Purpose and Considerations of Osmosis Pretreatment

Special attention should be paid to raw water pretreatment when using a reverse osmosis system. To avoid clogging the system, raw water should be pretreated to remove suspended solids and reduce turbidity. Furthermore, sterilization should be performed to prevent the proliferation of microorganisms.

Because reverse osmosis has very high requirements for the amount of suspended solids in the raw water, a fouling index (FCI) is commonly used to assess water quality. This method essentially measures the degree of fouling of the reverse osmosis system by suspended solids in the water. The FCI of the water entering the reverse osmosis system should ideally not exceed 5, and a value generally less than 3 is recommended. The pH value of the feed water should also be considered during pretreatment. Various semi-permeable membranes have their optimal operating pH values, so the pH of the feed water must be adjusted according to the requirements of the reverse osmosis membrane. The temperature of the feed water should also be considered during pretreatment. Membrane permeability increases with increasing water temperature, but excessively high temperatures will accelerate the hydrolysis of cellulose acetate membranes and soften the organic membrane, making it more prone to compaction. Therefore, for organic membranes, it is generally advisable to control the temperature in the range of about 20-40℃, and for composite membranes, it is advisable to control the temperature in the range of about 5-45℃.