Integration of nano fi ltration and bipolar electrodialysis for valorization of seawater desalination brines: Production of drinking and waste water treatment chemicals

Nanofiltration (NF), as a selective Mg(II) and Ca(II) separation and concentration treatment, and electrodialysis with bipolar membranes (EDBM) were evaluated for the valorization of seawater desalination reverse osmosis brines (60 NaCl/L) to produce both rich Mg(II) and Ca(II) brines for phosphate recovery and HCl and NaOH as chemicals for desalination treatments. A NF pilot plant, using NF270 membranes at 20 bar, provided a rich Mg(II) (8.3 g Mg(II)/L) and Ca(II) (2.1 g Ca(II)/ L) brine on the concentrated stream with enrichment factors of 3.2 for Mg(II) and 2.5 for Ca(II). The NF permeate stream containing 50 ± 2 g NaCl/L was treated to remove residual Mg(II) (760 mg/L) and Ca(II) (415 mg/L) by chemical precipitation with Na2CO3 and NaOH before the EDBM unit. Divalent cations free brine containing NaCl (50 g NaCl/L) were fed into the EDBM stack in order to produce NaOH and HCl under recirculation configuration. Constant voltage and acid and base concentrations at different initial conditions were evaluated to obtain the maximum acid and base concentration (approximately 1 M NaOH/HCl) at 9 V. No substantial effect of initial acid and base concentrations on the overall performance was observed. An energy consumption of 2.6 kWh/kg NaOH and current efficiency of 77 ± 3% were calculated. © 2015 Elsevier B.V. All rights reserved.

Aromatic polyamide NF membrane (NF270) provided a rich Mg(II) (8.3 g Mg(II)/L) and Ca(II) (2.1 g Ca(II)/L) concentrate stream from SWD-RO brine working at 20 bar with enrichment factors of 3.2 for Mg(II) and 2.5 for Ca(II). Rejection of polyvalent ions (Ca(II), Mg(II), S(VI), Sr(II), Cu(II), Ni(II), and Al(III)) was higher than univalent ions (Na(I), Cl(I), and K(I)), mainly dependent on the hydrated ionic radii and speciation. Rejection of undesired ions increased with pressure, undesired NaCl rejection also increased but maximum value of 12% was reached. Results are comparable with the results reported in literature for the use of nanofiltration for seawater pre-treatment. NaCl concentration slightly decreased after the NF treatment. Results demonstrated that higher quality of brine was obtained when NF was used as a pre-treatment: with purities of 98% of NaCl which would benefit its subsequent reuse. Nevertheless, further a purification step for removal of Ca(II) and Mg(II) was needed to meet the EDBM requirements. Removal values of more than 95% of calcium and magnesium were achieved using precipitation treatment by Na2CO3 (pH 10.6) and NaOH (pH 11.6) before EDBM process. Divalent cation free brines containing NaCl (50 g NaCl/L) were fed into the EDBM stack in order to produce NaOH and HCl under Table 4 Experimental results of concentration and energy consumption. U (V) Ĩ (kA/m2) Electrode (Na2SO4) Salt (NaCl) Acid (HCl) Time (h) Base (NaOH) Cinitial (g/L) 8.98 8.91 8.91 0.41 0.44 0.47 Cfinal (g/L) Cinitial (g/L) Cfinal (g/L) Cinitial (M) Cfinal (M) Cinitial (M) 43.59 43.85 45.03 53.60 52.98 52.88 0.01 0.01 0.02 0.06 0.12 0.53 0.90 0.88 1.16 0.05 0.10 0.44 0.66 0.71 1.06 Ec (kWh/kg NaOH produced) Ie (%) 2.44 2.54 2.68 0.031 0.028 0.023 2.345 2.576 2.888 85.6 77.3 68.9 Cfinal (M) 45.50 44.66 46.86 kg HCl produced recirculation configuration. According to the results, EDBM is a suitable method for desalinating brines producing acid and base, in this study HCl and NaOH were produced from NaCl. Conversion percentage of HCl and NaOH from NaCl were higher than 70%. It was found that an increase of the applied voltage represented higher NaOH and HCl concentrations. Furthermore, initial acid and base concentrations represented no substantial effect on the overall performance. For energy consumption and current efficiency, values around 2.6 kWh/kg NaOH and 77%, respectively, were obtained. Acknowledgments This research received support from the Center for the Development of Industrial Technology in Spain (www.sostaqua.com), the ZeroDischarge project (CTQ-2011-26799) and the Waste2Product (CTM2014-57302-R) financed by “Ministerio de Economía y Competitividad” and the Catalan Government (Project Ref. SGR201450-SETRI), Spain. The work of Mònica Reig was supported by the Spanish Ministry (MINECO) within the scope of the grant BES-2012-051914. We want to thank Dr. S. Asensio (Solvay Iberia) for his contribution in the discussion of the results and also to M. Martinez for her contribution to the project.