It has been said that “water is the next oil” [1]. Just like oil, water — specifically, clean drinking water — is a resource that is rapidly depleting. Every year, 1.2 billion people do not have access to safe drinking water, and millions of people die, including almost 5000 children a day, from various waterborne diseases [2]. These numbers are increasing as the world population keeps growing.
Are we really running out of water?
Unlike oil, the total volume of water on earth (estimated to be ~ 1020 gallons) is constant [3]. In principle, water is a “renewable” resource as it never leaves the water cycle: it simply gets shuffled around through the processes of evaporation, condensation, and precipitation. If one looks at the statistics for water consumption, one realizes that there is, in fact, plenty of fresh water available. Out of the ~ 1018 gallons of fresh water on Earth, the average global water footprint (the amount of water used each year) is only 1015 gallons [3,4]. As long as this water cycles quickly enough, we should not run out of it, at least not globally.
So what is the challenge really about?
First, demand for water expands much faster than population growth. It is not difficult to project that at some point in time our consumption of water will overtake what is available. In fact, scientists recently predicted that water demand could exceed water supply in just 2 decades [8].
Second, water is not always distributed to where it is needed in an efficient manner (Fig. 1). Canada, for example, has ample supplies of clean water; but the Middle East and North Africa are constantly in need. Re-routing large volumes of water from regions with an excess of water to regions of water scarcity is possible but difficult, and requires large-scale changes in infrastructure.
Third, pollution and contamination can make water unusable. Various agricultural and industrial processes introduce toxic substances, such as animal wastes, pathogens and heavy metals, into water sources (e.g. rivers, lakes). Contamination like this effectively lowers the supply of fresh water because natural water cycles are not fast enough to efficiently clean the contaminated water.
Figure 1. Areas of physical water scarcity (click to expand). Source: [5]
Providing Clean Water
It is possible to purify dirty water and make it safe for human consumption. Many countries already do this with centralized water treatment plants. However, it is difficult to provide clean water where such infrastructure is absent. New developments in nanotechnology may make it possible to purify water on a much smaller scale, without the need for centralized water treatment plants.
Researchers have recently come up with a new way to sterilize water using nanofiltration devices [6]. These devices are made out of carbon nanotubes and silver nanowires coated on cotton. They are “nano” because the size of the wires and tubes is roughly a billion times smaller than a meter. The cotton backbone of the filter provides mechanical support to the nanowires and nanotubes, while the carbon nanotubes dispersed throughout the cotton backbone make the whole filter electrically conductive. Silver is known to be antibacterial. Silver nanowires dispersed in the filter thus act as an anti-bacterial mesh. Electricity is also applied to these nanowires creating strong electric fields at the tips of the wires that further assist in killing bacteria.
This device kills ~98% of the bacteria in water. It can run on a battery consuming only one fifth the energy required to operate standard filtration systems. It can thus be an attractive candidate for disinfecting water in resource-limited areas.
Figure 2. Schematic of a filter incorporating silver nanowires (AgNWs) and carbon nanotubes (CNTs) in cotton. Source:[6]
From salt water to fresh water
Given the large amount of water available in the oceans, turning salt water into potable fresh water seems to be an obvious solution to the water shortage problem. Salt water is salty because it contains many electrically charged particles called ions. Desalination, the removal/separation of ions from water, is not simple, however. A common method for desalination operates by pushing water through a membrane filter that allows only water molecules to pass through, but not the ions. This process requires a lot of energy and money to install and operate, and therefore is usually designed to process large volumes of water (500 million cubic meters of fresh water every year). Efficient smaller-scale desalination systems that operate at the village level with minimal infrastructure and energy sources are rare.
Recently, researchers have developed a small-scale, portable desalination device made possible by advances in nanotechnology [7]. Instead of using a physical membrane that blocks ions, the filter is more like an electrical barrier that repels ions. Only neutral water molecules can pass this barrier to be collected as fresh water.
The electrical barrier is made from a nanoscale fluidic channel. When the walls of this nanochannel are negatively charged and its diameter is small enough, only positively charged ions (cations) can pass through it. Negatively charged ions (anions) build up at the entrance to the nanochannel. These anions repel other ions from entering this region, thus creating an electrical barrier that only neutral water molecules can pass through. The water that passes through this barrier is therefore collected as desalinated, fresh water.
This device is the first of its kind to effectively desalinate small volumes of water (~12 liters/hour) in a portable device that can be operated in places with limited access to basic infrastructure. The efficiency of recovering fresh water is ~ 50%, close to that of large scale desalination plants. The energy needed to run this device now is slightly more than that needed by standard methods, but can be reduced if multiple devices are used in parallel. The biggest advantage of this system is that it requires minimal infrastructure for operation.
Looking ahead
In a way, the water problem is more serious than the oil problem: we are starting to find alternative energy sources to replace oil, but there is no substitution for water because all living things need water to survive. Most waterborne diseases and related deaths occur in developing countries that lack the basic infrastructure for cleaning water. What is critically needed is small to medium scale methods for purifying water in a cost-effective way. Instead of cleaning water in a centralized location and distributing it, the more practical option may be to clean water right where it is needed. Current efforts have already resulted in new water purification concepts that are smaller in scale and cheaper to run than traditional centralized treatment plants. Nanotechnology thus holds great promise to tackle the global water challenge.
Sindy K. Y. Tang is a postdoctoral fellow at the Wyss Institute for Biologically Inspired Engineering and School of Engineering and Applied Sciences at Harvard University.
References
1. http://www.forbes.com/2010/03/22/disease-drought-conservation-technology-ecotech-water.html
2. Nanotechnology, water and development (http://sites.merid.org/nano/waterpaper/NanoWaterPaperFinal.pdf); Montgomery et al., “Water and sanitation in developing countries: including health in the equation. Environ.” Sci. Technol. 41, 17–24 (2007).
3. http://ga.water.usgs.gov/edu/earthwherewater.html
4. According to http://www.waterfootprint.org, “water footprint is an indicator of freshwater use that looks at both direct and indirect water use of a consumer or producer. Water use is measured in terms of water volumes consumed (evaporated) and/or polluted per unit of time.”
5. http://maps.grida.no/go/graphic/areas-of-physical-and-economic-water-scarcity
6. Schoen et al., “High Speed Water Sterilization Using One-Dimensional Nanostructures” Nano Lett. 2010, 10, 3628–3632
7. Kim et al., “Direct seawater desalination by ion concentration Polarization” Nature Nanotechnology 5, 297 – 301 (2010)