황산칼슘 스케일 플러싱으로 인한 역삼투막의 손상

Abstract

Recovery of reverse osmosis (RO) process can be limited by scale formation. Sparingly soluble chemicals such as calcium sulfate precipitate to form scale if the concentration exceeds solubility by concentrating RO feed water. Generally, the effect of scale formation is believed to be similar to that of membrane fouling, which increases hydraulic resistance of RO membrane due to scale formed on the membrane surface. In this work, the effect of gypsum scale which is one of common scalants in RO processes, were investigated through both 4-inch RO module test s and lab-scale coupon experiments. In the 4-inch RO module tests with a constant flux (18.3 LMH), feed water was concentrated until 300% of calcium sulfate solubility reached by discharging permeate and recirculating concentrate into feed water tank. After that, both permeate and concentrate were recirculated to maintain a constant osmotic pressure to identify the effect of gypsum scale on the transmembrane pressure (TMP). During that period, TMP increased from 33 to 47 bar due to the gypsum scale formation as expected. After the scale formation experiment was finished, the RO module was flushed with pure water (with no chemical cleaning agents) by 0.03 m/s of crossflow velocity for several minutes. Interestingly, the pure water permeability was restored to the value before the scale formation test. However, salt rejection test revealed that the tested RO membrane removed only 96 % of sodium chloride of 1000 mg/L (more than 99 % of salt rejection before the scale formation test). By the lab-scale coupon tests followed, it was proved that the degradation of salt rejection ability of the tested membrane came from the pure water flushing after the scale formation tests. This means RO membrane can be irreversibly damaged by gypsum scale flushed way.