Better desalination technology key to solving world's water shortage

Over one-third of the world's population already lives in areas struggling to keep up with the demand for fresh water. By 2025, that number will nearly double. Some countries have met the challenge by tapping into natural sources of fresh water, but as many examples -- such as the much-depleted Jordan River -- have demonstrated, many of these practices are far from sustainable.

A new Yale University study argues that seawater desalination should play an important role in helping combat worldwide fresh water shortages -- once conservation, reuse and other methods have been exhausted -- and provides insight into how desalination technology can be made more affordable and energy efficient.

"The globe's oceans are a virtually inexhaustible source of water, but the process of removing its salt is expensive and energy intensive," said Menachem Elimelech, a professor of chemical and environmental engineering at Yale and lead author of the study, which appears in the Aug. 5 issue of the journal Science.

Reverse osmosis -- forcing seawater through a membrane that filters out the salt -- is the leading method for seawater desalination in the world today. For years, scientists have focused on increasing the membrane's water flux using novel materials, such as carbon nanotubes, to reduce the amount of energy required to push water through it.

In the new study, Elimelech and William Phillip, now at the University of Notre Dame, demonstrate that reverse osmosis requires a minimum amount of energy that cannot be overcome, and that current technology is already starting to approach that limit. Instead of higher water flux membranes, Elimelech and Phillip suggest that the real gains in efficiency can be made during the pre- and post-treatment stages of desalination.

Seawater contains naturally occurring organic and particulate matter that must be filtered out before it passes through the membrane that removes the salt. Chemical agents are added to the water to clean it and help coagulate this matter for easier removal during a pre-treatment stage. But if a membrane didn't build up organic matter on its surface, most if not all pre-treatment could be avoided, according to the scientist's findings.

In addition, Elimelech and Phillip calculate that a membrane capable of filtering out boron and chloride would result in substantial energy and cost savings. Seventy percent of the world's water is used in agriculture, but water containing even low levels of boron and chloride -- minerals that naturally occur in seawater -- cannot be used for these purposes. Instead of removing them during a separate post-treatment stage, the scientists believe a membrane could be developed that would filter them more efficiently at the same time as the salt is removed.

Elimelech cautions that desalination should only be considered a last resort in the effort to provide fresh water to the world's populations and suggests that long-term research is needed to determine the impact of seawater desalination on the aquatic environment, but believes that desalination has a major role to play now and in the future.

"All of this will require new materials and new chemistry, but we believe this is where we should focus our efforts going forward," Elimelech said. "The problem of water shortage is only going to get worse, and we need to be ready to meet the challenge with improved, sustainable technology."

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by Yale University, via EurekAlert!, a service of AAAS.

Journal Reference:

Menachem Elimelech, William A. Phillip. The Future of Seawater Desalination: Energy, Technology, and the Environment. Science, 5 August 2011: Vol. 333 no. 6043 pp. 712-717 DOI: 10.1126/science.1200488

New desalination process developed using carbon nanotubes

A faster, better and cheaper desalination process enhanced by carbon nanotubes has been developed by NJIT Professor Somenath Mitra. The process creates a unique new architecture for the membrane distillation process by immobilizing carbon nanotubes in the membrane pores. Conventional approaches to desalination are thermal distillation and reverse osmosis.

"Unfortunately the current membrane distillation method is too expensive for use in countries and municipalities that need potable water," said Mitra. "Generally only industry, where waste heat is freely available, uses this process. However, we're hoping our new work will have far-reaching consequences bringing good, clean water to the people who need it."

The process is outlined by Mitra and his research team in the current issue of the American Chemistry Society's Applied Materials & Interfaces. Doctoral students Ken Gethard and Ornthida Sae-Khow worked on the project. Mitra is chairman of the department of chemistry and environmental science.

Membrane distillation is a water purification process in which heated salt water passes through a tube-like membrane, called a hollow fiber. "Think of your intestines," said Mitra. "It's designed in such a way that nutrition passes through but not the waste." Using a similar structure, membrane distillation allows only water vapor to pass through the walls of the hollow tube, but not the liquid. When the system works, potable water emerges from the net flux of water vapor which moves from the warm to the cool side. At the same time, saline or salt water passes as body waste would through the fiber.

Membrane distillation offers several advantages. It's a clean, non-toxic technology and can be carried out at 60-90oC. This temperature is significantly lower than conventional distillation which uses higher temperatures. Reverse osmosis uses relatively high pressure.

Nevertheless, membrane distillation is not trouble free. It is costly and getting the membrane to work properly and efficiently can be difficult. "The biggest challenge," said Mitra, "is finding appropriate membranes that encourage high water vapor flux but prevent salt from passing through."

Mitra's new method creates a better membrane by immobilizing carbon nanotubes in the pores. The novel architecture not only increases vapor permeation but also prevents liquid water from clogging the membrane pores. Test outcomes show dramatic increases in both reductions in salt and water production. "That's a remarkable accomplishment and one we are proud to publish," said Mitra.

Another advantage is that the new process can facilitate membrane distillation at a relatively lower temperature, higher flow rate and higher salt concentration. Compared to a plain membrane, this new distillation process demonstrates the same level of salt reduction at a 20°C lower temperature, and at a flow rate six times greater.

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by New Jersey Institute of Technology.

Journal Reference:

Ken Gethard, Ornthida Sae-Khow, Somenath Mitra. Water Desalination Using Carbon-Nanotube-Enhanced Membrane Distillation. ACS Applied Materials & Interfaces, 2011; 3 (2): 110 DOI: 10.1021/am100981s