Water is a renewable resource, but it is not an infinite one. All around the globe, various communities and entire nations struggle to find an adequate supply of fresh water.
Just recently in the U.S., the California government declared a State of Emergency after four continuous years of drought and placed restrictions on water use throughout the state. Since 2011, two of the largest reservoirs in California have had their surface areas decrease by half. Even more worrying in that region is the status of Lake Mead, the primary aquifer for 20 million people, which has been in a drought for the last 14 years. The reservoir has dropped to about two-fifths of its full capacity and water levels are currently only 80 feet away from rendering two major intake tunnels—which supply water to California, Arizona, Nevada, and Mexico—useless.
To solve this global problem of increased freshwater scarcity, scientists and engineers are looking for ways to better make use of the water available. There are two primary ways in which this can be done: remove the salt from (or desalinate) seawater, or recycle used water. Both desalination and water reclamation require the use of specialized filters, but current methods are expensive and energy intensive.
To solve this problem, scientists are working to create more efficient and cost-effective filter membranes using nanotechnology.
Historically, the challenge with water filtration has been an issue of scale. The particles suspended in water that make it dangerous to drink when left untreated are generally just too small to remove with current commercialized membrane technology. Even when it is possible to remove these particles, which include pathogens, bacteria, and certain ions (electrically charged atoms that make up salts), it costs a lot of money to supply the energy needed to power the process.
In order to be commercially viable for water treatment, the membranes within water filters must be permeable enough that large quantities of water can flow through easily and swiftly, yet small and selective enough that unwanted molecules, even as small as single-atom ions, are blocked from passing through.
Maintaining this delicate balance of permeability and rejection requires membranes that are made of highly engineered materials. Currently, scientists and engineers around the globe are researching ways to use nanotechnology—namely, carbon nanotubes—to improve upon current polymer membrane designs. Scientists hope that constructing membranes out of nanoscopic materials will improve the efficiency of removing these particulates as well as lower the cost of operation.
It is important that this membrane technology is adaptable for both desalination and water recycling because what is beneficial in one community may be impossible in another. Coastal areas like California that lack freshwater but have easy access to the sea could make full use of desalination to augment their water supply. However, in landlocked places like central Australia, it makes much more sense both environmentally and economically to recycle the water they already have, rather than try to transport desalinated water over long distances.
To get a better idea of how these membranes work, it is necessary to first understand current water treatment methods. One of the most common methods of desalination is Reverse Osmosis (RO), wherein pressure is applied to treated seawater, forcing it through a membrane that blocks salt ions from crossing. However, this process is expensive because it requires high amounts of energy to pressurize the water through the membrane.
Where desalination takes undrinkable water from nature and turns it into something that can be consumed, water recycling involves reusing contaminated water before it is returned to nature. Normally when water leaves a home or facility, it flows through the sewers until it eventually reaches a municipal water treatment plant. There, the wastewater undergoes multiple stages of treatment during which solid debris, microbes, and unwanted particulates are removed. After that, the treated water is generally released into a nearby river or lake, and any remaining contaminants become greatly diluted.
When water is recycled for direct use it is no longer expelled into a freshwater body, but instead undergoes an extended purification process to ensure that it is safe to use. During this process, filters are vital for removing contaminants from the water. These particulates, such as metal ions and bacteria, are extremely small but can pose health risks if found in large enough quantities. In order to effectively remove these particles from the water, filters that have membranes with extremely fine pores are needed.
The use of nanotechnology allows for a large degree of control over the filtration membrane design. Scientists can control the diameter of the carbon nanotubes within nanometer precision by varying the composition of the chemicals used to make them. A larger diameter might be desired during the earlier stages of microfiltration to block out bigger molecules. Filters with smaller diameters are more selective and, therefore, able to reject much smaller ions. This alterable feature allows for a more versatile range of filters that can be used in water purification.
Filters with smaller diameters are especially important in desalination. But, even nanotechnology has its limits. The radii of the ions that make up ocean salt are on the scale of Angstroms, which is about equivalent to the diameter of a Hydrogen atom and significantly smaller than nanotubes can currently be made. This means that scientists must do more than just decreasing diameter size in order to filter out unwanted ions.
One way to do this is by changing the chemical composition of the nanotube openings so that they have either a positive or negative charge. By changing the electric charge at the tips of carbon nanotubes, these membranes can be made to repel certain ions, acting as gatekeepers and letting only water pass through. Because salt contains both cations (positively charged) and anions (negatively charged), multiple membranes are needed for desalination: one to filter out the positive, one to filter out the negative.
In addition to the versatility of nanotube filters, these new membranes require less power than those currently used in energy-intensive water treatment applications, which use highly pressurized water. Carbon nanotubes can be made so that the inside of the tubes are hydrophobic, or water-repellent. This limits the amount of friction caused between the water molecules and the walls of the nanotube, allowing water to pass through easily.
At the moment, this technology is being tested out small-scale in laboratories and is not commercially viable. But, by continuing research, scientists and engineers plan on making these membranes operate on a large, commercial level. As more and more communities face water crises around the world, there will be a growing urge to fund such research. While many of these researchers are focusing on applications for desalination, this technology has a promising future in water recycling as well.
Ultimately, investing in membrane technology now will prepare us for a future where clean freshwater becomes an increasingly rare commodity. With improved membrane technology comes more ways that we can take full advantage of the water resources that we have.