Organic fouling during reverse osmosis (RO) process

Abstract

Reverse osmosis (RO) membrane processes have been used in seawater desalination for over couples of decades. In recent years, new applications of RO membrane processes for reclamation of treated effluent have become popular due to its good performance in rejecting contaminants of very small size. A major obstacle for RO membrane processes is membrane fouling caused by natural organic matter (NOM). A review of literature revealed that the characteristics of NOM and solution chemistry play important roles on RO membranes performance. Operation parameters, such as operating pressure, are also reported to greatly influence the rate and extent of membrane fouling. In this study, a lab-scale RO membrane system was set up to systematically investigate the roles of operating pressure and NOM hydrophobicity on membrane performance. In addition, the effects of pH, ionic strength and divalent cations (Ca2+) on the fouling potential of fractionated NOM components were also studied. Secondary effluent was used as feed water to evaluate the influence of operating pressure on the performances of three RO membranes. It was observed that each membrane had a threshold operating pressure, below which the membrane fouling can be effectively controlled. Results of hydraulic cleaning showed that although under high operating pressure the normalized permeate flux after 24-hr running decreased to a great extent (>40% decline), it could almost be completely restored to the initial value. This indicated that short-period membrane fouling is reversible, and backwash commonly adopted in industry is a necessary and effective method for fouling control. To have a more clear insight of NOM fouling, synthetic NOM solutions were used in the second stage of research. A commercial humic acid, the representative of NOM, was treated by a hydrophobic resin (DAX-8) to be fractionated into its hydrophobic and hydrophilic components, respectively. Fouling experiments with the resulting two factions were conducted under different solution chemistries. 24-hr operation results showed that the hydrophilic NOM had a higher fouling potential (28.65% normalized permeate flux decline) than that of hydrophobic fraction (22.94% decline). When pH value was decreased from 7 to 4, fouling potentials of both hydrophobic and hydrophilic fractions increased to a great extent (approximately 38% normalized permeate flux decline for each fraction). Calcium ions exhibited a contrary influence on the performances of fractionated NOM. With 10-3M calcium ions added, normalized permeate flux of hydrophobic NOM decreased from 77.06% to 70.75%. In contrast, normalized permeate flux of hydrophilic NOM increased from 72.35% to 81.90%.