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Water is the most vital and ubiquitous resource for humanity. For this reason, its application and maintenance should be understood and treated with reverence as the most significant and valuable interaction between human beings and Mother Nature.

In 1903, the first Nobel prize in chemistry was granted to Jacob Hendricus van’t Hoff for his pioneering work in osmosis using a membrane, a filtering material that can reject salts from, for example, seawater. If two water streams of a low and high salt concentration, such as a river and an ocean near an estuary, are flowing across a membrane, water molecules spontaneously move from the river water to the seawater side across the membrane until the two concentrations become equal.

As a consequence, the pressure increases and the concentration decreases in the seawater side. To prevent this water transport from the low to the high concentration regions, a hydraulic pressure needs to be applied to the seawater side. This (minimum) pressure that completely stops the spontaneous water movement from the low to high concentration region is called the osmotic pressure.

In early 1962, President John F. Kennedy told the nation that, “If we could ever competitively, at a cheap rate, get fresh water from salt water, it would be in the long-range interests of humanity which would really dwarf any other scientific accomplishments.”

A few years later, Sydney Loeb and Srinivasa Sourirajan, two researchers at the University of California, Los Angeles, developed the first commercial reverse osmosis membrane. Applying hydraulic pressure, often two to three times higher than the seawater osmotic pressure, allows water molecules in seawater to move toward the freshwater side. This process is called seawater destination.

The largest reverse osmosis desalination plant in the world is located at Sorek, Israel, and its maximum daily production rate is 624,000m³ at a cost below 50 cents per ton. By contrast, the Board of Water Supply in Hawaii charges $4.42 for the first 13,000 gallons, equivalent to $3.53 per ton.

The replacement period of an reverse osmosis membrane is about three to five years depending on the pre-treatment quality of the input water (called feed stream). The state of Hawaii recently adopted the reverse osmosis technology, which will be installed in the Honouliuli wastewater treatment plant in the near future.

If 4 million gallons of seawater are treated using reverse osmosis with 60 percent of it being recovered, then 600 gallons of fresh water and 400 gallons of brine (having more than twice the higher concentration of seawater) will be produced. The drawback of the high-quality reverse osmosis technology is the high electricity bill for the high-pressure pumping and the brine disposal back to the ocean.

In the last two decades, there has been active research to reduce the reverse osmosis power consumption. One method is re-making the osmosis forward instead of reversing it. Instead of the river-water, if we use a draw solution that has a higher concentration that is more than twice that of the seawater concentration, the water molecules in the seawater will spontaneously move (from the seawater) to the draw solution.

Currently, membrane distillation is a promising method to achieve this goal. However, developing a hybrid separation system using both thermal distillation and membrane technology is a challenging task.

Reducing Environmental Impacts

If reverse osmosis brine and wastewater are used in the forward osmosis system as draw and feed solutions, respectively, then water transport will be from the wastewater to the brine, which will be diluted before subsequent disposal.

Therefore, we can reduce environmental impacts by the direct disposal of brine to the ocean. If a liquid stream of juice, wine, milk and so forth are inputted to the forward osmosis system, then water from these tasty but rather dilute solutions will move to the seawater side.

This leads us to ask the question of whether we can use this extra pressure to rotate a turbine and generate electricity. The fundamental answer is yes, but it requires new membrane materials for higher performance. This process is called pressure-retarded osmosis, on which research was started in the late 1980s, around the time that the forward osmosis was first proposed in food engineering.

Recently, the Natural Energy Laboratory of Hawaii Authority received a $2 million grant from the U.S. Department of Energy’s Solar Energy Technologies Office to prove that desalination using forward osmosis is effective. Their development can be a ground-breaking step to produce a stable amount of high-quality fresh water at a cheaper cost than pumping freshwater from an aquifer. It would be interesting to see the (almost) first plant-scale application of reverse osmosis at Honouliuli Wastewater Treatment Plant and forward osmosis-based desalination of NELHA at Kailua-Kona, Hawaii island.

Based on my knowledge from researching membrane science for two decades, I strongly believe that the ultimate solution for the global water scarcity won’t come from a single novel development of a brand-new technology, but rather it should start from the rigorous technical analysis of the current infrastructure, locally inexpensive energy resources including renewable ones, and the accurate cost-analysis of future water projects and maintenance.

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