Wastewater treatment plant to reduce the salt concentration before discharge . RO is widely used in food processing as a concentration process and recovery process of useful components in food wastewaters ; it is conceivable that RO can also be used to reduce the volume of the wastewater in food wastewater treatment (for example , RO may be used to treat nutrients such as nitrate or phosphate ) or to produce recycled water for reuse in food processing operations. Like RO, electrodialysis (ED) is used to desalt impaired waters or re-move ions such as nitrate, arsenic ,and phosphate .ED may also be used to separate acids from food wastewaters. Recovery of carboxylic acids (such as acetic, citric, and lactic) is a known application of ED in food and agricultural processing. As explained in chapter 3, there are four common types of membrane module designs available for membrane processes; however, not all module types are suitable for all membrane processes. Table 5.1 provides a guideline for selecting modules for membrane processes. One current interest in application of membrane processes in food wastewater treatment is recovery of valuable commodities from food wastewater streams. Chapter 8 is devoted entirely to the recovery of useful materials and energy from food and agricultural wastewaters including using membrane-based technologies to achieve the objectives of the recovery. One problem that has hindered the widespread use of membrane technology is the noticeable occurrence of concentration polarization/fouling in membrane processes. The detrimental effect of concentration polarization and/or membrane fouling add significant costs to the operator. In membrane filtration processes, concentration polarization is formed as the result of the rapid accumulation of retained solutes near the membrane surface to the point that the concentration of macromolecule solute reaches the gel-forming concentration and the retained molecules diffuse back into the bulk fluid. The cause of concentration polarization in perva-poration or electrodialysis is slightly different from that of membrane filtration in that it is triggered by the relatively slow diffusional mass transfer rates of solutes or ions from the bulk to the membrane surface.
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