C. Linder, S. Belfer, J. Gilron and O. Kedem
The Institutes for Applied Research, Ben-Gurion University of the Negev Beer-Sheva, Israel

The use of membrane processes in difficult applications, such as the treatment of contaminated water streams for general reuse, would be made economically viable if high recoveries could be achieved. High recoveries and waste minimization are also very important for many applications in arid and semi arid regions such as inland desalination, nitrate removal from drinking water, and waste stream desalination for recycling. Electodialysis removes ionic contaminants from the feed stream, and thus has intrinsically higher recoveries than pressure driven processes, which removes the water. This intrinsic advantage can only be realized in contaminated streams, if ED with low fouling and low polarization can be achieved beyond commercially available products. In this regard we shall describe newly developed ED membranes and ionic spacers having reduced fouling and which minimize polarization. High recovery for RO processes is also limited by fouling. A chemical treatment of commercial RO membranes, which significantly reduces fouling and allows for easy cleaning, will also be discussed.

The use of anion exchange spacers was shown to have a significantly greater improvement on ED performance when compared to conventional non-ionic polyolefin or cation exchange spacers. In effect the use of high capacity ionic spacers can reduce both pumping and electrical energy consumption by reducing polarization, allowing lower flow rates over the membrane surface, lowering resistance, and increasing efficiency and output. Stable reproducible heterogeneous anion exchange spacers with capacities from 0.03 to 1.1 meq/g were made. The greater output with the same or reduced power consumption was found to increase with increasing ion exchange capacity. For example the output for marginal desalination (going from 20 to 5 mM/l NaCl using commercial homogeneous membranes, 1.0V) increased over that of the non-modified spacer by 6% and 9% for spacers with 0.8-to 1.1-meq/g capacity respectively. This improvement was achieved without an increase in power consumption. Though not yet tested, it is expected that the currently studied, new high capacity spacers, will permit the use of lower surface flow rates without polarization effects, further reducing energy consumption.

The membranes are a unique development of this project. They allow the use of, heat sealable, inexpensive heterogeneous ion exchange membranes; comprising a polyolefin matrix (polyethylene) matrix with dispersed ion exchange particles. Commercial heterogeneous membranes are inadequate for many applications because of high polarization and fouling, but with the use of a novel coating, performances have been achieved with low polarization and an output equal to the best commercially available homogeneous membranes, plus the addition of high fouling resistance. The surface modification consists of a reactive anion exchange polymer, a neutral hydrophilic polymer, polyamines and cross-linkers. The result is a stable composite membrane which for the initial times they have been tested are equivalent to the best commercial homogeneous ED membranes with low and polarization at a fraction of the cost.

The fouling of RO membranes cause flux decline and in some cases loss of rejection. In any case fouling has severe economic consequences for RO desalination processes. The decrease of water flux in all pressure driven processes has been attributed to accumulation of contaminants and/or dissolved molecules to the membrane surface. We have been developing the surface modification of commercial RO membranes to increase surface hydrophilicity, and thus reduce fouling and facilitate the cleaning of foulants when it does occur. The synthetic route of choice is radical grafting of hydrophilic polymers on the polyamide backbone of commercial membranes. From a practical point of view the mild conditions of chemical reactions are especially important: the surface reaction takes place in aqueous solution at room temperature. Surface grafting was characterized by ATR-FTIR and ESCA. It was found that membranes were modified without damage to their transport properties and with improved fouling resistance and ease of cleaning.