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Membranes 2020, 10, 221 24 of 72 3.5.3. Is There One Optimal PEF Condition? The choice of PEF parameters (pulse/pause durations, ratio and frequency) is, however, delicate and significant differences between processes are visible depending on the parameters chosen (Table 1). Indeed, the effects of variations in the duration of each of the two periods, their ratio as well as the frequency of PEF has been highlighted during electrodialytic treatment for fouling mitigation, increasing energy efficiency and improving mass transfer. The following analyses are based on data available in the literature and whose authors have tested at least three different PEF conditions to find potential trends. For Fouling Mitigation In the case of protein solutions [103,104], peptides [105], lignin [106], and organic fouling [84], it was found that the use of long pauses during the application of PEF could remove macromolecule fouling at the surface of IEM (Table 1). This positive effect of a longer pause duration is explained by the better detachment of macromolecules from the membrane surface and/or electrostatic interaction decrease by the solution flux due to the washing effect. Regarding solutions with divalent potential scaling ions [110], the application of pulse/pause regimes with ratio 1, 10 s/10 s was proved beneficial for decreasing scaling formation. However, no entire scaling prevention was obtained due to the strong interactions of scaling ions with membrane ion-exchange groups. Subsequent studies conducted by Mikhaylin et al. revealed that short pulse duration in PEF mode (between 1 and 3 s) allow better mitigation of concentration polarization and scaling phenomena [112]. Indeed, the application of the optimal pulse/pause mode of 2 s/0.5 s allows a 40% decrease of the CEM scaling and complete inhibition of AEM scaling (just traces of AEM scaling were detected) in comparison with a continuous current mode [112]. The working principle behind the effectiveness of PEF, for scaling mitigation, is related to reducing concentration polarization, in this case, to avoid the accumulation and close packing of foulants at the surface of the membrane combined with the cleaning effect of the solution recirculation. Very recently, on more complex solutions, such as polymer-flooding produced water and acid whey, the impact of the pulse/pause durations was also demonstrated (Table 1) and the best conditions were between 0.01 and 0.25 Hz. Hence, for polymer-flooding produced water [84], the authors pointed out that the 10 s/30 s and 1 s/3 s regimes, same ratio of 3, were the best for mitigating the negative effects of concentration polarization and organic fouling. For acid whey, it was demonstrated that decreasing the pulse/pause durations from 35 s/35 s to 5 s/5 s, same ratio of 1, decreased drastically the scaling (77% less scaling) (Figure 13). Furthermore, decreasing the pulse duration from 35 s to 15 s for a same pause duration (35 s) led to a scaling decrease of 85% while decreasing the pause duration from 35 s to 15 s for a same pulse duration of 25 s, increased the scaling by 6 folds [83] (Figure 13). It appears that there is no real consensus according to PEF parameters to apply in order to eliminateor decrease the fouling or scaling of IEM, as well as the type of parameter to be taken into account (pulse and pause durations, pulse/pause ratio or frequency). However, from all the studies reported in the literature, short pulse and pause duration as well as ratio around 1 would be more effective on scaling while short pulses, with long pause would be more effective for eliminating the different types of fouling (protein, lignin, peptide, etc . . . ). Concerning optimum frequency, the use of pulsations with a frequency of the order of 0.1 Hz and an extended relaxation time would allow the mitigation of fouling. However, the optimum frequency for fouling mitigation is depending on the membrane structural difference [102].PDF Image | Electrodialytic Processes
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