Foulants analyses for NF membranes with different feed waters
Two different NF membranes were operated to remove natural organic matter (NOM) originating from Dongbok Lake in Korea. Coagulation/sedimentation and sand filtration treated waters as membrane feed waters were used. Tested NF membranes such as RF (Saehan, Korea) and NF90 (Filmtec, USA) membranes were exhibited slightly different pore size distribution, hydrophobicity and surface charge in terms of fractional rejection method, contact angle measurement and electrophoresis method, respectively. Chemical cleaning using a NaOH solution at pH 12 was conducted periodically to prevent organic/inorganic fouling during nanofiltration. The tested NF membranes were autopsied to compare the fouling propensity from different feed waters using pure water and a NaOH solution. Organic/inorganic foulants onto membrane surface were analyzed in terms of molecular weight (MW) distribution, structure, and IR analysis, and fouled membranes were also characterized in terms of pore size distribution, surface charge, and SEM-EDS analysis. Polysaccharides and/or N-acetyl aminosugar groups with MW ranging from 30,000 to 50,000 g/mol were identified using HP-SEC and IR analysis. Inorganic foulants (i.e., Si and Al) were also fouled onto the membrane surface and/or pores, and it is effectively removed by caustic cleaning, not pure water. Caustic cleaning was proven to be effective to remove both fouled NF membranes as a basis of flux recovery, and it could efficiently desorb the hydrophobic NOM constituents or proteinlike substances from the relatively hydrophilic and less negatively-charged NF membranes.