Paper to be presented at the US-Korea Conference
California, August 2005
Nanofiltration of Natural and Synthetic Organic Compounds in Water Supplies: Fouling Control by Advanced Oxidation (H2O2/UV) Pretreatment and Chemical Cleaning
Warner (Wonho) Song, Ph.D.
LEE & RO, Inc.
1199 South Fullerton Road
City of Industry, CA 91748-1232
Membrane technologies can be cost-competitive in water treatment due to their effectiveness in removing a broad spectrum of organic and inorganic contaminants, as well as pathogenic microorganisms. However, permeate flux deterioration caused by restricted membrane transport due to fouling could significantly affect their overall performance and economics. Membrane fouling due to natural organic matter (NOM), exemplified by humic substances, polysaccharides, and proteins, is a major cause for flux decline. Therefore, appropriate source water pretreatment (for destruction or transformation of NOM) and membrane cleaning for flux recovery are important considerations. Advanced oxidation processes (AOPs), such as H2O2/UV oxidation, have shown potential for complete destruction of synthetic organic chemicals (SOCs), and for the transformation of humic or hydrophobic substances into nonhumic or less hydrophobic components. In this regard, H2O2/UV pretreatment simultaneously confers a two-fold advantage: (i) mitigation of flux decline due to NOM fouling and (ii) removal of contaminants such as SOCs and hydrogen sulfide. Additionally, the nonselective reactivity of hydroxyl radical vis-à-vis molecular ozone might reduce accumulation of undesirable nonhumic substances, such as polysaccharides and biodegradable oxidation byproducts, in the pretreated water. Furthermore, UV irradiation and residual hydrogen peroxide, as well as generated hydroxyl radicals, may significantly mitigate biological fouling. Nonetheless, pretreatment of source water might slow down but cannot entirely eliminate fouling. Therefore, effective chemical cleaning is necessary to detach different classes of foulants from the membrane and restore the permeate flux characteristics. Furthermore, the selection of appropriate cleaning agent is critical because incompatible combinations of cleaning agent and membrane material will lead to irreversible flux loss, poor solute rejection, unnecessary costs through excessive chemical use, and/or reduction in membrane life spans.
This study investigated methods directed at improving performances of nanofiltration processes with respect to permeate flux and permeate quality. In order to accomplish these goals, the H2O2/UV pretreatment of source water prior to nanofiltration, as well as membrane cleaning strategies, was investigated. The groundwater containing NOM obtained from a deep aquifer in Southern California was used as feed for a bench-scale crossflow membrane experiments. The destruction of alachlor, an acetanilide pesticide, and hydrogen sulfide was investigated during the H2O2/UV oxidation. It further examined membrane surface characteristics with regard to the nature and mechanisms of membrane fouling by natural organic matter (NOM) adsorption, and membrane cleaning using different chemical agents. Several surface characterization techniques were employed including attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and scanning electron microscopy (SEM).
The results demonstrated that that performance efficiency and cost effectiveness of nanofiltration for removing NOM and SOCs could be greatly improved by source water pretreatment and membrane cleaning. NOM source water treated by the H2O2/UV oxidation showed less membrane-fouling potential based on flux monitoring. Also, fouling potential was inversely proportional to the degree of decomposition (or transformation) of NOM by the H2O2/UV process. Organic materials which are amenable to adsorption (e.g., polysaccharides, humic substances) were transformed into less adsorbable organic materials during the preoxidation. In addition, H2O2/UV pretreatment appeared to have great potential to control biological fouling. Additionally, the H2O2/UV treatment resulted in effective removals of hydrogen sulfide and alachlor. Membrane cleaning studies using different chemical agents demonstrated that the membranes fouled with H2O2/UV-pretreated water appeared to be more amenable to caustic cleaning than those fouled with raw water. This was presumably due to the lower sorption affinity of preoxidized NOM, increased amount of organic acid constituents, and transformation of non-desorbable NOM fractions into readily desorbable organic substances. The study showed gradual accumulation of residual foulants after repeated cleaning, and that flux deterioration could be mitigated by a cleaning strategy using a successive application of caustic and acid solutions. Correlations were established between the performance of virgin, fouled, and cleaned membranes, and their surface properties as characterized by several analytical techniques including ATR-FTIR, XPS, AFM and SEM. The analyses of membrane surface characteristics by the above techniques provided insight into the mechanistic aspects of NOM fouling, H2O2/UV pretreatment, and membrane cleaning.
Corresponding author:
Wonho Song,
LEE & RO, Inc.
1199 South Fullerton Road
City of Industry, CA 91748-1232
Phone: 626-912-3391 Ext: 208
Fax: 626-912-4015
E-mail: wonhoson@yahoo.com