State Water Resources Research Institute Program

Project ID: 2009DE157B
Title: Direct Contact Membrane Distillation
Project Type: Research
Start Date: 3/01/2009
End Date: 2/28/2010
Congressional District: At-Large
Focus Categories: Water Supply, Treatment, Water Use
Keywords: Desalination, technologys, membrane distillation
Principal Investigators: Dentel, Steven K (University of Delaware); Bryan, Victoria
Federal Funds: $ 1,500
Non-Federal Matching Funds: $ 3,000
Abstract: Fresh water is one of our most crucial resources. Its sources are limited in many places, including Delaware, nationally and internationally; only 2.5% of the total water on the Earth is fresh water, and the majority of that is found in glaciers and polar ice caps. The supply of fresh water is decreasing and, left unchecked, will continue to do so given the trends in world population growth. The demand for fresh water already exceeds the supply in many parts of the world. This is why it is so important to invest in new sources of fresh water, specifically desalination. Desalination is a process by which saline water (generally sea water) is converted to fresh water. The most common methods of desalination include flash distillation, reverse osmosis, and electrodialysis, but these are expensive compared to most alternatives and are therefore not practical in most situations. A promising new method of desalination, called direct contact membrane distillation (DCMD), provides a more cost-effective and convenient alternative by using waste heat to remove salt from water.

In membrane distillation, feed and permeate compartments are separated by a hydrophobic membrane. A temperature difference between two compartments leads to differing water vapor pressures, causing water vapor transport across the membrane. Vapor transport from the warm compartment produces distilled water in the cooler compartment. Since it is possible to warm the compartment well below the boiling point of feed, the use of low-grade thermal energy is possible. In many situations, this energy is available at minimal cost, so membrane distillation will not be as costly as typical desalination processes which are more energy intensive. Whereas reverse osmosis - pressure driven process relies on disproportionately higher pressure gradients to induce solvent flux at higher solute concentrations, DCMD's thermally-driven process shows only minor decreases in flux even at brackish concentrations of dissolved salts. Especially in the face of increasing energy costs, this translates into cost-savings over the traditional methods of desalination.One very interesting application of DCMD might be for the desalination of the vary saline waters that are found in very deep aquifers (over 1000m). These waters are also at the high temperatures (greater than 55oC) necessary to drive the DCMD process. The energy needed to pump the water to the surface should be much less than the pressure head based solely on the depth due to the subsurface pressure at the source water depth. The balance between desalination requirements and available temperature gradient should be examined as a function of depth to determine the best combination.

This project will be performed by Victoria Bryan, under advisement by Dr. Steve Dentel. The Dentel group is currently working in conjunction with United Technologies Research Centre on strictly industrial applications of DCMD technology as an alternative to reverse osmosis. However, the direction of the research is currently focused on priority industrial markets, such as flue-gas desulfurization and cooling tower blowdown to gauge whether or not the technology can be commercially successful upon launch. Considering the need of potable water in many areas of the world, the use of DCMD with deep aquifer sources may represent a unique low-energy water purification option. This includes the state of Delaware, where water supply limitations have led to consideration of reverse osmosis for conversion of saline water to potable water. The DCMD process represents a realistic alternative to reverse osmosis, and the use of deep aquifers would avoid water rights issues that are contentious in some areas.

Progress/Completion Report, 2009, PDF

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