RO 해수담수화를 위한 모래여과 공정의 최적화와 RO 생산수 후처리

Abstract

Seawater desalination is the technique of obtaining fresh water from seawater by removing salinity. Recently, reverse osmosis (RO) desalination for seawater application considered the most economical process available. It is necessary to perform pre-treatment and post-treatment to make seawater drinkable. In order to make a successful RO pre-treatment of raw water effective, a conventional pre-treatment which can be used widely, is inexpensive, and able to achieve the feed water quality is needed. Ferric chloride (FeCl₃) has been shown to be the most successful coagulant in RO pre-treatment application. According to the study, ferric chloride would cause accelerated fouling in the RO membrane when the oxidized form of iron is more than 0.05 mg/L. Most iron is removed by sand filtration, although some iron could pass through the sand filtration and negatively affect the RO membrane. Meanwhile, control of corrosiveness is necessary during post-treatment in order to drink RO permeate. In general, it is usual to use lime and CO₂ to control corrosion of RO permeate, and needs careful handling in the field. Therefore, this research is focused on the optimization of sand filtration as a pre-treatment, and control of corrosion as a post-treatment of seawater desalination by reverse osmosis. The sand filtration experiment is conducted by different coagulant doses, coagulation pH, mixing conditions, and filter media depths. And then the turbidity, SDI_(15), particle counts, Fe concentration, and RO permeate flux are evaluated. The experiment of corrosive control is conducted by different CO₂ injection methods, injection sequences of lime & CO₂, injection time differences, mixing ratios of tap water-RO permeates, and pH level. Then the turbidity, pH, hardness, alkalinity, temperature, electrical conductivity, and LSI are evaluated. The result of the sand-filtration experiment is similar in Fe ion concentration and SDI, regardless of coagulant dose and coagulation pH, except for the coagulant dose 30 mg/L at pH 6. The results also show a decrease in Fe ion concentration and SDI as the filter media depth is increased. With the flocculation, in addition, proper pre-treatment water quality through the RO process could be more easily achieved. The result of the post-treatment experiments is similar in turbidity, hardness, alkalinity and LSI regardless of CO₂ injection method and injection sequence of lime & CO₂. But there is great difference in turbidity as the injection time difference is changed. Also, results show an increase of turbidity as the ratio of tap water is increased in mixed water. Although the others conditions remained the same, the test shows that pH is an important factor in post-treatment, where the result of LSI is opposite given that there is a minus value when pH is 8 and a plus value when pH is 9.