High Pressure Membrane Filtration for Sustainable Water Treatment
The content of this article is designed to give engineers, researchers, and students an insight into membrane filtration. The topic is addressed using the knowlege and understanding of membranes and their application to the treatment of contaminated water .
In this article, we are referring to relatively high pressure membrane filtration, such as nanofiltration (NF), and tight ultrafiltration (UF) membranes.
Content Table
Understanding Membrane Filtration
Today, nanofiltration membrane is being increasingly considered as an option for water and wastewater treatment. This is particulalry evident in the case of micropollutants, including pharmaceuticals, endocrine disrupting chemicals, and personal care products (Westerhoff et al., 2005; Kim et al., 2007; Benotti et al., 2009). However, one single process cannot be applied to all contaminants. Even with the reverse osmosis membrane there are problems such as: high costs and fouling. Fundamental to the proper design of membranes, is the understanding of membrane filtration itself, in relation to hydrodynamic/mass transport of various contaminants, and chemical interactions between contaminants and membranes.
What controls the removal of contaminents by a high pressure membrane?
We may have to answer to this question when dealing with problems and various projects, otherwise, we have to select an inappropriate membrane which needs less water production or poor performance. Concentration polarization is the first thing for us to understand and use to predict and evaluate membrane performance, with regard to both contaminants removal and fouling. The concentration polarization is defined as an increased concentration of contaminants near the membrane surface, resultant from many different phenomenas, such as the transport of the contaminants and interactions between the contaminants and the membrane surface. The concentration polarization equation can be derived, as follows:
Figure 1. Schematic of the concentration polarization: J=flux, C=contaminant concentration, D=diffusion coefficient, x=distance from the membrane surface, Cp=concentration of permeate (treated water), CB=concentration of bulk solution (feed water), Cm=concentration at the membrane surface.
Thus, Concentration polarization equation here, k (mass transfer coefficient) = D/δ (diffusion coefficient/concentration polarization thickness)
The concentration polarization equation has two significant implications; firstly, flux is not connected to trans-membrane pressure in the equation, secondly, the ratio of J/k affects concentration Cm as compared to CB. Please recall that J (↓) is water flux toward the membrane surface, and k (↑) is mass transfer coefficient which has the same dimension as J, and of which direction is opposite to J.
The effects of J/k ratio on membrane performance are provided in the following table (Cho et al., 2002) and figure (Park and Cho (2008)). As summarized in Table 1, with data obtained with the two different waters, containing relatively hydrophilic (SL-SW) versus hydrophobic (IR-GW) organic matters, the same membrane provided different removals of the DOC with different J/k ratios; higher J/k ratio resulted in decrease in removal, which was more severe with the hydrophilic DOC than the hydrophobic one. With this, the paper also provided the different effective molecular weight cutoff (MWCO) values for tested membranes, with different water sources. Thus, MWCO for a membrane are expected to have many different values, along with the absolute one which is generally provided by the manufacturers; lower MWCO means high potential to remove corresponding contaminants, under specific filtration conditions. Difference in behaviors of removals by membranes, between the hydrophobic versus hydrophilic organic matters, is to be discussed later, using electrostatic removal mechanisms. With varying J/k ratio values, we could see change in removal trends in organic matters (increasing versus decreasing); increasing and decreasing mean diffusion versus convection dominant removal mechanisms, with the same membrane and the same water, which gives us a good case study to be able to control membrane performance for sustainable water treatment.
Table 1 Effective MWCO values using the same trans-membrane pressure (Cho et al. (2000))
Figure 2. The removal trends of different membranes against J/k ratio (Park and Cho, 2008).
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This article was written by:
Jaeweon Cho, School of Environmental Science and Engineering, GIST, Korea
e-mail: jwcho@gist.ac.kr
H.K. Shon, School of Civil and Environmental Engineering, University of Technology, Sydney, P.O. Box 123, Broadway, NSW 2007, Australia, e-mail: hkshon@eng.uts.edu.au
References
Benotti, M.J., Trenholm, R.A., Vanderford, B.J., Holady, J.C., Stanford, B.D., Snyder, S. (2009) Pharmaceuticals and endocrine disrupting compounds in U.S. drinking water, Environ. Sci. Technol., 43(3), 597-603.
Cho, J., Sohn, J., Choi, H., Kim, In S., Amy G. (2002) Effects of molecular weight cutoff, f/k ratio (a hydrodynamic condition), and hydrophobic interactions on natural organic matter rejection and fouling in membranes, Journal of Water Supply: Research and Technology-AQUA, 51(2), 109-123.
Kim, S.D., Cho, J., Kim, In S., Vanerford, B.J., Snyder, S.A. (2007) Occurrence and removal of pharmaceuticals and endocrine disruptors in South Korean surface, drinking, and waste waters, Water Research, 41, 1013-1021.
Park, N., Cho, J. (2008) Natural organic matter diffusivity for transport characterizations in nanofiltration and ultrafiltration membranes, Journal of Membrane Science, 315, 133-140.
Westerhoff, P., Yoon, Y., Snyder, S., Wert, E. (2005) Fate of endocrine-disruptor, pharmaceutical, and personal care product chemicals during simulated drinking water treatment processes, Environ. Sci. Technol., 39, 6649-6663.
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