Hold the salt: Engineers develop revolutionary new desalination membrane

April 6, 2010 By Wileen Wong Kromhout, University of California Los Angeles

(PhysOrg.com) -- The new reverse-osmosis membrane resists the clogging that typically occurs when seawater and brackish water are purified.

Researchers from the UCLA Henry Samueli School of Engineering and Applied Science have unveiled a new class of reverse-osmosis membranes for that resist the clogging which typically occurs when seawater, brackish water and waste water are purified.

The highly permeable, surface-structured can easily be incorporated into today's commercial production system, the researchers say, and could help to significantly reduce desalination operating costs. Their findings appear in the current issue of the Journal of Materials Chemistry.

Reverse-osmosis (RO) desalination uses high pressure to force polluted water through the pores of a membrane. While pass through the pores, mineral salt ions, bacteria and other impurities cannot. Over time, these particles build up on the membrane's surface, leading to clogging and membrane damage. This scaling and fouling places higher energy demands on the pumping system and necessitates costly cleanup and membrane replacement.

The new UCLA membrane's novel surface topography and chemistry allow it to avoid such drawbacks.

"Besides possessing high water permeability, the new membrane also shows high rejection characteristics and long-term stability," said Nancy H. Lin, a UCLA Engineering senior researcher and the study's lead author. "Structuring the membrane surface does not require a long reaction time, high reaction temperature or the use of a . The anti-scaling property, which can increase membrane life and decrease operational costs, is superior to existing commercial membranes."

The new membrane was synthesized through a three-step process. First, researchers synthesized a polyamide thin-film composite membrane using conventional interfacial polymerization. Next, they activated the polyamide surface with atmospheric pressure plasma to create active sites on the surface. Finally, these active sites were used to initiate a graft polymerization reaction with a monomer solution to create a polymer "brush layer" on the polyamide surface. This graft polymerization is carried out for a specific period of time at a specific temperature in order to control the brush layer thickness and topography.

"In the early years, surface plasma treatment could only be accomplished in a vacuum chamber," said Yoram Cohen, UCLA professor of chemical and biomolecular engineering and a corresponding author of the study. "It wasn't practical for large-scale commercialization because thousands of meters of membranes could not be synthesized in a vacuum chamber. It's too costly. But now, with the advent of atmospheric pressure plasma, we don't even need to initiate the reaction chemically. It's as simple as brushing the surface with plasma, and it can be done for almost any surface."

In this new membrane, the polymer chains of the tethered brush layer are in constant motion. The chains are chemically anchored to the surface and are thus more thermally stable, relative to physically coated polymer films. Water flow also adds 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," Cohen said. "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.

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," Lin said. "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."

Cohen's team, in collaboration with the UCLA Water Technology Research (WaTeR) Center, is currently carrying out specific studies to test the performance of the new membrane's fouling properties under field conditions.

"We work directly with industry and water agencies on everything that we're doing here in technology," Cohen said. "The reason for this is simple: If we are to accelerate the transfer of knowledge technology from the university to the real world, where those solutions are needed, we have to make sure we address the real issues. This also provides our students with a tremendous opportunity to work with industry, government and local agencies."

A paper providing a preliminary introduction to the new membrane also appeared in the Journal of Membrane Science last month.

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not rated yet Apr 06, 2010
They've added nano-fur ! That is ingenious !!
1 / 5 (1) Apr 06, 2010
I wonder how much the cost of RO can be reduced with this technology. RO is already nice and cheap relative to the cost of utility water, making it feasible as a standard utility method. A major improvement would make use even more widespread. It would help in locations which lack lots of skilled maintenance.
1 / 5 (1) Apr 07, 2010
stupid question here...wandering thought....whatever you want to call it...
But wouldn't it be cheaper just to boil the salt water and collect the vapor/condensation from it? Wouldn't that leave the salt in the pan and the water in the bag, so to speak?
Hmm, may have to do a home experiment...
not rated yet Apr 08, 2010
"..enabling the membrane to repel molecules of an opposite charge. "

Interesting, but perhaps its an error .. *same*-charges repel, no opposite ones. Error or not, if one charge is repelled, the particles with the different charge are attracted... Would this not lead to 'clogging'?

not rated yet Apr 08, 2010
stupid question here...wandering thought....whatever you want to call it...
But wouldn't it be cheaper just to boil the salt water and collect the vapor/condensation from it? Wouldn't that leave the salt in the pan and the water in the bag, so to speak?
Hmm, may have to do a home experiment...

There are no stupid questions. ;) .. only dumb answers .. But in this case, consider the cost of that 'boiling' .. that's why RO (which is under pressure) is more economical. Far less costly to run a pump than to buy/store/transport/burn fuel and then perhaps have to use energy to condense the vapor.

1 / 5 (1) Apr 08, 2010
that is true, I did not think of the boiling cost, as I was not thinking of it at a civilization level...merely a personal level.
I was thinking if I have a pan and an ocean, I can boil the ocean and make me some fresh water for myself and family...I was thinking very small scale...
1 / 5 (1) Apr 10, 2010

With sufficient sunlight, this process is workable with any kind of water- you only need enough sun to evaporate the water, and collect the condensate, which is distilled water.

Careful, though- distilled water is actually a strong solvent, and is known to leach essential minerals and nutrients out of the body, as it tries to attain it's ideal buffer/solute state.

You can address this by adding small amounts of calcium or some other readily-dissolved nutrient/mineral to it before consumption.

This truly is a giant step forward, if the membrane works as advertised, as it would finally open the door to real, industrial-scale desalinization, at a price point that might be affordable for those that need it the most.

Big Question: Would this work in a gravity-fed configuration?
1 / 5 (1) Apr 11, 2010
stupid question here...wandering thought....whatever you want to call it...
But wouldn't it be cheaper just to boil the salt water and collect the vapor/condensation from it? Wouldn't that leave the salt in the pan and the water in the bag, so to speak?
Hmm, may have to do a home experiment...

All you need is sufficient excess generating capacity and flashing the water to steam or even electrolysis becomes feasible, inexpensive and much less dependent on fragile technologies like RO.
not rated yet Apr 24, 2010
Big Question: Would this work in a gravity-fed configuration?

Depends on how deep you dig your hole, or how far under the ocean you place your filters.

Water pressure scales rather smoothly from the stratosphere to the bottom of a deep-sea trench (tho there is that nonlinear bit in the middle where you go 'splash').

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