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Qilin's Research Group
Civil and Environmental Engineering
Rice University


Membrane Fouling and Fouling control strategies


1. Fouling Mechanisms of NF and RO membranes in Wastewater Reclaimation and Seawater Desalination

  • Membrane fouling is a major issue impeding the application of membrane technology to water treatment processes. In our lab, complex mechanisms of combined colloidal and organic fouling of nanofiltration (NF) and reverse osmosis (RO) membranes are studied via experimental measurement and mathematical modeling(links to publications 1, 2). The study investigates the interactions between foulants in solution, the adsorption of the foulants on the membrane surface, and effective cleaning protocols for removing combined fouling. The roles of various membrane surface chemical funcationalities are investigated using self-assembled monolayers (SAMs), and the differential adsorption of common wastewater and seawater organic foulants is quantified using quartz crystal microbalance with dissipation monitoring (QCM-D). A membrane surface chemistry parameter is being developed as an indicator for adsorptive fouling by organic foulants and the effectiveness of subsequent chemical cleaning. The effects of important cationic species on organic foulant adsorption are determined.

    Figure 1: Illustrating the effect of hindered back diffusion of organics and solutes as a mechanism contributing to combined fouling.

      Figure 2: Zeta potential measurements of solutions containing the model organic, 1mg/L organic with silica colloids, and 20mg/L organic with silica colloids. Zeta potential (with standard error) of silica colloids alone is marked by the black bar. BSA is the only organic that interacts with the silica colloid solution to change the surface chemistry, indicating that BSA can contribute to combined fouling mechanisms directly.

    Figure 3: Cake layer resistances formed during single and combined fouling experiments with silica and a) Dextran (Dex), b) humic acid (HA), c) sodium alginate, and, d) bovine serum albumin (BSA). Combined resistances are compared to the calculated sum of the individual components in order to reveal combined fouling mechanisms, which are present in the HA, alginate and BSA experiments. The three hypothesized mechanisms: increased resistance of the mixed fouling layer, hindered back diffusion, and organic foulant adsorption were shown to have varying effects on combined fouling, depending on the specific organic foulant. The greatest synergism was observed in the presence of an interacting organic foulant, BSA, which can adsorb on silica colloids as well as the membrane surface to reduce repulsive interaction between foulants and the membrane as well as that among foulants.

    (Figure 1,2 & 3 from publication of Contreras, A., 2009)


      adsor rates
    algnate   cleaning

    Figure 4. Calculated adsorbed mass of a) BSA, and b) sodium alginate on different SAMs at equilibrium. Adsorption occurred in 10 mM NaCl (solid symbols) and 7 mM NaCl/1 mM CaCl2 (open symbols); c) Calculated initial adsorption rate of alginate and BSA on different SAMs under different solution conditions; d) Stacked bar graph indicating % mass of adsorbed layer removed by DI (solid) and 2% SDS cleaning (pattern). The figures showed the influences of solution conditions as well as surface functionalities. For adsorption of BSA, specific interactions between functional groups play an important role in addition to the non-specific forces. The amount of adsorbed mass of alginate showed closer correlation with surface hydrophobicity than that of BSA. Ca2+ significantly increased the equilibrium adsorption mass of alginate. It also increased the initial adsorption rate on nearly all surfaces, except on hydrophobic surfaces for BSA adsorption. Cleaning results showed that BSA and 每NH2 may contribute more to irreversible fouling of RO and NF membranes.

    (Figure 4 a,b,c, & d from publication Contreras, A., 2011)

    alg adsorp
      alg cleaning
    alg rates

    Figure 5 a) Equilibrium adsorption/deposition mass of alginate on different SAM surfaces under designed solution conditions: 0.628M NaCl (Na+); 0.4851M NaCl + 0.0478 MgCl2 (Na++Mg2+); 0.6014M NaCl + 0.0089M CaCl2 (Na++Ca2+); and 32g/L Instant Ocean solution (Seawater); b) Initial adsorption/deposition rates of alginate; c) Cleaning efficient of surfactant for the adsorbed mass under different conditions. Under seawater condition, adsorption on hydroxyl surface had the least amount of adsorption, while the differences between the adsorption amounts on other SAMs were not so obvious. The influence of Ca2+ on the total adsorption mass was dominant over other ions in seawater. The initial adsorption/deposition rate of alginate was greatest on 每COOH surfaces under seawater condition. But without the presence of Ca2+ the rate was highest on 每NH2 surfaces, possibly due to the lack of charge neutralization of Ca2+ to 每COO- in alginate molecules. Cleaning results showed that 每CONH2 had the most irreversible fouling without the presence of Ca2+, and 每NH2 was relatively less efficient to be cleaned by surfactants compared with other surfaces under seawater condition.

  • Collaborator: Dr. Roni Kasher, Ben-Gurion University of the Negev
  • Funding source: NSF, Rice University

2. Role of polymer flocculant in Coagulation MF

  • Polymers, which are often used as coagulant aids or flocculants in conventional water treatment processes to improve floc size, structure and strength, are also used in the coagulation/flocculation pretreatment for microfiltration (MF) of surface water . However, little is known about their impact on membrane performance. Because MF can remove significantly smaller particles than sedimentation/sand filtration and polymers themselves are potential membrane foulants, polymer flocculants may not necessarily effect positively on MF systems. This study aims at elucidating the role of polymer flocculants in treatment efficiency and membrane fouling during MF of surface water, and developing guidelines for use of polymer flocculants in MF systems

    Fig.4 每 Polymer flocculants can severely foul MF membranes at concentrations as low as 0.05 mg/L.

    (Wang, S., et al., Water Research (2011))

    Fig. 5 每 Polymer flocculants form aggregates on the membrane surface, blocking membrane pore openings. Shown in the figure are SEM images of PVDF membranes fouled by pDADMAC: (a), surface; (2), cross-sectional

    (Wang, S., et al., Water Research (2011))


  • Collaborator: Dr. Charles Liu, Pall Corporation
  • Funding source: Pall Corporation


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-----Rice University
-----6100 Main ST MS-519
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-----Houston TX 77005

-----Email: Qilin.li@rice.edu
-----Phone: (713) 348-2046
-----Office: Keck Hall 225




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