Engineers Use Rocket Science To Make Wastewater Treatment Sustainable

Now two Stanford University engineers are developing a new sewage treatment process that would actually increase the production of two greenhouse gases - nitrous oxide (aka, "laughing gas") and methane - and use the gases to power the treatment plant.

"Normally, we want to discourage these gases from forming," said Craig Criddle, a professor of civil and environmental engineering and senior fellow at the Woods Institute for the Environment at Stanford. "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."

Criddle, an expert in wastewater management, has joined forces with Brian Cantwell, a professor of aeronautics and astronautics, who has spent the last five years designing rocket thrusters that run on nitrous oxide.

With support from a Woods Institute Environmental Venture Projects grant, Cantwell and Criddle are applying that rocket technology to sewage treatment, with the goal of making the process energy neutral and emissions free.

"We want to reduce the cost of wastewater treatment, increase energy generation and eliminate greenhouse gas emissions," Cantwell said.

"For too long we've thought of treatment plants as places where we remove organic matter and waste nitrogen," Criddle added. "We need to view these wastes as resources, not simply something to dispose of."

Microbial zoo

For Criddle and Cantwell, the first step in building a green treatment plant is raising the right kind of bacteria. "We're really managing a zoo," Criddle said. "To get the right microbes, we need to encourage the growth of bacteria that produce nitrous oxide gas."

One way to accomplish that is by reducing the bacteria's oxygen supply, he said. Conventional treatment plants pump air into wastewater sludge - a process called aeration. The idea is to convert nitrogen waste into harmless nitrogen gas by promoting oxygen-loving bacteria that thrive on sugars and other organic matter in the sludge.

But aeration is a costly and energy-intensive process. As an alternative, the Stanford team wants to create 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's 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."

Using less oxygen also could reduce costs, Cantwell added. "In a typical treatment plant, aeration is responsible for about half of the operating expenses," he said. "So pumping less oxygen could save a lot of money."

Rocket science

In recent experiments, the researchers demonstrated that under laboratory conditions nitrous oxide gas could be produced from wastewater using a low-oxygen technique. But there's a downside to the process. Nitrous oxide is a significant greenhouse gas that's more than 300 times more potent than carbon dioxide.

That's where Cantwell's rocket thruster comes in. Designed for use in spacecraft, the thruster runs on nitrous oxide - a surprisingly clean-burning propellant.

"When it decomposes, nitrous oxide breaks down into pure nitrogen and oxygen gas," Cantwell explained. "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."