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Biofouling of Organic and Chemically Modified Ro/Nf Membranes

Arun Subramani, University of California, A242 Bourns Hall, Chemical & Environmental Engineering,, Riverside, CA 92507, Xiaofei Huang, Civil & Environmental Engineering, University of California, 5732G Boelter Hall, University of California, Los Angeles, CA 90095, and Eric M. V. Hoek, Civil & Environmental Engineering, UCLA Water Technology Research Center and California NanoSystems Institute, Henry Samueli School of Engineering and Applied Science, 5732G Boelter Hall, University of California, Los Angeles, Los Angeles, CA 90095.

Nanofiltration (NF) and reverse osmosis (RO) membranes are increasingly being used for reclamation of non-traditional water sources. In California, for example, the production of reuse water (reclaimed municipal wastewater) will double to nearly one million acre-feet per year by 2015. Bacteria and biopolymers (glycoproteins and lipopolyssacharides) are the predominant colloidal foulants present in reuse water to be treated by reverse osmosis and nanofiltration membranes. Adsorption of natural organic matter onto membranes, and subsequent chemical cleaning, results in modification of the membrane surface properties. Hence, long-term fouling by microbial, colloidal, and dissolved solutes is governed by the modified membrane surface properties. The goal of this work is to elucidate the relative rate and extent of microbial deposition onto initially clean, organic matter fouled, and chemically cleaned NF/RO membranes. The results provide valuable insight into RO/NF membrane fouling as well as the short-term/long-term benefits and drawbacks of various chemical cleaning strategies.

Bovine serum albumin (BSA) and alginate acid (AA) are chosen as model organic foulants. Sodium dodecyl sulfate (SDS), caustic (NaOH), and bleach (NaOCl) are chosen as chemical cleaning agents. Four commercially available polyamide composite membranes and a gram-negative bacterium, Pseudomonas putida, are used as model membranes and biological foulants. Both clean and organic-modified (fouled) membranes are exposed to the chemical cleaning agents. Physicochemical properties of clean membranes, modified membranes, and bacteria cells are characterized. Membrane and foulant surface (zeta) potentials are determined from electrokinetic measurements. Surface tensions of clean membranes, modified membranes, and microbial cells are determined from equilibrium sessile drop contact angles using one apolar and two polar liquids. Morphological and chemical characterizations of membranes and foulants are performed using AFM, SEM, EDX, and ATR-IR. Direct microscopic observation is employed to determine the rate and extent of P. putida deposition onto clean and modified membranes.

Model organic foulants, BSA and AA, exhibit spherical diameters of about 5-10 nm and P. putida is about 1 micron in unbuffered 10-20 mM aqueous electrolyte solutions at pH of about 5.8. The organic and bacterial foulants and selected membranes are all significantly negatively charged at the experimental solution chemistries. Contact angle data shows that BSA and P. putida are thermodynamically stable in water (hydrophilic), whereas AA is thermodynamically unstable (hydrophobic). Both model foulants are kinetically stabilized by electrostatic repulsion. Two of the membranes (HL and NF270) are relatively hydrophilic and smooth, while the other two membranes (NF90 and ESPA4) are relatively hydrophobic and rough. An extended DLVO analysis of the adhesive free energy between membranes and cells provides valuable insights into differences in observed deposition behavior onto various membrane surfaces. Modeled foulant-membrane and foulant-foulant interfacial forces help explain the influence of initial, fouled, and cleaned membrane properties on the deposition rate of microbial cells. Direct microscopic observation data and model calculations will be presented at the conference to provide a fundamental explanation for the potential role of organic fouling and chemical cleaning on long term membrane biofouling.