Prevention of colloidal-fouling in electrodialysis by chlorination

The effect of chlorination on the colloidal-fouling of electrodialysis membranes by solutions of sodium alginate and sodium humate was determined. Chlorination was found to be more efficient for the prevention of fouling for the alginate. At higher levels of chlorination, fouling by both substances was substantially reduced. Chlorination of water in order to prevent fouling in electrodialysis stacks is not mentioned in the literature. It has been shown in our laboratories l that humate and alginate are colloids that cause serious fouling of anionic membranes. The effect of various chlorination techniques on solutions containing these two materials was studied in accelerated fouling tests. The multicompartment apparatus used for the experiments was formed by the series connection of perspex plates. Each cell-member was a circular hole in a 1-cm-thick perspex plate with a diameter of 2 cm, and contained tangential inflow and outflow tubes of 3 mm diameter. Two measuring electrodes were inserted in each member so that they were near the membrane surfaces adjoining the members. The membranes were clamped tightly between two cell members. The set of cells and membranes was closed by two open electrode cells which were separated from the first and last cell by cation membranes. The fouling experiments were performed with solutions containing 0.05 N NaCl and 1000 ppm sodium humate or sodium alginate. The current density was 10 mA/cm² and the linear flow velocity 2 cm/sec. Eight A.M.F. anionic membranes were inserted in each experiment. Voltage-time curves were utilized to detect fouling. Chlorine gas and sodium hypochlorite were used alternately and the pH of the solution was always adjusted to neutral. This means, of course, that eventually the ratio of Cl₂:NaOCl was always the same. The degree of chlorination was varied between 0.27 and 1.2 g Cl₂ per gram humate and 0.024 to 0.6 gram Cl₂ per gram alginate. With humate the time of chlorination was about 20 minutes when the Cl₂:humate ratio was 0.27. When more chlorine was used to enhance chlorination, it became necessary to wait a day before it was completed. The time of chlorination for alginate was very short. At the higher levels of chlorination the waiting time was approximately 30 minutes; less than 10 minutes were sufficient at lower levels of chlorinalion. The fouling curves after chlorinations for humate and alginate are shown in Figs. 1 and 2 respectively. Low level chlorination of humate solutions had almost no effect on membrane fouling (Fig. 1). However, further increases in the chlorination level resulted in a significant reduction of fouling. The chlorination of sodium alginate is much more effective and prevents fouling even at such low levels of chlorination as 0.024 gram Cl₂ per gram alginate (Fig. 2). It is evident (Figs. 1 and 2) that the degree of protection from fouling {A figure is presented} 1)0.1% humate 2)0.1% humate + 0.27g Cl₂/g humate 3)0.1% humate + 0.6g Cl₂(as NaOCl)/g humate 4)0.1% humate + O.81g Cl₂/g humate 5)0.1% humate + 1.2g Cl₂(as NaOCl)/g humate {A figure is presented} 1)0.1% alginate 2)0.1% alginate + 0.024g Cl₂ (as NaOCl)/g alginate 3)0.1% alginate + 0.18g Cl₂(as NaOCl)/g alginate 4)0.1% alginate + 0.6g Cl₂ (as NaOCl)/g alginate increases with increasing levels of chlorination. It is recognized that residual free chlorine can result in membrane damage. It thus becomes necessary to determine the optimal conditions, namely, maximum fouling prevention consistent with no risk of membrane damage.