In-Home Systems: Utilizing RO and NF Elements

Dec. 28, 2000

According to the Water Quality Association (WQA), more than 15 percent of American households now have an in-home water purification system utilizing either reverse osmosis (RO) or nanofiltration (NF) membrane technology. These systems are used to purify water from multiple sources including ground water, surface water and even brackish water.

Innovations in membrane technology, which has been used in major industries and hospitals, are driving the industry into the home. Under-the-sink water purification systems are now more cost-efficient and easier to install than ever. By incorporating RO elements, these systems are a reliable means for purifying drinking water.

Historically, membrane elements were too expensive and inefficient for residential use. However, synthetic materials have been responsible for a lower cost and increased reliability, making them a more practical solution.

RO Elements in In-Home Systems

RO membranes are designed to act as an ultra-fine filter in order to reject solids as small as one angstrom in diameter. No other filtration method can achieve this level of purification.

In water treatment, the membrane is permeable to water molecules but is not permeable to molecules of dissolved solids such as salt. Reverse osmosis effectively removes these salts, bacteria and even viruses.

Traditional RO membranes date back to the 60s when they were constructed of cellulose acetate and required pressures greater than 600 pounds per square inch (psi). Those antiquated membranes removed as much as 95 percent of the solids from water. The tremendous amount of energy required by these systems made them impractical for in-home use.

The WQA lists undersink and countertop residential systems, known as low pressure RO systems, as those systems with water feed pressures of less than 100 pounds per square in gauge (psig).

In addition to removing between 95 percent to more than 99 percent of dissolved salts, RO membranes also reduce bacteria passage as well as the passage of bacterial byproducts by 99.5 percent or better. RO also removes silica and total organic carbon (TOC) from incoming water. These often are important additional considerations. Other contaminants RO membranes can remove include arsenic, nitrates, lead, mercury and fluoride. In addition, RO membranes reduce odor, color and foul taste.

NF Elements in In-Home Systems

NF membranes do not remove the level of dissolved solids that RO membranes do and are not used as widely in in-home systems. Instead, NF technology is found more often in municipal water treatment plants. NF membranes are designed to reject solids larger than one nanometer (10 angstroms) in diameter with molecular weights above 200. In exchange, they are capable of working at much lower pressures than RO systems, reducing the amount of energy they need.

About the Author

Jon Goodman is global marketing manager for FilmTec membranes. FilmTec is a wholly owned subsidiary of The Dow Chemical Co.


RO Elements: Low Energy Membranes

In 1977, The FilmTec Corp., which is now a wholly owned subsidiary of The Dow Chemical Co., introduced the first low-energy element, FILMTEC FT 30, that could effectively eliminate 98 percent of dissolved solids, utilizing pressures as low as 150 psi. Over the following 23 years the process has evolved to provide an even higher level of rejection, eliminating more than 99.9 percent of all dissolved solids in industrial systems. Reverse osmosis membranes typically are incorporated as part of a whole, home filtration device that also includes carbon filters and micro screens. Howeverm it is the reverse osmosis technology that cleans the water down to the molecular level .

A new generation of FILMTEC NF elements have been designed only to remove specific contaminants while permitting the passage of other desirable molecules. For example, in Paris there are more than 9,000 FILMTEC NF membranes removing atrazine from river water but leaving magnesium and calcium in as part of the local health requirement.

About the Author

Jon Goodman

Image courtesy Institute of Chemical Research of Catalonia (ICIQ).
All images courtesy of Ecosorb.