conclusions
5
0.0
0 0
50
100
150
200
250
300
350
time, min
Fig. 5. The evolution of the concentration in acid compartments and the conductivity in saline water compartments during the BMED-based reclamation process. 65
8.5
Current efficiency, %
7.5 55 7.0 50
6.5
45
6.0
40
5.5
Energy consumption, Kwh/Kg
8.0
60
5.0 35 4.5 0.5
0.7
0.9
1.1
1.3
Acid concentration, M
Fig. 6. The current efficiency and energy consumption of a BMED process corresponding to the different product concentrations.
Fig. 5 demonstrates the gradual desalination process in every BMED cycle and the conversion between the two paths of saline water when one reached the endpoint of desalination. Along with the continuous BMED process, the product concentration increased gradually. It can be noticed that the relation between the product concentration and operating time is nonlinear, which is significantly different from the typical relation displayed by BMED running in a constant-current mode. That is, during the BMED process running in the constant-voltage mode, the current density decreased with the decrease of salt contents in saline water. Obviously, both higher treating ability at relatively high current density and lower salt content of saline water for further treatment can be simultaneously achieved when the BMED in a constant-voltage mode was adopted. On the other hand, the concentration that the product can reach should also be paid more attention. Generally speaking, a higher concentration of recovered acid and base will be favored for practical applications. Fig. 6 shows the corresponding current efficiency and energy consumption during a BMED process under different product concentrations. It can be seen clearly that the work performance deteriorated remarkably along with the increase of product concentration. Obviously, these should be attributed to the leakage of H þ and OH through ion-exchange membranes and current leakage through acid and base solution when their concentration increased remarkably. Undoubtedly, some improvements in BMED equipment, for example, the adoption of AEMs – which can block proton leakage – and spacers – which can prevent current leakage – were needed urgently. In this
With respect to the dilemma that some unsolved applicationoriented problems impeded the BMED technology to be put into practice for the reclamation of industrial brine, a novel hybrid BMED-based scheme was put forward in this study. That is, the industrial brine produced during surface water desalination process by IE was fed into ED installed with monovalent selective CEMs for the synchronous achievement of concentrated saline water and preliminary hardness removal. Besides the improvements of the subsequent BMED performance due to the concentration of the saline water, the recovery ratio of water for the whole surface desalination process can be increased because of the reuse of the low-salinity water from the ED process. After being further softened by IE, the concentrated saline water was desalinated to produce acid and base by a BMED in a constant-voltage mode. Therein, the saline water having been partly desalinated will be directly returned to ED for the next concentration, with the synchronous achievement of desalination of industrial brine. This can effectively simplify some relevant pretreatment operations for BMED, especially, the removal of divalent cations, silica, and organic matter. Moreover, the RSM analyses showed that some initial factors, including the exerted voltage, the concentration of the saline water pretreated by ED, the initial concentration of acid/base, and their interactions significantly influenced the current efficiency and energy consumption of the BMED process. When compared with that of the BMED process with a constantcurrent mode, it was worth noting that the optimized initial concentration of acid and base was recommended as 0 M in the BMED with a constant-voltage mode. Finally, a continuous BMED experiment on a real solution confirmed the feasibility of the reclamation scheme. The experimental results indicated that the required product concentration remarkably affected the work performance of BMED. In this case, an acceptable current efficiency and energy consumption can be obtained when the product concentration was set as 0.9 M. Obviously, the product concentration of 0.9 M is adequate for the regeneration of ion-exchange resins. Of course, if the BMED equipment can be improved, for example, the use of protonblock AEMs and specified spacers for preventing current leakage, better work performance can be expected.
Acknowledgments This research was supported in part by the National Science Foundation of China (No. 21276245) and Natural Science Foundation of Shandong Province (ZR2011EMQ004). This is MCTL Contribution no. 29. References [1] Q. Schiermeier, Water: purification with a pinch of salt, Nature 452 (2008) 260–261.
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