Membrane Filtration As an Alternative: Part 2

About the author:

Dr. Mohamed Lahlou is the technical assistance specialist for the National Drinking Water Clearinghouse. This article originally appeared as a tech brief for the NDWC.

Dr. Mohamed Lahlou

Nanofiltration membranes have a nominal pore size of approximately 0.001 microns and a molecular weight cut-off (MWCO) of 1,000 to 100,000 daltons. Pushing water through these smaller membrane pores requires a higher operating pressure than either microfiltration (MF) or ultrafiltration (UF). Operating pressures usually are near 600 kPa (90 psi) and can be as high as 1,000 kPa (150 psi). These systems can remove virtually all cysts, bacteria, viruses and humic materials. (See Figure 2 and Table 1.) They provide excellent protection from disinfection byproducts (DBP) formation if the disinfectant residual is added after the membrane filtration step. Since NF membranes also remove alkalinity, the product water can be corrosive. Measures such as blending raw water and product water or adding alkalinity may be needed to reduce corrosivity. NF also removes hardness from water. Therefore, they are sometimes referred to as "softening membranes." Hard water treated by NF will need pretreatment to avoid precipitation of hardness ions on the membrane. More energy is required for NF than MF or UF, which has hindered its advancement as a treatment alternative.

NF membranes have been observed to operate on the principle of diffusion rather than sieving as with MF and UF membranes.

Operational parameters of membranes include the physical and chemical properties of the membrane, the pore size or (MWCO) and configuration.

Reverse osmosis systems are compact, simple to operate and require minimal labor, making them suitable for small systems. They also are suitable for systems where there is a high degree of seasonal fluctuation in water demand.

These systems can effectively remove nearly all inorganic contaminants from water. RO also can effectively remove radium, natural organic substances, pesticides, cysts, bacteria and viruses. (See Figure 2 and Table 1.) RO is particularly effective when used in series. Water passing through multiple units can achieve near zero effluent contaminant concentrations. Disinfection also is recommended to ensure the safety of the water.

Some of the advantages of RO are

• Removes nearly all contaminant ions and most dissolved non-ions,

• Relatively insensitive to flow and total dissolved solids (TDS) level, and thus suitable for small systems with a high degree of seasonal fluctuation in water demand,

• RO operates immediately, without any minimum break-in period,

• Low effluent concentration possible,

• Bacteria and particles also are removed, and

• Operational simplicity and automation allow for less operator attention and make RO suitable for small system applications.

Some of the limitations of RO are

• High capital and operating costs,

• Managing the wastewater (brine solution) is a potential problem,

• High level of pretreatment is required in some cases,

• Membranes are prone to fouling, and

• Produces the most wastewater at between 25 to 50 percent of the feed.

RO removes contaminants from water using a semipermeable membrane that permits only water–not dissolved ions such as sodium and chloride–to pass through its pores. Contaminated water is subject to a high pressure that forces pure water through the membrane, leaving contaminants behind in a brine solution. Membranes are available with a variety of pore sizes and characteristics.

Typical RO units include raw water pumps, pretreatment, membranes, disinfection, storage and distribution elements. These units are able to process virtually any desired quantity or quality of water by configuring units sequentially to reprocess waste brine from the earlier stages of the process. The principal design considerations for reverse osmosis units are

• operating pressure,

• membrane type and pore size,

• pretreatment requirements, and

• product conversion rate (the ratio of the influent recovered as waste brine water to the finished water).

Waste stream disposal is a significant problem in many areas. Unlike conventional treatment processes in which approximately 5 to 10 percent of the influent water is discharged as waste, membrane processes produce waste streams amounting to as much as 15 percent of the total treated water volume. (See Table 2.) Since little or no chemical treatment is used in a membrane system, the concentrate stream usually contains only the contaminants found in the source water (although at much higher concentrations). Therefore, the concentrate sometimes can be disposed of in the source water. Other alternatives for disposal include deep well injection, dilution and spray irrigation, or using the municipal sewer. These alternatives usually are necessary for NF wastes that contain concentrated organic and inorganic compounds. Regardless of the type of membrane, disposal must be carefully considered in decisions about the use of membrane technology. Applicable local discharge regulations must be respected.

One of the most critical aspects of using membrane technology is ensuring that the membranes are intact and continuing to provide a barrier between the feedwater and the permeate or product water. There are several different methods that can be used to monitor membrane integrity, including

• Turbidity monitoring,

• Particle counting or monitoring,

• Air pressure testing,

• Bubble point testing,

• Sonic wave sensing, and

• Biological monitoring.

1. American Water Works Association and American Society of Civil Engineers. 1998. Water Treatment Plant Design. New York: McGraw-Hill.

2. Bergman, A.R. J.C. Lozier. 1993. "Membrane Process Selection and the Use of Bench and Pilot Tests." Membrane Technology Conference Proceedings. Baltimore: American Water Works Association.

3. Jacangelo, J. G., J-M. Laine, E.W. Cummings, A. Deutschmann, J. Mallevialle, M.R. Wiesner. 1994. Evaluation of Ultrafiltration Membrane Pretreatment and Nanofiltration of Surface Waters. Denver: American Water Works Association and AWWA Research Foundation.

4. Jacangelo, J. G., S. Adham, J-M. Laine. 1997. Membrane Filtration for Microbial Removal. Denver: American Water Works Association Research Foundation and American Water Works Association.

5. Mallevialle, J., P.E. Odendaal, and M.R. Wiesner, 1996. Water Treatment Membrane Processes. New York: McGraw-Hill.

6. National Research Council. 1997. Safe Water From Every Tap. Washington, D.C.: National Academy Press.

7. U.S. Environmental Protection Agency. 1990. Environmental Pollution Control Alternatives: Drinking Water Treatment for Small Communities. Washington, D.C.: Office of Water. EPA/625/5-90/025.

8. U.S. Environmental Protection Agency. 1989. Technologies for Upgrading Existing or Designing New Drinking Water Treatment Facilities. Washington, D.C.: Office of Water. EPA/625/4-89/023.

9. U.S. Environmental Protection Agency. 1998. Small System Compliance Technology List for the Surface Water Treatment Rule and Total Coliform Rule. Washington, D.C.: Office of Water. EPA/815/R/98/001.