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Novel Desalination Process Promises Lower Energy Cost, Higher Recovery, and Less Brine Discharge

New Haven, CT — Yale researchers are building a pilot-scale plant to demonstrate a novel forward osmosis desalination process. The project, led by Professor Menachem Elimelech and graduate students Robert L. McGinnis and Jeffery R. McCutcheon, will feature a process that differs from existing desalination technologies in that it uses osmotic pressure, rather than hydraulic pressure or thermal evaporation, to separate fresh water from a seawater or brackish water source. This approach promises significant reductions in energy consumption and cost, as well as high feed-water recoveries and greatly reduced brine discharge streams.

How it works
The key to the ability of the forward osmosis (FO) process to achieve efficient desalination is in the composition of the osmotic “draw” solution used. It is a well-known fact that water will flow from a dilute to a concentrated solution, (when these solutions are separated by a semi-permeable membrane), and that a very concentrated solutiosn will draw water from a brackish or seawater saline source. The difficulty of utilizing this phenomenon in practice has been identifying a concentrated solution that contains solutes which can be efficiently and entirely removed. A concentrated sugar solution could be used, for instance, to effect desalination of brackish water, but this would result only in a less concentrated sugar solution, not fresh water.

The FO process developed at Yale uses a unique group of removable solutes to create a draw solution for desalination (1). When ammonia and carbon dioxide gases are dissolved in water in the correct proportion, they favor the formation of a highly concentrated solution of ammonium salts. This solution can have a very high osmotic pressure, which makes it ideal for drawing water from saline feeds, but what makes this solution most advantageous for use in FO is the ability of the salts to decompose from solution, when heated, into ammonia and carbon dioxide gases again, thus allowing for their efficient and complete removal and reuse (Figure 1). This process is therefore both a membrane and a thermal process, such that the separation is achieved using a semi-permeable membrane, but the energy used for that separation is in the form of heat.

Energy Cost
The heat used by the FO process can be minimized in quantity, minimized in cost, or set to some balance between the two. In most cases, FO will use less than half of the thermal energy needed by multi effect distillation (MED), the most efficient existing thermal desalination technology. FO has the further ability to use heat at much lower or higher temperatures than MED or MSF. FO can use heat as low in temperature as 40?C, which is just above that typical of steam entering condensers in an electrical power plant. At this temperature, the cost of thermal energy is very low. FO can also use higher temperature heat sources, which gives the benefit of greatly reducing the total amount of heat required. At 200°C to 250°C, for instance, FO may achieve a Gained Output Ratio (GOR, commonly used to compare thermal desalination efficiencies, with higher numbers indicating less heat used), of close to 30. MED desalination typically has a GOR of 8-12, depending on its configuration.

The electrical consumption of FO is also much lower than that of existing desalination technologies. In most cases, FO requires less than 0.25 kWh/m 3 of water produced. This is approx. 21% of what is required by MED and 9% of what is required by RO. The combination of low cost heat and minimal electrical consumption promises to make FO the best desalination process with respect to energy cost.

High Recovery
The high osmotic pressures characteristic of the ammonium salt draw solution allows for fresh water recovery from highly concentrated saline feeds. Laboratory tests show effective desalination of 3.4 M NaCl solutions, a salinity corresponding to approx. 85% recovery from a typical seawater source. Realization of this potential will require investigation of appropriate pre-treatment strategies, but the capability of FO to produce very high recoveries is unmatched by existing desalination methods. One significant impact of this increased capability will be the reduction in the volume of brine discharge streams from desalination plants. In the case of brackish water desalination, the very high recoveries made possible by FO may allow for zero liquid discharge (ZLD) operation, a capability critical to the adoption of desalination in inland environments. ZLD may also one day be possible in seawater desalination, provided that a use can be found for the large quantities of salt that would be produced.

The combination of low energy costs, high feedwater recoveries, and minimized brine discharge promise to make FO a highly useful desalination process. It is expected that the total water cost of FO will also be much lower than that of RO or MED, but only pilot testing will provide the necessary information for detailed estimates of total cost. Yale researchers have begun the construction of the FO desalination pilot, with funding provided by the Office of Naval Research, with completion expected by Spring of 2007.