Apr 07, 2006

Life Expectancy of POU Membrane Elements

Shortly after Water Quality Products published its 2005-2006 Membrane Filtration—A Guide to Current Technologies and Applications supplement, we received several questions from one of our readers, Quinn L. Coats, senior electrical engineering technician for GE Infrastructure, Water & Process Technologies. We asked YuJung Chang, Ph.D., to provide an expert response.

Quinn L. Coats: What is the life expectancy of a standard membrane element in a typical under-the-sink home RO system?

YuJung Chang: Usually, these RO membrane elements are replaced every two to three years.

Coats: What variables affect this?

Chang: RO membrane life expectancy depends on the following factors:

Water quality - Water parameters such as pH, turbidity, organics, iron, manganese, calcium, sulfate and other ions could form precipitates (scale) on membrane surfaces. If the RO feedwater contains a high concentration of material that tends to foul the membrane, the life expectancy will be shorter.

Level of pretreatment - With appropriate pretreatment, such as a filter cartridge with the right pore size and GAC filters, the membrane life expectancy can be extended.

Operating conditions - If the membrane system is operated outside the specified conditions, such as operating pressure and the daily quantity of water treated, the membrane life could be shortened. Also, membrane life can be extended if proper maintenance and monitoring are performed according to the manufacturer’s schedule.

Coats: What is the membrane life expectancy if the supply water is not filtered or softened and has approximately 20 gpg of hardness?

Chang: A hardness of 20 gpg is equivalent to 344 mg/L of CaCO3, which is very hard water. Also, solid particles (turbidity) could severely foul the RO membranes. If no filtration or softening is provided to this water, the RO membranes will likely need to be replaced every one to three months, depending on the actual water quality and whether regular chemical cleaning is performed. When treating hard water, usually antiscalant chemicals would be added to the RO feedwater to reduce scaling. Regardless of hardness, prefiltration (usually cartridge filters) should be provided since particulate fouling on RO membranes cannot be easily removed.

Coats: How often should membranes be changed if you have zero hard water vs. hard water (10 to 20 gpg)?

Chang: This question cannot be answered easily since there are many other water quality parameters that could affect RO membrane life expectancy, such as natural organic matter, barium, turbidity and the presence of oil/grease. Assuming there is no other fouling source, RO membranes treating water with zero hardness could last for at least three to five years. (Biological fouling would be another concern.) The membranes treating very hard water could last for less than six months to one year.

Keep in mind that the effect of hardness strongly depends on water recovery ratio, or concentration factor. For a high efficiency RO system, in which most of the water fed to the RO system ends up in the production line, the concentration factor in the RO reject side would be very high; therefore, the scaling and fouling level would be very high.

On the other hand, if most of the water fed to the RO system is used to flush out the RO reject, and only a small portion of feedwater is treated by RO membranes, the concentration factor in the RO reject line will be very low, and as a result, the scaling would not be very severe. Therefore, adjusting the water recovery ratio is another operation strategy for hard-water RO applications. Usually IX process is provided as a pretreatment to RO if the feedwater is hard.

About the author

YuJung Chang, Ph.D., is vice president and national technical director of Water Supply & Treatment for HDR, Inc., Seattle, Wash. He can be reached at 425.450.6275, or by e-mail at [email protected].