A growing number of countries are turning to desalination plants to bolster dwindling water supplies. Most of the new facilities make use of reverse osmosis technology, but unfortunately these systems are susceptible to clogging and membrane damage, which places higher energy demands on the pumping system and necessitates costly cleanup and membrane replacement. Now researchers have unveiled a new class of reverse-osmosis membrane that resists the clogging that typically occurs when seawater, brackish water and waste-water are purified.

Reverse Osmosis

In reverse osmosis desalination plants high pressure is used to force polluted water through the pores of a membrane. While water molecules pass through the pores, mineral salt ions, bacteria and other impurities cannot. However, over time these particles build up on the membrane's surface, leading to clogging and membrane damage, which places higher energy demands on the pumping system and necessitates costly cleanup and membrane replacement.

The new membrane developed by researchers from the UCLA Henry Samueli School of Engineering and Applied Science has a novel surface topography and chemistry that allow it to avoid such drawbacks. The highly permeable, surface-structured membrane can easily be incorporated into today's commercial production system, the researchers say, and could help to significantly reduce desalination operating costs.

Synthesized through a three-step process the researchers created a polymer “brush layer” on a polyamide surface. The polymer chains of the tethered brush layer are in constant motion with water flow adding to the brush layer’s movement, making it extremely difficult for bacteria and other colloidal matter to anchor to the surface of the membrane.

"If you've ever snorkeled, you'll know that sea kelp move back and forth with the current or water flow," said Yoram Cohen, UCLA professor of chemical and biomolecular engineering. "So imagine that you have this varied structure with continuous movement. Protein or bacteria need to be able to anchor to multiple spots on the membrane to attach themselves to the surface — a task which is extremely difficult to attain due to the constant motion of the brush layer. The polymer chains protect and screen the membrane surface underneath."

Another factor in preventing adhesion is the surface charge of the membrane. Cohen's team is able to choose the chemistry of the brush layer to impart the desired surface charge, enabling the membrane to repel molecules of an opposite charge.

Next Step

The team's next step is to expand the membrane synthesis into a much larger, continuous process and to optimize the new membrane's performance for different water sources. "We want to be able to narrow down and create a membrane selection system for different water sources that have different fouling tendencies," said Nancy H. Lin, a UCLA Engineering senior researcher. "With such knowledge, one can optimize the membrane surface properties with different polymer brush layers to delay or prevent the onset of membrane fouling and scaling.

"The cost of desalination will therefore decrease when we reduce the cost of chemicals [used for membrane cleaning], as well as process operation [for membrane replacement]. Desalination can become more economical and used as a viable alternate water resource."

The UCLA team is currently carrying out specific studies to test the performance of the new membrane’s fouling properties under field conditions.

A paper detailing the new membrane, “Polymer surface nano-structuring of reverse osmosis membranes for fouling resistance and improved flux performance,” appears in the Journal of Materials Chemistry.

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