Staging of ED Stacks

With feed and bleed or batch recycle systems some or all of the water that has already been processed by the stack is mixed with feed and returned via a recirculation pump. Recycle is inherently less efficient than once-through flow in both stack utilization and in energy consumptions since the same solution must be pumped and desalted repeatedly and then remixed with a more concentrated solution. However, the increased flexibility in process control makes recycle systems attractive for small-scale operations where stacks are over-sized to handle varying loads. Feed and bleed is useful for operations where the enriching stream needs to be as concentrated as possible. Ten-fold enrichment or 90% recovery of some feed waters as diluate can be achieved by concentrate recirculation if solubility limits of the dissolved substances are not exceeded in the concentrate stream. 

Outside of Japan, the largest application of ED has been in the desalination 

ED desalting of seawater has also been demonstrated on a commercial scale. The power consumption has been held to tolerable levels (28 watt-hours/gal) by the use of ion-exchange membranes with low resistance.5 Still the greatest use of ED treatment of seawater has been for the recovery of NaCI. Japan has no natural deposits of NaCI and had formerly obtained the salt by evaporation of seawater. The use of ED to produce a concentrated brine up to 20% TDS has substantially reduced energy costs (less than 200 kwh per ton of salt). Moreover, the use of membranes that are selective for univalent ions has improved salt purity to 97% and reduced precipitation problems. The approach has been so successful that essentially all the table salt in Japan is prepared from ED enriched brines.6 

Whey processing required several modifications to ED systems. To retard spoilage, the liquid streams had to be cooled. The inevitable accumulation of solids on the membranes and spacers required routine cleaning in place and periodic disassembly of the stacks for mechanical cleaning. These operations are now well established and can be applied to the treatment of other food products. 

Cows milk is more salty than milk from human mothers, and this limits its use in the preparation of infant formula. Desalting of cows milk by ED allows larger quantities of cows milk solids to be used for these purposes. Research has shown that desalting by ED to remove calcium improved the protein stability of frozen skim milk and its concentrates.7 The lumpy texture of thawed frozen milk has been attributed to clumping of micellar casein, and calcium removal led to the dissociation of micellar to serum casein. 

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