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It is a known fact that wastewater treatment plants are full of microbes, due to byproducts of the water filtration system. This latest study presents a method in which diverse species of bacteria convert solid and liquid wastes into gases – some of which contribute to global warming.
Craig Criddle, a professor of civil and environmental engineering and senior fellow at the Woods Institute for the Environment at Stanford, and Brian Cantwell, a professor of aeronautics and astronautics, have combined to design this unique system. “Normally, we want to discourage these gases from forming, but by encouraging the formation of nitrous oxide, we can remove harmful nitrogen from the water and simultaneously increase methane production for use as fuel,” says Criddle.
Cantwell and Criddle are applying rocket technology to sewage treatment, with the goal of making the process energy neutral and emissions free. They are funded by a grant from the Woods Institute, and their main objective is “to reduce the cost of wastewater treatment, increase energy generation, and eliminate greenhouse gas emissions,” according to Cantwell.Most treatment plants in the United States are using old technology, dating as far back as 1970, and are in dire need of an overhaul. “In the U.S., we haven’t invested much in wastewater treatment in recent decades,” Criddle comments.
The team’s first step in building a green treatment plant is growing the right kind of bacteria. “We’re really managing a zoo; to get the right microbes, we need to encourage the growth of bacteria that produce nitrous oxide gas,” said Criddle. One way to accomplish that is by reducing the bacteria’s oxygen supply. Conventional treatment plants use a process called aeration: pumping air into wastewater sludge.
In their paper, the researchers describe a process in which nitrogen waste changes into harmless nitrogen gas by promoting oxygen-loving bacteria that thrive on sugars and other organic matter in the sludge. However, aeration is a costly and energy-intensive process. Therefore, the Stanford team wants to create an alternative in the form of a low-oxygen environment in the treatment plant, where nitrous oxide-producing bacteria are favored while aerobic species die off.
These nitrous oxide producers consume relatively small amounts of organic matter. That is good news for other anaerobic microbes that produce methane gas by feasting on organic compounds. “When bacteria make nitrous oxide, less organic matter is oxidized, so more can be converted into methane – potentially two or three times more than is possible in a typical treatment plant,” Criddle said. “That extra methane can be used as fuel to run the plant independent of outside power sources.” One way of reducing costs is using less oxygen: “In a typical treatment plant, aeration is responsible for about half of the operating expenses,” Cantwell says. “So pumping less oxygen could save a lot of money.”
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Although in recent laboratory experiments it was demonstrated that nitrous oxide gas could be produced from wastewater using a low-oxygen technique, there is a significant downside: nitrous oxide is a greenhouse gas that is more than 300 times more potent than carbon dioxide. This is where Cantwell’s rocket knowledge came in handy. His rocket thruster – designed for use in spacecraft – runs on nitrous oxide, a surprisingly clean-burning propellant.
When asked to explain the mechanics of his system, Cantwell says, “When it decomposes, nitrous oxide breaks down into pure nitrogen and oxygen gas. At the same time, it releases enough energy to heat an engine to almost 3,000 degrees Fahrenheit, making it red hot, and it shoots out of the engine at almost 5,000 feet per second, producing enough thrust to propel a rocket.”
The unusual combination of space propulsion and environmental biotechnology started in 2008, when one of Cantwell’s graduate students, Yaniv Scherson, embarked on his doctoral thesis. “We wondered whether nitrous oxide could be exploited as an emissions-free source of energy,” Cantwell says, nostalgic look in his eyes. “Since the product of the decomposition reaction is simply oxygen-enriched air, energy is generated with zero production of greenhouse gas. But first we needed to find a cheap, plentiful source of nitrous oxide.”
Eventually, Scherson turned to Criddle, a known expert that studied thoroughly the subject of microbial communities in wastewater treatment plants. He says that wastewater sludge contains bacteria that naturally convert nitrogen wastes into nitrous oxide. For Scherson, it was a cheap resource for the gas.
The total design is comprised of two phases. First, operators reduce oxygen levels at the treatment plant, to encourage the production of nitrous oxide and methane gas. Then, the extra methane powers the plant and a small rocket thruster, which breaks down the nitrous oxide into clean, hot air. “A single thruster about the size of a basketball could potentially consume every ounce of nitrous oxide produced by a typical treatment plant,” Cantwell said.
Cantwell envisions a new generation of plants that are energy self-sufficient. “You even have the prospect of installing a wastewater facility where there is no energy source,” he says. The team envisions a new generation of plants, and finds such technology extremely important in the Third World, “where millions of people live with contaminated water.”
According to the researchers, their innovative technology has promising applications beyond wastewater treatment. For example, they wish to explore ways to recover energy from nitrate-contaminated groundwater beneath fertilized agricultural fields. “We’re thinking very broadly about all the ways nitrogen gets into the environment and how we can exploit it,” Cantwell said, and Criddle adds, “If successful, this technology could be a game changer, with the potential for worldwide impact on several fronts. For too long we’ve thought of treatment plants as places where we remove organic matter and waste nitrogen; we need to view these wastes as resources, not simply something to dispose of.”
TFOT has also covered a new technique to convert wastewater into clean energy, designed by Waltham Technologies, and the development of a new catalyst that uses visible light to disinfect bacteria and viruses, researched at the University of Illinois. Another related TFOT story is the production of solar energy using an “artificial leaf”, made using bacteria as well.
For more information about the usage of rocket science in order to improve wastewater treatment, see Stanford University’s press release.
