Membrane Filtration
Membrane is a thin layer of material that is capable of separating compounds as a function of physical and chemical properties when a driving force is applied across the membrane, which has been widely used in a wide range of applications in water industry [1]. Membrane can be classified by the range of compounds separated and the driving forces employed. For example, microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO) are four membrane processes that use pressure to transport water across the membrane. MF membranes are capable of removing suspended particles, colloids, and bacteria, while UF and NF membranes can remove macromolecules/natural organic matter and dissociated acids/divalent ions/sugars/pharmaceuticals, respectively. RO membranes retain many solutes as water permeates through the membrane.
Figure 1. Membrane Classifications Based on Pore Size
Content Table
- Membrane History in Water in the US
- Removal of Natural Organic Matter by NF and UF Membranes
- Removal of Inorganic Contaminants by RO and NF Membranes
- Removal of Organic Contaminants by RO and NF Membranes
- Removal of Endocrine Disrupting Compounds and Pharmaceuticals, and Personal Care Products by NF and UF Membranes
- References
- Further Readings
- Related Articles
- Links
Membrane History in Water in the US
- Prior to 1990 mostly RO in industrial applications
- Historically, smaller facilities (< 1 mgd)
- 1st Significant MF/UF System in North America in 1993 (Saratoga, CA – 3.6 mgd)
- Membrane bioreactor emerged in early 1990’s
- In-land brackish desalination in mid 1990’s
- Over 250 membrane WTP now on-line
- Trend is to more, and larger facilities ( Minneapolis – 70 and 95 mgd, Singapore – 72 mgd).
Removal of Natural Organic Matter by NF and UF Membranes
With the concern of disinfection by-products (DBPs) as an important drinking water quality issue, removal of natural organic matter (NOM) is important since they act as the precursors to DBPs. NOM can be effectively rejected during filtration by NF and UF membranes requiring relatively low pressure. In addition, organic matter is a significant source of flux decline due to its fouling in NF and UF filtrations. Size exclusion, electrostatic exclusion, and hydrophobic interactions are the well-known mechanisms for NOM rejection and flux decline. Numerous studies investigated the effects of chemical and physical properties of NOM on NOM rejection and flux decline with NF and UF membranes. NOM rejection can be controlled by size exclusion, electrostatic repulsion, and aromaticity/hydrophobicity interactions between NOM and membrane surface and pores. Natural organic matter is composed of various types of organic compounds. The quantity of NOM is commonly represented by the amount of dissolved organic carbon (DOC). Humic substances are the major constituents of NOM, accounting for approximately 50 percent of the DOC. About 30 % of the DOC is comprised of hydrophilic acids.
Removal of Inorganic Contaminants by RO and NF Membranes
Toxic ions such as Cr(VI), As(V), and perchlorate (ClO4-) have been an important drinking-water quality issue. River systems in the United States have been found to have chromium concentrations that range from < 1 to 30 ug L-1. The United States Environmental Protection Agency (USEPA) has implemented the maximum allowable contaminant levels for arsenic (10 ug L-1) and chromium (total–100 ug L-1) in public drinking water supplies. Although perchlorate is not listed as a USEPA drinking water contaminant, the California Department of Health Service (CDHS) has recently revised a notification level of 6 ug L-1. The removal of chromate, arsenate, perchlorate from drinking source water is critical for the protection of human health. Several different technologies have been investigated to remove these toxic ions from drinking source water and/or wastewater, including ion exchange, coagulation, and sorption-based metal oxide. Nevertheless, membrane filtration using RO and NF plays an important role in the removal of chromate, arsenate, and/or perchlorate and has also been a promising technology for water treatment.
Removal of Organic Contaminants by RO and NF Membranes
The effective removal of organic compounds has always been a major challenge for the production of potable water, since the United States Environmental Protection Agency assessed the hazard of over 85,000 chemicals. Although there are currently no federal regulations for most of these chemicals in drinking water, drinking water must be essentially free from organics in order to be fit for human consumption. However, there are few studies of how to remove the many unregulated chemicals based upon conventional and advanced drinking water treatment technologies including coagulation, softening, activated carbon, ion exchange, oxidation (e.g., chlorination and ozonation), and membrane filtration. For the last few decades, the use of membrane technology has grown significantly in the water industry compared to other water treatment technologies, since membrane filtration requires minimal addition of aggressive chemicals and produces no problematic by-products. In particular, RO including low pressure RO (LPRO) and NF are broadly used membrane processes for both potable water treatment and wastewater reuse.
Rejection characteristics of organic and inorganic compounds were examined for six RO membranes and two NF membranes that are commercially available. A batch stirred-cell was employed to determine the membrane flux and the solute rejection for solutions at various concentrations and different pH conditions. The results show that for ionic solutes the degree of separation is influenced mainly by electrostatic exclusion, while for organic solutes the removal depends mainly upon the solute radius and molecular structure. In order to provide a better understanding of rejection mechanisms for the RO and NF membranes, the ratio of solute radius (ri,s) to effective membrane pore radius (rp) was employed to compare rejections. An empirical relation for the dependence of the rejection of organic compounds on the ratio ri,s/rp is presented in Fig. 2. The rejection for organic compounds is over 75% when ri,s/rp is greater than 0.8. In addition, the rejection of organic compounds is examined using the extended Nernst-Planck equation coupled with a steric hindrance model. The transport of organic solutes is controlled mainly by diffusion for the compounds that have a high ri,s/rp ratio, while convection is dominant for compounds that have a small ri,s/rp ratio.
Figure 2. Effect of solute radius/effective membrane pore radius on the rejection of various compounds by RO (AK), LPRO (ESPA), and NF (ESNA) membranes. Operating conditions: DP=800 kPa; stirring speed=400 rpm; fc=2.5. ●, sodium chloride; ▲, urea; □, 2-(2-butoxyethoxy)ethanol; △, caprolactam; , creatine; ▽, formaldehyde; ○, methanol; à, 2-propanol; ¨, water). Solid curve: regression fit to data for organic compounds; dashed curve: regression fit for inorganic compounds [5].
Removal of Endocrine Disrupting Compounds and Pharmaceuticals, and Personal Care Products by NF and UF Membranes
Reports of endocrine disrupting compounds (EDCs) and pharmaceuticals and personal care products (PPCPs) in wastewater effluents and surface waters used as drinking water supplies have raised substantial concern in the public and regulatory agencies. EDC/PPCPs have been detected in wastewater effluents and raw drinking source waters around the world at concentrations of sub ug/L. In light of potential risk to humans and wildlife even at those trace levels, removal of EDC/PPCPs will likely become important in water industry to protect the environment and eliminate refractory organics. Numerous studies have shown the removal of conventional micropollutants such as pesticides and alkyl phthalates and NOM by the NF and UF membranes. In addition, these previous studies investigated existing separation mechanisms (e.g., size/steric exclusion, hydrophobic adsorption, and electrostatic repulsion). The rejection of uncharged trace organics by NF membranes is influenced by steric hindrance, while the rejection of polar trace organics can be explained by electrostatic interactions with the charged membranes.
In a previous study, a general separation trend due to hydrophobic adsorption as a function of octanol-water partition coefficient was observed between the hydrophobic compounds and porous hydrophobic membrane during the membrane filtration in unequilibrium conditions [6]. The results showed that the NF membrane retained many EDC/PPCPs due to both hydrophobic adsorption and size exclusion, while the UF membrane retained typically hydrophobic EDC/PPCPs due mainly to hydrophobic adsorption. However, the transport phenomenon associated with adsorption may depend on water chemistry conditions and membrane material. Fig. 3 compares the average percentage retention of EDC/PPCPs by UF membrane versus the percentage by NF membrane.
Figure 3. Summary of average percentage retention across four waters spiked with EDC/PPCPs (SRW, CRW, ORW, and PVW) by the NF and UF membranes: (a) Group I and (b) Group II.
References
This article was authored by:
Yeomin Yoon.
Department of Civil and Environmental Engineering, University of South Carolina, 300 Main Street, Columbia, SC 29208, USA
Resources
- Mallevialle, J., P. Odendaal, and M. Wiesner, Water Treatment: Membrane Processes. 1996: McGraw Hill.
- Chang, Y., 2006 Surface Water Treatment Workshop, Farbo, ND. 2006.
- Yoon, Y.M., et al., Effects of retained natural organic matter (NOM) on NOM rejection and membrane flux decline with nanofiltration and ultrafiltration. Desalination, 2005. 173(3): p. 209-221.
- Yoon, J., et al., Removal of toxic ions (chromate, arsenate, and perchlorate) using reverse osmosis, nanofiltration, and ultrafiltration membranes. Chemosphere, 2009. 77(2): p. 228-235.
- Yoon, Y. and R.M. Lueptow, Removal of organic contaminants by RO and NF membranes. Journal of Membrane Science, 2005. 261(1-2): p. 76-86.
- Yoon, Y., et al., Nanofiltration and ultrafiltration of endocrine disrupting compounds, pharmaceuticals and personal care products. Journal of Membrane Science, 2006. 270(1-2): p. 88-100.
Further Readings
1. Membrane Filtration Guidance Manual (United States Environmental Protection Agency)
2. Membrane Filtration (National Drinking Water Clearinghouse)
3. Membrane Filtration Handbook: Practical Tips and Hints (Osmonics; Wagner J.)
4. Membrane Filtration (Powerpoint Presentation) (OzScientific)
5.Bacteriologic Water Quality: Membrane-Filtration
6. Membrane Filtration Technology Fact Sheet
7. Pretreatment Optimize Membrane Filtration
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