Integration of electrically driven membrane separation processes for water treatment and resources recovery
Nowadays, due to the growing fresh water demand, several processes are used to purify seawater by means of desalination or industrial brackish water by different treatment processes. The main limitation of these techniques is the production of rejected brines. For this reason, new management techniques for brines valorization are being studied to achieve the maximum water recovery, avoid liquid streams disposal and recover the valuable compounds from the concentrated streams. In this thesis, four membrane technologies were used to promote resources recovery, including water, depending on the valorization way of the concentrated stream: electrodialysis (ED) was used for its concentration, nanofiltration (NF) for its purification, selectrodialysis (SED) for its ions separation and ED with bipolar membranes (EDBM) for acid and base production from the brines. The integration of these membrane techniques provided brines reuse and promoted potential circular economy based on solutions where a waste is transformed into a resource. Seawater reverse osmosis (SWRO) brine was treated by ED in order to concentrate NaCl for the chlor alkali industry. An ED pilot plant was used to concentrate the brine up to 150 250 g NaCl/L, depending on temperature and current density conditions. Then, a mathematical algorithm was developed to predict the concentration evolution during the ED process. The model was able to describe the NaCl concentration evolution and the energy consumption taking into account temperature changes and longtime operation. Moreover, monovalent selective cationic (MVC) membranes were synthetized using several mixtures of polyvinylidene fluoride (PVDF) and sulfonated PVDF (S PVDF) Then, surface polymerization of polyaniline (PANi) doped with p toluene sulfonic acid (pTSA) or L valine was applied in order to improve their cationic monovalent selectivity. Results indicated that sodium selectivity increased when using doping agents (higher sodium selectivity when using valine than pTSA) or increasing the voltage applied. Besides, NF was used as a purification treatment for the SWRO brine. Different membrane configurations (flat sheet (FS) and spiral wound (SW) were tested to study ions rejection behavior. The solution diffusion electromigration film model (SDEFM) was successfully applied in order to fit the experimental rejections and calculate the membrane permeances to each ion. Ions rejection and permeances calculated for both membrane configurations were similar. These results indicated that lab scale results could be used for the NF scale up. Also, the dominant salt effect on the trace ion rejection was determined by means of a FS membrane indicating that a higher initial dominant salt concentration implied a lower rejection for the dominant salt itself and also for the trace ions. Furthermore, two ED based technologies were used. SED was utilized to separate chloride from sulfate ions of an industrial wastewater rich in sodium chloride and sodium sulfate, achieving separation factors around 80 90 EDBM was employed to produce sodium hydroxide/hydrochloric acid from sodium chloride and sodium hydroxide/sulfuric acid from sodium sulfate.Finally, ED, NF and SED were used as pre treatments for EDBM. With the NF and EDBM system it was possible to purify the SWRO brine working with NF membranes at 20 bar. However, the permeate stream was treated by chemical precipitation in order to diminish the calcium and magnesium concentration before being introduced in the EDBM system. Maximum NaOH and HCl concentrations of 1 M were obtained. ED was used prior to the EDBM in order to concentrate the SWRO brine up to 200 gNaCl/L and be able to produce 2 M acid and base. SED was used to separate chloride from sulfate ions of an industrial wastewater. Both streams, sodium chloride rich and sodium sulfate rich were introduced in the EDBM stack and pure sodium hydroxide, hydrochloric acid (87 and sulfuric acid (93 were produced.