| Seawater desalination, an effective means to obtain fresh water, eases the worriesabout the water resource crisis. The technology includes multistage flash (MSF),multi-effect distillation (MED), reverse osmosis (RO) and electrodialysis (ED), andeach technique plays a special function for different water quality and different areas.Regardless of the technique, an acid pretreatment is necessary to avoid scalingproblems in the desalination system, and this is especially the case for RO. However,the transportation and storage of a large amount of sulfuric acid or hydrochloric acidlead to some hidden risks, especially for desalination plants located far from sulfuricor hydrochloric acid plants. The same question is found in the posttreatment ofdesalted seawater. Before pumped to water supply pipeline, the desalted seawaterneed to be remineralized with CaCO3disslution method because of the low TDS. Butthis method still need a large amount of acid. Besides, Seawater concentrate generatedfrom the desalination process is a useful resource which contains a large amount ofdissolved ions, which is approximately two times higher than that of natural seawater.If discharged directly, that will not only increases the salinity of offshore areas, butalso squanders resources.The emergence of bipolar membrane electrodialysis offers a good solution forthe three questions above. When a direct electrical potential is established, the watermolecule at the interphase of bipolar membrane will be split into H+and OH-. H+willcombine with Cl-and SO42-in seawater concentrate to generate acid in the acidcompartment, while OH-combine with Na+in seawater concentrate to produce thecorresponding base in the base compartment. The acid solution produced can be usedfor the pretreatment and posttreatment in seawater desalination, and the base can beused for the extraction of magnesium resources. The method can solve the source of the acid and utilize the resources better, simultaneously.Based on the acid and base production by bipolar membrane electrodialasis,Whether this technique can be used for the three questions above will be investigatedin this article. The research contents and results are as follows:1. Before the acid and base production using seawater concentrate by bipolarmembrane electrodialysis, the experiment studied the acid and base production of themain compositions (NaCl and Na2SO4) in seawater concentrate firstly.There-compartment bipolar membrane electrodialysis was used to produce sulfuricacid and hydrochloric acid in situ from sodium sulfate and sodium chloride. Theresults show that both salts have a high acid yield, and can be used for acid productionin seawater desalination pretreatment.2. After the feasibility of acid and base production using the main compositionsin the seawater concentrate was verified, the article next use the seawater concentrateto conduct an experiment for aicd and base production. Prior to applying BMED tothe seawater concentrate, aseries of initial tests was conducted to select the idealoperatingconditions for the process. The results show that1mol/L mixed acid andsodium hydroxide could be produced continuously at a constant current density of57mA/cm2and aflow rate of0.30L/h in the continuous-mode BMED. After theproduction of the acid and base solution, membrane fouling was inspected bymeasuring the stack resistance and using electron microscopy and surface elementalanalysis. No visible fouling deposits on the surface of the CEMs (AEMs) wereobserved, thus proving that the BMED process for mixed acid production withpretreated RO concentrate is suitable for a long-term run.3. The water produced by reverse osmosis seawater desalination, is usually,characterized by very low salinity and alkalinity, and the corresponding posttreatmentis needed. This research aims at proposing a remineralisation method based on thelimestone dissolution according to water quality criteria published in Isreal.Simultaneously, the water produced by the salt compartment of bipolar membraneelectrodialysis can be recycled effectively by the remineralisation process. Theresearch investigated different remineralisation processes when particle size of calcium carbonate varied from2mm to4mm and water flow rate varied from50L/hto100L/h. The results showed that the smaller particle size lead to higher dissolutionrate of calcium carbonate, and higher flow rate lead to less dissolution of calciumcarbonate. After the remineralized water was recombined with desalted waterbypassing the dissolution reactor,100mg/L sodium bicarbonate was added to meetthe final water alkalinity. |
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