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Principles and Process Selection for Reverse Osmosis System Pretreatment
Reverse Osmosis System Pretreatment Principle and Process Selection Overview of Reverse Osmosis System Pretreatment For reverse osmosis membrane elements, water sources cannot directly enter the membrane elements in most cases, because the impurities contained in them will contaminate the membrane elements, affecting the stable operation of the system and the service life of the membrane elements. Pretreatment is the process of treating raw water with appropriate processes according to the characteristics of impurities in it to meet the influent requirements of reverse osmosis membrane elements. It is called pretreatment because it is located before reverse osmosis in the entire water treatment process. 1. Purposes of Reverse Osmosis System Pretreatment: 1. Prevent fouling on the membrane surface, i.e., prevent suspended impurities, microorganisms, colloidal substances, etc. from adhering to the membrane surface or blocking the water flow channels of membrane elements. 2. Prevent scaling on the membrane surface. During the operation of reverse osmosis equipment, some insoluble salts such as CaCO3, CaSO4, BaSO4, SrSO4, and CaF2 are deposited on the membrane surface due to water concentration, so the formation of these insoluble salts must be prevented. 3. Ensure that the membrane is free from mechanical and chemical damage, so that the membrane has good performance and a sufficiently long service life. 2. Process Selection for Reverse Osmosis System Pretreatment Design When the suspended solids content in surface water is less than 50mg/L, the in-line coagulation filtration method can be adopted. When the suspended solids content in surface water is more than 50mg/L, coagulation, clarification and filtration methods can be adopted; When the iron content of groundwater is less than 0.3mg/L and the suspended solids content is less than 20mg/L, the direct filtration method can be adopted. When the iron content of groundwater is less than 0.3mg/L and the suspended solids content is more than 20mg/L, the in-line coagulation filtration method can be adopted. When the iron content of groundwater is more than 0.3mg/L, oxidation for iron removal should be considered first, followed by direct filtration or in-line coagulation filtration. When the raw water has a high organic matter content, chlorination, coagulation, clarification and filtration can be used. If this treatment still cannot meet the requirements, activated carbon filtration can be used simultaneously to remove organic matter. When the raw water has high carbonate hardness and CaCO3 will still deposit on the reverse osmosis membrane after chemical dosing treatment, softening or lime treatment can be adopted. When other insoluble salts scale and precipitate in the RO system, scale inhibitor dosing treatment shall be carried out. It is worth mentioning that barium and strontium are not always present in raw water analysis. However, despite their low concentrations, they tend to scale and precipitate on the membrane surface easily as long as the sulfate content in the water is greater than 0.01mg/L. Such scales are difficult to clean and remove, so their formation on the membrane surface should be prevented as much as possible. When the raw water has a high silicate content, lime and magnesium oxide (or dolomite powder) can be added for treatment. When the silicon concentration in the RO feed water is greater than 20mg/L, scaling tendency assessment must be conducted. Since silicon scale is difficult to clean, it is essential to prevent it from scaling on the membrane.
