3. MEMBRANE PROCESSESa. Electrodial

3. MEMBRANE PROCESSES

a. Electrodialysis. In this procedure, stacks of alternating positively and negatively charged membranes in single and multi-stage configurations constitute the semipermeable membrane. Different impurities are removed in different degrees according to the selectivity of the membrane.
(1) Pretreatment. Plugging of the membrane can be prevented by pretreatment with carbon adsorption or filtration. The water to be treated is held in large tanks, with the membranes serving as obstacles to flow. All correctly charged particles in the water are attracted to the membrane obstacle, but only certain ones are able to pass through. This allows for selective distribution of the particles and a cleaner water. Chlorine oxidation disinfects the matter.
(2) Efficiency factors. Factors such as air temperature, water temperature, water pH, and electrical power applied to the equipment determine the efficiency of this process.
b. Reverse Osmosis.
(1) Use of reverse osmosis. This process is used primarily to remove salt from sea water (desalination) for drinking or manufacturing purposes. Reverse osmosis operates at pressures of 500 to 1,500 pounds per square inch with hydraulic application rates of 3 to 50 gallons per day per square foot. The reverse osmosis membrane will reject from 90 to 99 percent of the total dissolved solids presented to it. Either process will remove 100 percent of the suspended solids. High temperature conditions allow for more water to be treated with this process.
(2) Eguipment. Reverse osmosis equipment is usually designed as hollow-fiber, spiral- wound membrane, or as tubular units. Pressure can be applied to the high salt- strength water in the tubes. This forces the water present through the barrier membrane while trapping the salt and solids in the tubes.
4. ION EXCHANGE

a. Types of Processes. Ion exchange operations are either batch or continuous type. In the batch process, the reaction is confined to the contents of one vessel at a time. In the continuous type, there is a constant feed and a constant drain. In the batch process, the resin is stirred with the water to be treated until the equilibrium reaction is complete. Used resin is removed by settling. In the continuous process, the exchange material is placed in a bed or column and the water is passed through it.
b. Resins. The extent of removal of the ions in the water depends upon the chemical equilibrium state between the ions in the liquid phase and those in the solid resin material. Both natural and man-made ion-bearing resins are available. Regenerant and separate restorant chemicals are used to remove the undesirable inorganic ions and organic materials from the used resin in order to prepare the resin for reuse.