by: K. Good*, I. Escobar, X. Xu, M. Coleman

The University of Toledo

Due to its mass production, impact on the environment, and ever-changing environmental regulations, wastewater and its treatment has become a major concern for many municipalities. Conventional treatment systems require a large amount of space, long treatment time, and often are unable to produce effluent that meets quality levels needed for discharge. These concerns have created a need for wastewater treatment technologies that are more efficient than the standard primary and secondary treatment processes currently utilized.

A new technology in the water treatment field is the application of membranes. Membrane filtration is the separating of solvent and solute by passing a solution through a selective semi-permeable substance. As shown in figure 1, membranes typically have two layers. The bottom layer is a porous support that provides mechanical strength and stability. This porous support is covered by a thin selective layer, often referred to as the film or skin. The selective layer is responsible for providing the membrane with its separation properties. Operated under pressure or a vacuum, water is allowed to pass through the selective layer, while other materials, such as pollutants, are rejected, thus presenting the basis of separation. The measure of a membrane’s ability to separate solute from solvent is called selectivity, while the measure of volume of solvent that passes through membrane per area of membrane per unit time is referred to as flux. See figure 2 for a schematic illustrating the theory of membrane filtration. The mechanism for separation, which varies with the type of membrane, can be size exclusion, charge repulsion, differing diffusion rates through the membrane, or a combination of these.

Their separation ability makes membranes an attractive alternative for solving wastewater treatment problems. Membrane processes can remove inorganics, including total dissolved solids (TDS) and hardness causing substances. They also can remove bacteria commonly found in wastewater and can even remove small easily assimilable organics, which in many systems are the limiting nutrients for bacterial growth (LeChavallier et al., 1991). Not only could membranes remediate to the extent that there is little to no impact on the discharge body of water, they could remediate to the extent that reuse of the effluent is possible.

A main problem with the application of membrane systems to water treatment is the phenomena known as fouling. Fouling is the process by which membrane resistance to flow is increased through deposition of substances onto the membrane. There are two types of fouling: biofouling and abiotic fouling. Biofouling, often called the cancer of membranes, results from the accumulation and growth of microorganisms on the membrane. Abiotic fouling is the formation of a layer of rejected materials, mainly natural organic matter (NOM) on the membrane surface. Fouling leads to a decline in flux, frequent cleanings, and eventually to total blockage of the membrane. The pollutant content of wastewater typically leads to rapid fouling, often making membrane treatment economically unfeasible. Additionally, in order to make membrane filtration an economical alternative, water must be produced rapidly (i.e. high flux) and the water produced must be free of contaminants (i.e. high selectivity). Unfortunately, flux and selectivity are mutually counteractive properties. Modifying a membrane to increase its flux typically decreases selectivity and vice versa.

Therefore, modifying a membrane to increase the resistance to fouling without decreasing flux and/or selectivity is desirable. This may only be achievable by modifying the membrane surface and microstructure. One method of surface modification is through the use of ion beam irradiation. Ion beam irradiation is the bombardment of ions through the membrane. As the ions penetrate the membrane, they lose energy to the membrane by two main processes: interacting with target nuclei and interacting with target electrons. This addition of energy to the membrane results in bond breaking, bonds forming, crosslinking, and formation of volatile molecules that alter the microstructure of the membrane. One result of these alterations to the microstructure is smoother surfaces (Xu et al., 1997). This is significant because membranes with smoother surfaces are usually less susceptible to fouling (Elimelech et al., 1997). The rearranged microstructure may also impact pore size distribution, leading to variations in permeability (flux) while preserving or improving selectivity.

The objective of this study is to determine if ion beam irradiation of commercial water treatment RO and NF membranes can reduce the severity of fouling while preserving or improving its transport properties. This is being studied through bench-scale filtration experiments in which irradiated membranes and their non-irradiated counterparts are tested to determine the effects of irradiation on flux, resistance to fouling, and finished water quality. The extent of surface modification is determined through atomic force microscopy (AFM) analysis (Nanoscope IIIa Scanning Probe Microscope, Digital Instruments, Santa Barbara, CA). The transport characteristics and fouling levels of the irradiated membranes are compared to that of their nonirradiated counterparts. Through comparison of the initial and final mass transfer coefficient (MTC), recoverability of MTC after cleaning, permeate water quality, permeate biostability, and severity of membrane fouling, the effects of irradiation on membrane application to wastewater treatment is determined.

Preliminary experiments were performed on a nanofiltration (NF) membrane (TFC-S, Koch Membranes, San Diego, CA), which is commonly used in municipal water treatment when softening and THMFP reduction are the main objectives. This membrane consists of a polysulfone support layer, which is covered by an aromatic polyamide selective layer. It was irradiated with 1x10¹³ H+ ions/cm² at energy of 150 keV using a 1.7 MV high current Tandetron Accelerator from the University of Western Ontario, Canada. AFM analysis revealed that the irradiated membrane had a significantly lower surface roughness compared to the non-irradiated control, as shown in figures 3 and 4. The bench scale filtration experiments indicate that the initial clean water permeate flux of the irradiated membrane was 280% higher than that of the non-irradiated membrane. After filtration of a synthetic wastewater, the permeate flux of the irradiated membrane decreased by 45.4%, while the non-irradiated membrane’s flux decreased by 89.5%. Biofouling on the modified membrane surface was 41% less than on the unmodified membrane. Table 1 shows the irradiated membrane permeate had lower UV-254, turbidity, and DOC levels than the non-irradiated membrane permeate. Thus, preliminary studies indicate that irradiation induced microstructure and surface modifications resulted in increased membrane flux, lower susceptibility to fouling, and improved selectivity. This could have a major impact on the economics involved with membrane treatment of wastewater.

Current work in this study includes examining the effect of varying irradiation conditions, such as different implantation energies and different ion densities, on membrane flux, fouling, and selectivity. Membranes being tested are: (1) TFC-S, (2) 4040-UHA ESPA (Hydranautics, San Diego, CA), and (3) NTR 7450 (Hydranautics, San Diego, CA).

References

[1] I.C. Escobar, S. Hong, A. A. Randall, Removal of assimilable organic carbon and biodegradable dissolved organic carbon by reverse osmosis and nanofiltration membranes, Journal of Membrane Science 175 (2000) 1-17.

[2] S. Hong, M. Elimelech, Chemical and physical aspects of natural organic matter (NOM) fouling of nanofiltration membranes, Journal of Membrane Science 132 (1997) 159-181.

[3] M. Elimelech, X. Zhu, A.E. Childress, S. Hong, Role of membrane surface morphology in colloidal fouling of cellulose acetate and composite aromatic polyamide reverse osmosis membranes, Journal of Membrane Science 127 (1997) 101-109.

[4] E. M. Vrijenhoek, S. Hong, M. Elimelech, Influence of membrane surface properties on initial rate of colloidal fouling of reverse osmosis and nanofiltration membranes, Journal of Membrane Science 188 (2001) 115-128.

[5] X. L. Xu, M. R. Coleman, U. Myler, P. J. Simpson, Postsynthesis Method for Development of Membranes Using Ion Beam Irradiation of Polimide Thin Films, Membrane Formation and Modification, Oxford University Press (2000) 205-227.

[6] E. H. Lee, Ion-beam modification of polymeric materials – fundamental principles and applications, Nuclear Instruments and Methods in Physics Research B 151 (1999) 29-41.

[7] X. Xu, M. R. Coleman, Atomic Force Microscopy Images of Ion-Implanted 6 FDA-pMDA Polyimide Films, Journal of Applied Polymer Science 66 (1997) 459-469.

[8] X. Xu, M. R. Coleman, Ion beam irradiation-an efficient method to modify the sub-nanometer scale microstructure of polymers in a controlled way, Mat. Res. Soc. Symp. Proc. 540 (1999) 255-260.

[9] M.W. LeChevallier, W. Schulz, R. G. Lee, Bacterial nutrients in drinking water, Appl. Environ. Microbiol. 57 (1991) 857-862.

 

Table 2. Permeate water quality results.

axis spans 800 nm)

Figure 1. Cross-sectional view of membrane layers.                                                                       Figure 2. Membrane filtration theory.

Figure 4. AFM image of non-irradiated TFC-S membrane. (x and y axis spans 10 mm and z axis spans 800 nm)

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