Sea water desalination using electrodialysis

Most widely applied and commercially proven desalination technologies fall into two categories of thermal (evaporative) and membrane based methods. Membrane methods are less energy intensive than thermal methods and since energy consumption directly affects the cost-effectiveness and feasibility of using desalination technologies membrane methods such as reverse osmosis (RO) and electrodialysis (ED), are attracted great attention lately. In this paper water desalination using a laboratory ED setup was described and evaluated. Taguchi method was initially used to plan a minimum number of experiments. A L9 orthogonal array (four factors in three levels) was employed to evaluate effects of temperature (at 25, 40, and 55°C), voltage (at 5, 7, and 9 V), flow rate (at 0.07, 0.13, and 0.25 mL/s) and feed concentration (at 10,000, 20,000, and 40,000 ppm) on separation percentage of salt ions. Maximum percentage of desalination was obtained at the lowest feed concentration and flow rate levels (10,000 ppm and 0.07 mL/s, respectively) and the highest voltage and temperature levels (9 V and 55°C, respectively). Analysis of variance (ANOVA) was applied to calculate sum of square, variance, ratio of factor variance to error variance and contribution percentage of each factor on response. The results showed that all factors have significant effect on the response. It was found that, feed concentration is the most influential factor on ED performance (its contribution percentage was calculated to be 82.4%). It was finally found that, contrary to the case of waste water treatment (concentrations of lower than 1000 ppm) which flow rate is the influential factor, in desalination of sea water (concentrations of upper than 10,000 ppm) feed concentration is the key parameter.

In this study, the effect of operating conditions (temperature, concentration, flow rate and voltage) on performance of an electrodialysis cell was studied. Taguchi data analysis method was utilized to study four parameters in three levels (L9 OA). Effect of concentration (10,000, 20,000, 40,000 ppm), temperature (25, 40, 55°C), flow rate (0.07, 0.13, 0.22 mL/s) and voltage (5, 7, 9 V) on separation percent of individual ions in the solution was scrutinized. As a result, higher temperature and voltage and fewer flow rate and concentration were recommended to modify performance of ED cell. ANOVA analysis was applied to evaluate the relative importance of the effects of various factors. It was realized that all factors have significant effect on the response and feed concentration has the largest contribution to the total sum of squares and correspondingly has a major effect on separation percent. It has also found out that, simultaneous increase in voltage and temperature as well as decrease in feed concentration and flow rate optimize the separation percent. Electrodialysis was found to be very effective for seawater desalination specifically at lower concentrations.