Every day, people visit the toilet. Or, in developing nations, the field, pit or other location where they can relieve themselves. With more than 7 billion people on the planet, that’s a lot of waste. Mixed with water, these wastes are known as sewage. Because they host germs, they can’t just be left lying around. If they taint the water people use for eating, drinking and bathing, those germs can spread disease. So cleaning up the daiiy production of human wastes is an important part of keeping people healthy.
Most people don’t stop to think about what happens to these wastes as they get flushed down a toilet. But scientists do. Around the world, they’re doing more than just cleaning up our mess. They’re turning these wastes into the latest source of renewable energy.
People in Bristol, England, power the local bus from their trips to the toilet. In the United States, some airlines have begun using farm manure as an eco-friendly jet fuel. South Korea has a water treatment plant that uses human wastes to power itself. And people have even used pee to charge up a cell phone.
Once we get beyond the ick factor, sewage is full of energy waiting to be harnessed. And that’s why scientists are hard at work putting poop and pee to work in all sorts of ways.
Getting around town
When the GENeco Bio-Bus pulls up to a stop in Bristol, everyone knows it’s running on human waste: The bus is covered in tastefully drawn images of people sitting on toilets. Toilet wastes, along with discarded food, is used to create the gas that powers this bus. But first the end product of a good meal has to go through a series of steps before it’s ready to power people across town.
WESSEX WATER/JULIAN JAMES PHOTOGRAPHY
When flushed, sewage travels to the Wessex Water treatment plant. There the plant separates solid materials from the rest of the wastewater. The solid material, called sludge, is fed into ananaerobic digester. Inside this structure, bacteria chow down on — or digest — the waste. The bacteria work in an oxygen-free, oranaerobic, environment.As the bacteria eat the sludge, they release gas, which is mostly methane. Methane is a greenhouse gas. Such gases help to trap heat close to Earth’s surface. Their buildup in Earth’s atmosphere has been causing global warming. But when methane is captured, it can be burned as a renewable source of energy. It’s a type ofnatural gas, which burns cleaner than gasoline.
The sludge-digester plant captures methane from food waste using the same process. Then it combines gases from the two sources. This mix is used to fuel up the Bio-Bus.
Homes along the route that this bus takes produce enough waste to drive the bus a whopping 4.1 million kilometers (2.5 million miles) a year, says Mohammed Saddiq. He’s the managing director of GENeco. That’s far more than is needed to run the bus. So there’s plenty left over to heat nearby homes, as well.
Letting waste do the work
A wastewater treatment plant in Ontario, Calif., captures methane using a similar process. Run by the Inland Empire Utilities Agency, this plant purifies the gas and pipes it into a fuel cell. This device converts chemical energy into electric energy. The fuel cell is heated before the gas comes in. When the biogas (gas collected from the breakdown of waste) enters the cell, it reacts with oxygen. This produces water, carbon dioxide and electricity.
The electricity powers the water-treatment plant. Excess heat is routed back to the anaerobic digester. There it warms the sludge, which is good because warm microbes work faster.The fuel cell cuts down on the coal or other fossil fuels needed to run the plant, says Jesse Pompa. He is an engineer at Inland Empire. But in this case, the electricity the fuel cell creates doesn’t cover all of the water-treatment plant’s needs.
Such uses of biogas are common in wastewater treatment plants, says Perry McCarty. An environmental engineer, he works at Stanford University in Palo Alto, Calif. For decades, people have used anaerobic digesters to produce biogas for energy, he notes. But normally those digesters do not capture all of the energy stored in wastewater. That’s because all the waste in that water doesn’t settle out into sludge. Some is left behind.
Many wastewater treatment plants also use membrane bioreactorsto treat wastewater. These systems use oxygen-breathing bacteria — or aerobes — to clean the leftover water and break down any waste that remains in it. Most of these systems don’t produce methane to generate electricity. In fact, they useelectricity to add oxygen to the wastewater. That’s because the bacterial workers in this system require oxygen to break down theorganic wastes in the water.
PERRY MCCARTY/STANFORD UNIVERSITY
To cut the need for that extra electricity, McCarty and some colleagues in South Korea decided to create a system that wouldn’t require adding oxygen. They started with a metal cylinder filled with small (1 millimeter) bits of activated carbon. (Activated carbon is a type of carbon also used to clean up water for drinking and aquariums.) Then the engineers added anaerobic bacteria.These microbes colonized the pieces of carbon, covering each one. As wastewater flowed through the bioreactor, massive numbers of bacteria chowed down on the dissolved waste particles. While doing so, they generated methane.
McCarty and his team built a treatment plant in South Korea to test the process. And it worked better than the scientists had expected. The methane produced by this system, when added to the methane produced during the anaerobic digestion of sludge, is enough to power an entire wastewater-treatment plant. So it needs no fossil fuels. And the more waste the bacteria ate, the healthier they were, making the system self-sustaining.
Electricity from pee
Methane produced by bacteria is one source of energy people can harness from human wastes. But some types of bacteria produce electricity directly. Electricity is the movement of electrons through a circuit. As bacteria break down food, some types release electrons. Lucky for us, these bacteria love to dine on the stuff we flush down the toilet.
At the University of the West of England in Bristol, Ioannis Ieropoulos puts these bacteria to work. As a bioengineer, he uses living organisms to solve engineering problems. He grows the microbes on a mesh made from carbon fibers. There they form a biofilm.
Now, the engineers pipe in urine. As the bacteria chomp down on the organic matter in this liquid waste, they release electrons. These negatively charged particles then travel through the carbon mesh to wires. Such electricity-producing set-ups are known asmicrobial fuel cells. They provide electricity and can clean up wastewater.
IOANNIS IEROPOULOS/UNIVERSITY OF THE WEST OF ENGLAND, BRISTOL
How much power a microbial fuel cell can generate depends on its size (and the number of bacteria it houses). Little fuel cells hold just 50 milliliters (1.7 ounces) of urine. They produce only a little energy each. Big fuel cells — holding up to 40 liters (10.6 gallons) of urine — provide a lot more energy. But their size limits the number of places they can be used.Ieropoulos and his team have been working to make the fuel cells smaller and easier to carry. One goal is to have these fuel cells create enough electricity to power the lights in refugee camp tents and buildings. So far, the researchers have built many small units (each holds just 6.25 milliliters, or 0.2 ounces, of urine). Then they stack them up. The engineers have shown that a stack of 40 microbial fuel cells can charge up a cell phone battery. They also have used the fuel cells to light up bathrooms. Using 288 stacked fuel cells, the scientists powered the lights in a urinal on the Bristol campus. It took 432 to power a urinal that was used about 1,000 times a day at the nearby Glastonbury Music Festival.
The bacterial workers in these fuel cells are remarkably resilient, says Ieropoulos. Once they form a biofilm, they can last about 3 weeks without anyone stopping to visit the toilet and provide them with additional food. And when someone does, the bacteria jump to immediate action, generating electricity.
Ieropoulos is currently developing the technology so that it can be used pretty much anywhere. “We want to be able to drop it into a hole in the slums,” he says. There, it will clean wastewater, improving public health. At the same time, it could provide electricity for people who currently lack it.
Feeding a bacterial battery
Back at Stanford, environmental engineer Craig Criddle is also putting electron-producing bacteria to work. His team has created a microbial battery. Similar to the microbial fuel cell, bacteria inside his team’s new battery grow on carbon filaments.
They attach themselves to the filaments using nanowires. These tiny strings hold the bacteria in place. They also act as electrical wires to carry electrons away from the microbes.
XING XIE/STANFORD ENGINEERING
Engineers place the bacteria-coated filaments inside a small container filled with wastewater. As the bacteria dine on the waste, electrons pass through their nanowires into the anode (or electrically negative end of the battery). From there, they travel to an outside circuit where they can power something (such as a light bulb) or charge a portable battery.The microbial battery captures energy from waste more efficiently than a microbial fuel cell can. In the fuel cell, energy is lost when some electrons react with oxygen and hydrogen to make water, Criddle explains. But in the microbial battery, the electrons are transferred to an electrode that prevents such reactions from occurring. When the electrode fills up with electrons, the battery stops working. But the researchers simply remove the electrode, releasing the excess electrons into the air. When reinserted in the microbial battery, electrons flow freely and the battery goes back to work.
Like McCarty’s bioreactor, these batteries could be used to capture energy from dissolved particles in wastewater. Or they could be powered by food wastes. For example, manufacturers of fruit juice or soda might use these batteries to clean the water left over from making and bottling those drinks, Criddle says. At the same time, these batteries could provide power for the factory.
XING XIE/STANFORD ENGINEERING
The microbial battery probably won’t be portable. It won’t plug into a device the way a typical battery does. But it could be used to charge those types of batteries. It also could be used to capture energy from the bottom of the ocean, Criddle adds. That’s because the electricity-producing bacteria naturally live there.Mention powering lighting or laptops with pee or poop and most people shudder. They see the germy, smelly wastes, not the resources they hold. Indeed, there’s energy gold in those wastes. And fortunately, cadres of non-squeamish engineers are willing to harness them as a sustainable source of fuel or power.