Following a major disaster, the first resource that begins to run out is usually fresh, clean, pure water. Even in areas like Haiti or Sri Lanka, which are surrounded by water, the amount of drinkable water is very low, because the salt water from the surrounding oceans is undrinkable. Traditional desalination methods require large operating plants that are too large and complicated to transport to the disaster-stricken location, and would require more electricity than a developing country could provide. Following the earthquake in Haiti, graduate students led by Steven Dubowsky at the Massachusetts Institute of Technology set out to design a compact, self sufficient desalination system that rescue workers could take with them into the field and operate in all kinds of harsh conditions. The system would also have to be simple enough for people unfamiliar with engineering to assemble and operate. The desalination system would combine the efforts of mostly electrical, chemical, computer, and mechanical engineers to develop an operational device which could be used to purify salt water.
The first hurdle that the design team had to face was which method of desalination to use. The three main methods of purifying salt water are reverse osmosis, vacuum distillation, and multi-stage flash distillation. In reverse osmosis, a force is applied to a solution with particles in it, such as water with salt. This force pushes the solution up against a selective membrane, with pores large enough to allow the solvent (water) through, but too small to allow the solute (salt) through. Vacuum distillation is essentially boiling the salt water and collecting the evaporated fresh water when the salt is left behind. Multi-stage flash distillation is the process most large desalination plants use in their purification process. This process consists of evaporating the salt water in different stages at gradually increasing temperatures and pressures to optimize the distillation process. The engineers considered reverse osmosis to be the method best suited to their needs, because with some tweaking to the engineering of the process, it could be done using much less energy than the other two processes, and would be much safer as well as simpler.
Once they had decided on a process, the next question was how to power it. Most of the developing countries where the system was expected to see most of its use would not be able to supply large amounts of reliable power. Because of this, the team had to create a self-sufficient device. This job fell primarily to electrical and chemical engineers. The possible choices were to draw power from some renewable energy source, such as solar, water, or wind power, or to rely on batteries. Batteries were quickly eliminated as an option because they would mandate a certain amount of time the system would work for before it had to be brought back to recharge, which might not always be possible. Of the possible types of renewable energy, solar power is by far the most reliable, because the device would probably not be operated near moving water, due to the nature of its work, and it would be very hard to generate enough power from a wind turbine that was also small enough to be easily transported. The problem with solar cells is that when the sky is cloudy or overcast, they generate much less electricity. Weather conditions could therefore inhibit the desalination process. Computer engineers stepped in with a partial solution to this problem. With the help of an onboard microchip, the pumps could be programmed to move different amounts of water based on how much power they were receiving. In this manner, the desalination system could always maximize whatever energy it was able to derive from the sun. What this means is that when there is less sun, less fresh water would be produced. While this method is not perfect, it is the most effective of the available options.
After all the systems and processes are in place, the only component left to design is the physical form factor or the system. Arguably, this is the most difficult part of the design process for this particular problem. Limitations on the design include that everything must be able to withstand severe weather conditions, operate in all kinds of terrain, be easily portable, easy to disassemble and reassemble, and most importantly, easy to understand by people without any engineering knowledge. This last condition is by far the most crucial, because even if the desalination system is perfectly engineered it is useless if rescue teams cannot operate it. This last problem requires the work of all the different engineers involved with the project. Electrical engineers work on the wiring and solar panels, computer engineers work on the processor, chemical engineers work on the desalination devices, and mechanical engineers work with the physical components and how they interact. The team at MIT is currently at the stage of testing different prototypes in many different locations to find the most effective one. A possible solution to this problem would be to have rescue teams specifically trained to assemble and operate the water purification systems. This idea is flawed, however, because the goal is to ship the units with nothing but a set of instructions to disaster areas where they can be assembled by any rescue team. Another possible solution would be a modular design, which would allow the unit to be assembled in whatever shape best suited the situation, and parts could be connected by hoses with colored numbers indicating which parts should be attached and where they should be connected. This design, however, would probably not be very durable. Ultimately, the more durable the machine is, the harder it will be to take apart and put together, and vice versa. It will be up to the engineers to decide what combination of durability and flexibility best suits their needs.
A possible improvement that could be made over the current design is in the water filtration method. While reverse osmosis is probably the most effective method that is currently fully useable, there is research going on into other, better methods. One of the most competitive of new desalination technologies is actually being developed by researchers at MIT as well. This new method is called ion concentration polarization. In this method, water is processed through very small devices that operate on the microscopic level to clean the water. While just one processor is not enough to clean any significant amount of water, with thousands of them operating at once the system becomes quite feasible. With ion concentration polarization, the water purification portion of the desalination operation would take up much less space, and part of the device would be driven by gravity. When salt is filtered out in this way, bacteria and other contaminants are also filtered, eliminating the need for another machine to purify the fresh water. Currently, ion concentration polarization takes more energy than reverse osmosis, but the researchers at MIT and more in Korea are making large strides towards making the technology cheap and efficient. Because the water processors would be very small and all part of one small device, the overall size and complexity of the entire system would be much smaller and simpler, respectively. The water processor could be made very durable and would easily fit into a modular design, giving users the best of both worlds with flexibility and durability.
The potential benefits of an efficient desalination system are immeasurable. With a steady fresh water supply already in place in a disaster area with no need for replenishment, resources could be diverted from moving water into the area to bring other essentials in, such as medical equipment. The desalination systems would be useful in areas other than disaster locations as well, as they could be used in any environment suffering from a lack of water. According to the research team at MIT, a large unit could produce about one thousand gallons of water every day, and one C-130 cargo plane could carry twenty-four units. Just one shipment of units could provide water for ten thousand people, and one unit could keep an entire village afloat. If ion concentration polarization were used, this water would also be safe to drink without any further treatment, potentially saving thousands of lives in third-world countries.
The portable desalination system is a very complex and difficult problem to solve. It requires the input of all kinds of engineers, and professionals from different fields as well, such as meteorology for weather predictions and effects and anthropologists to facilitate relations with the people using the systems. After all the obstacles are overcome, however, the value of the system to both the environment and the local population is limitless. Hopefully, the few remaining obstacles can be resolved to help those in need as quickly as possible.
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