10 Innovations in Water Purification

If we could tap the vast oceans as a source of drinking water, everyone would have more than enough. But that means removing the salt, which is inefficient and costly using existing technology. That's why a new process, developed by New Jersey Institute of Technology chemical engineering professor Kamalesh Sirkar, has such dazzling promise.

In Sirkar's direct-contact membrane distillation (DCMD) system, heated seawater flows across a plastic membrane containing a series of hollow tubes filled with cold distilled water. The DCMD's tubes have tiny pores, which are designed so that they can be penetrated by the water vapor which collects on them, but not by salt. The vapor diffuses through the pores and is drawn off, to be condensed again into liquid water.

According to Sirkar, his system is extremely efficient -- it can produce 80 liters (21 gallons) of drinking water per 100 liters (26 gallons) of seawater, about twice what existing desalination technology can produce. One potential downside of DCMD is that it requires a steady, inexpensive source of heat in order to prevent the water temperature on either side of the membrane from equalizing. But there's the possibility that DCMD systems could someday recycle waste heat from shore-based factories and offshore oil drilling operations, making it a win-win for everybody [source: Greenmeier].

Clay ceramic filters work in a fashion similar to the desalination technology described in the previous section. Basically, water flows through clay that contains a lot of really tiny holes, which are big enough to let water molecules though, but too small for biological contaminants, dirt, and other bad stuff [source: Doulton USA]. The first such device was developed by a British potter, Henry Doulton, back in the early 1800s for purifying water drawn from the Thames, which was so contaminated with raw sewage that cholera and typhoid were continual dangers [source: Brodrick].

Since Doulton, other inventors have made improvements to his basic concept, such as adding silver coatings to kill bacteria, so that today's ceramic filters do an even better job of getting rid of dangerous pathogens. The really revolutionary development, though, is that humanitarian non-governmental organizations have set up factories to make and give away large numbers of inexpensive ceramic filters in the developing world.

A 2006 study found that Cambodians who used the simple filters, which are portable and require no energy to run, reduced the incidence of diarrheal disease by 46 percent, and E.coli contamination in their water by 95 percent from 2003 rates [source: Resource Development International – Cambodia ]

One drawback with these ceramic filters is the speed of filtration. The water seeps out the clay filter at a rate of just 2 liters (2.11 quarts) per hour. But the process needs to be slow in order to give the silver solution time to kill pathogens. The filter also does not remove harmful chemicals like arsenic.

In the U.S., water companies add a small amount of fluoride -- between 0.8 and 1.2 milligrams per liter -- to drinking water as a way to protect teeth from decay. But in some parts of the world, including India, the Middle East and some African countries, water already has a lot of naturally-occurring fluoride, and the levels can be so high that they're dangerous to health. In one Indian village, for example, a naturally occurring level of 5 to 23 milligrams (.00017 to .008 ounces) per liter has caused residents to suffer severe anemia, stiff joints, kidney failure and stained teeth [source: World Health Organization].

Fortunately, Indian researchers offered a possible solution in a March 2013 International Journal of Environmental Engineering article. The researchers have developed a filter system that uses a common medicinal herb, Tridax procumbens, to absorb excess fluoride from drinking water. The plant, which has also been used to extract toxic heavy metals from water, attracts fluoride ions when water passes through it at a temperature of about 27 degrees Celsius (80.6 degrees Fahrenheit).

The filter could potentially provide an inexpensive, easy-to-use way of making water safe in places where the supply contains excessive fluoride. But it also may be used by people in the U.S. and other countries who don't like the idea of fluoride being added to their water [source: Science Daily].

Sand and gravel have been used to purify water for thousands of years, and in 1804, a Scotsman named John Gibb designed and built the first filter that strained water through grains of sand to remove bigger particles of contamination. His technology worked so well that pretty soon, London and other big cities in Europe were using rapid sand filters to make river water look clearer and taste better.

By the late 1800s, scientists figured out that filtering made water safer to drink as well, since the particles stopped by the filtering were the ones that helped to transmit the microbes that caused water-borne diseases. The value of filtering was demonstrated in 1892, when the city of Hamburg -- which got its drinking water from the River Elbe -- suffered a cholera epidemic that killed 7,500 people, while the neighboring city of Altona -- where water from the same river was filtered -- escaped almost untouched [source: Huisman and Wood].

But recently, researchers have figured out how to coat sand grains with graphite oxide to create "super sand" that can reportedly filter harmful contaminants, such as mercury, from water five times as effectively as ordinary sand. Efforts continue to find ways to make super sand absorb even more dangerous contaminants, and eventually use it in developing countries where water supplies are heavily polluted [source: Science Daily].

If you've seen the 1940s cinematic black comedy "Arsenic and Old Lace," in which a couple of well-meaning spinsters take it upon themselves to put lonely old men out of their misery by giving them elderberry wine laced with arsenic, you know that the latter substance is pretty bad stuff. When it contaminates drinking water, arsenic can cause bladder, lung and skin cancer, as well as harm the nervous system, heart and blood vessels [source: National Resources Defense Council].

Unfortunately, almost 100 million people in developing countries today are exposed to dangerously high levels of arsenic in their surface water, and they can't afford the complex, expensive water treatment plants used in the U.S. to get rid of it. However, a new technology may offer a solution. Monmouth University (N.J.) chemistry professor Tsanangurayi Tongesayi has developed an inexpensive arsenic-removing system in which chopped-up pieces of ordinary plastic beverage bottles are coated with cysteine, an amino acid.

When the plastic pieces are added to water, the cysteine binds to the arsenic, removing it and rendering the water drinkable. In tests, he's been able to take water containing dangerous arsenic levels of 20 parts per billion, and reduce it to 0.2 parts per billion, which meets the U.S. Environmental Protection Agency's standard [source: Science Daily]

In impoverished countries where people can't afford to build expensive water purification plants, they sometimes rely upon a free resource -- sunlight. A combination of heat and ultraviolet radiation from the sun will wipe out most of the microbes that cause diarrhea, an ailment that claims the lives of 525,000 children in Africa every year [source: WHO]. One complication: In order for the process to work, the water has to be clear, which is a problem in rural areas where people get their water from rivers, streams and boreholes that yield water filled with suspended clay particles.

But Joshua Pearce, an associate professor of materials science and engineering at Michigan Technological University, and colleague Brittney Dawney from Queens University in Ontario have a solution. In a 2012 article in the Journal of Water, Sanitation and Hygiene for Development, they proposed a solar disinfection regimen that first treats the water with a process called flocculation.

The process involves adding a small amount of table salt is added to the water to draw out the clay. While the resulting drinking water has higher levels of salt than Americans are used to, it's still got less in it than Gatorade. "I've drunk this water myself," Pearce said in an interview. "If I were somewhere with no clean water and I had kids with diarrhea, and this could save their lives, I'd use it, no question" [sources: Science Daily, Dawney and Pearce].

For travelers in developing countries, exposure to unsafe water can be a big risk. Wouldn't it be great if you could just dip a magic wand into water and purify it? Now, essentially, you can. A handheld device called the SteriPEN, marketed by Katadyn Group in Switzerland, uses ultraviolet light to eradicate disease-causing microorganisms.

The device employs the same purification technology used by bottled-water plants, but it's been miniaturized, so that it weighs just 6.5 ounces (184 grams) and fits into a backpack. Stick it into a liter of stream or pond water for 48 seconds, and voila -- it's safe to drink [source: Stone]. Such a portable water purification process can destroy bacteria, viruses, and protozoa, such as giardia and cryptosporidium, which can cause sickness [source: New York Times].

The big market for SteriPENS is backpackers and travelers, but they're also used by the U.S. military. SteriPEN also has donated some of the devices to game wardens who have to work in remote wilderness areas where they don't have access to tap water [source: Stone]. One caveat with ultraviolet purification: Water that's cloudy must be pre-filtered first in order to remove particles that are in suspension [source: Centers for Disease Control and Prevention].

Slow sand filters are a convenient, inexpensive way to purify water in developing countries. But a University of Virginia-based nonprofit humanitarian organization called PureMadi -- "Madi" is the Tshivenda South African word for "water" -- has come up with an additional, easy-to-use technology that can purify a container of water simply by being immersed in it [source: Samarrai].

The MadiDrop is a small ceramic disk, about the size of a hamburger patty, which contains microbe-killing silver or copper nanoparticles. Nanoparticles are basically really, really tiny objects specially designed by scientists to behave as a single unit [sources: Samarrai, Mandal].

The MadiDrop is cheaper, easier to use, and easier to transport than the larger ceramic flowerpot filters (pictured on the first page) that PureMadi already is making, according to James Smith, a civil and environmental engineer who is one of the project's leaders. The one downside, again, is that the MadiDrop doesn't remove suspended particles that make water cloudy. So ideally, users will put water through a two-step purification process, by first using the flowerpot filter to get rid of sediment and then eradicating the microbes with MediDrop [source: Samarrai].

Many of us probably think of algae as that gross stuff that we have to clean out of our fish tanks every now and then, but they can be a serious threat to health as well. Blooms of blue-green algae, called cyanobacteria, are found in both fresh and salt water throughout the world. They produce toxins called microcystins which are easily ingested by people who drink, swim or bathe in water that's contaminated with them.

Once microcystins get into your body, they can attack your liver cells. That's obviously not something that you want to happen. Unfortunately, conventional water treatment processes, such as chlorination or a sand filtration process, don't get rid of these tiny menaces. That's why a new water purification process developed by researchers at Scotland's Robert Gordon University has so much promise.

The researchers have identified more than ten different strains of bacteria that like to have microcystins for lunch, and are capable of metabolizing them so that they break down into harmless, non-toxic materials. If the algae-killer bacteria are introduced into water sources, they should be able to get rid of the microcystins and make the water safe to drink without using any potentially harmful chemicals [source: Science Daily].

We've already mentioned an innovative new device, the MadiDrop, which utilizes silver or copper nanoparticles to make water safe for human consumption. But nanotechnology -- that is, the engineering of really, really small objects and structures, smaller than the width of a human hair -- has a lot more potential to help clean up the world's drinking water.

Researchers at India's D.J. Sanghvi College of Engineering say that filters fashioned from carbon nanotubes and alumina fibers, for example, could be capable of removing not just sediment and bacteria, but even traces of toxic elements such as arsenic.

One advantage of using nanofilters, as they're called, is that they're more efficient than conventional water filtration systems, and don't require as much water pressure. And even though their pores are a lot smaller than conventional filters, they have a similar or faster flow rate [source: Science Daily]. Plus, they could be fitted with chlorine dioxide for additional disinfection.

At Massachusetts Institute of Technology, researchers are even looking at using nanotechnology for desalination. They're experimenting with using sheets of graphene, a form of carbon that's just a single-atom thick, to filter seawater. With nanotechnology, it's possible to create sheets filled with miniscule holes, just a billionth of a meter thick, which can block particles of salt but allow water molecules to pass through [source: Chandler].