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Colloidal particles can be abundant in sea or brackish waters and they are often negatively charged. Their deposition on the membrane surface is driven by the electric field pushing the colloidal particles towards the positive electrode. Such migration is stopped by the presence of the membranes, acting as a mechanical barrier and being covered by a growing deposited layer of colloids on their surface. Similarly to scaling, colloidal-fouling prevention strategies include the reduction of recovery and pH adjustment. Besides, micro and ultrafiltration can be used as pre-treatments, while a higher fluid velocity inside the stack can help particles displacement from the membrane surface [153]. The most effective action for colloidal and organic foulants, however, is the use of a polarity reversal strategy, which will be presented in the following lines. Fouling due to organic matter can be very severe when ED treats food industry streams [155, 156] and in water reuse applications [151, 157]. In these cases, the presence of organic compounds can dramatically affect fouling phenomena leading to a huge decline in process performances. For this reason, several research works have adopted model foulants such as bovine serum albumin, humate and sodium dodecylbenzene-sulfonate in order to investigate the phenomenon in depth [152, 158]. The molecular size of organic particles can significantly affect their fouling behaviour. In fact, particles with a molecular weight of 200 - 700 Da can cause internal membrane fouling, being able to penetrate membrane pores. On the other side, larger molecules cannot enter inside pores, thus being blocked on the external surface, while, conversely, much smaller molecules pass freely through membrane pores, thus not generating any internal blocking and fouling in the IEM [157]. In order to reduce organic fouling pre-treatments such as microfiltration, ultrafiltration or activated carbon have been proposed and cleaning actions with NaOH solutions are also possible [153]. Regardless of the different classes of materials, most foulants present in feed waters exhibit electrostatic features which enhance the fouling risk for AEMs [151, 159]. In this respect, many efforts have been made through years in order to increase AEMs antifouling properties by surface modification processes [160–162]. Grebenyuk. et al. [160] modified AEMs by adding high molecular mass surfactants obtaining an increased resistance against organic deposition. Alternatively, modification with poly(sodium 4-styrene sulfonate) [161] or polydopamine [162] can also reduce fouling, while negligibly affecting other IEMs properties. Despite the different strategies proposed for reducing fouling in IEMs, standard ED operation always require in-place-cleaning procedures, resulting in a cost increase for the process [153]. For this reason, the development of the electrodialysis reversal (EDR) concept represented one of the main breakthroughs for the ED technology, succeeding in dramatically reducing the fouling tendency of IEMs in ED stacks for very long lifetimes. The EDR concept is based on the idea of reversing the 21PDF Image | Electrodialysis for water desalination
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