Reverse osmosis (RO) membranes have been widely applied in membrane filtration processes for water purification, since the high selective RO membranes are designed to reject all materials with particle diameter larger than 10 angstrom (Å) . However, this optimal selectivity leads to fouling that can greatly affect the performance and productivity of RO membranes. Biofouling remains as one of the major operational problems in RO processes and is caused by unwanted deposit and growth of microorganisms on the membrane. Numerous biofouling control strategies have been developed to restore the performance of RO membranes, but none of them are able to prevent or remove biofouling completely. A novel cleaning technique using a weak and monobasic acid (pKa=3.34, 25℃)named free nitrous acid (FNA) in combined with hydrogen dioxide (H2O2) was proposed.
The effects of FNA with or without H2O2 on biofouling of RO membranes were investigated in Chapter 4, five RO membranes with different degree of biofouling were cleaned using FNA solutions (10, 35 and 47 mg HNO2-N/L) at pH 2.0, 3.0 and 4.0 under cross-flow conditions for 24 hours. The cleaning efficiency of FNA solutions was compared with conventional cleaning solution sodium hydroxide (NaOH, pH 11). The cleaning tests demonstrated that FNA cleaning solutions were more efficient than NaOH at biomass removal and inactivation. At the optimum cleaning conditions (35 mg HNO2-N/L at pH 3.0),FNA has achieved higher biomass removal than NaOH for both heavily fouled (86-96% versus 41-83%) and moderately fouled (92-95% against 89-92%) membranes, respectively.In accordance to the biomass removal, 6-32% of viable cells remained on the moderately fouled RO membranes under the impact of FNA cleaning (pH 3), whereas 38-58% of viable cells stayed on the heavily fouled RO membranes. These results revealed that FNA cleaning is more effective for moderately fouled membranes, implying that early cleaning for biofouling is preferable. Although applying FNA alone, or combining it with H2O2 have shown better efficiency at biofouling removal than NaOH, the cleaning efficiency has not been significantly improved (<1% of enhancement) by adding H2O2 to FNA cleaning solutions. The effects of FNA on scaling of RO membranes were also studied using the same cleaning protocol developed for biofouling control. The results showed that FNA solutions at pH 2.0 and 3.0 were as efficient as conventional cleaning acids (hydrochloric acid and citric acid). The scaling layers which contain 32.4±1.7 g/cm2 of calcium were completely removed by all acidic cleaning solutions. Based on the results, FNA is shown to be a promising cleaning agent for RO membrane biofouling and scaling removal.
Further investigation focused on the effectiveness of FNA for biofouling prevention in RO processes (Chapter 5). The results showed that weekly FNA cleanings were unable to prevent fouling in the RO filtration systems, as the hydraulic performances (permeability and salt rejection) of RO membranes have gradually declined over two to three weeks filtration period. Although FNA cleaning was able to restore the permeability of RO membranes for one to two days, continuing declined permeability implied that the fouling rate was greater than the inhibition rate of FNA. The results of prevention tests also showed that FNA was more efficient at biomass inactivation and removal. The biomass contents and viable cells of the fouling layers formed in the experiment filtration unit (with FNA weekly cleaning) were less than half of that in the control filtration unit (without FNA weekly cleaning). Moreover, the results of live/dead cell staining revealed the abundance of viable cells in the control unit(57±5%) was four times higher than that in the experiment unit (13±2%). However, there was no significant difference in the concentration of macromolecules such as proteins and polysaccharides between control and experiment filtration units.