Two Stanford University engineers have developed a new sewage treatment process that increases nitrous oxide (laughing gas) and methane via microbial action but rather than release it to the atmosphere as a GHG, it will be used to power the treatment plant.
Prof. Craig Criddle, a civil engineer and a senior fellow at the Woods Institute for the Environment specializes in microbial action on wastewater treatment plants, explained: "…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, together with Prof. Brian Cantwell, an aeronautics and astronautics engineer who designs nitrous oxide fuelled rocket thrusters, applied the rocket engine technology to "…reduce the cost of wastewater treatment, increase energy generation and eliminate greenhouse gas emissions," and to "…view these wastes as resources, not simply something to dispose of."
They maximize the bacteria output by reducing their oxygen supply, which is opposite to the usual aeration process that is both costly and energy-intensive. This low oxygen environment kills off oxygen eating and nitrogen devouring bacteria, allowing the nitrous oxide producing bacteria to flourish. The nitrous oxide bacteria eat less organic compounds than the oxygen eating ones, resulting in more organic compounds left available for other anaerobic microbes to produce methane gas.
In fact, up to three times the methane content can be produced compared to a conventional waste treatment plant which can then be used to generate power. In addition, eliminating the aeration process can save almost half the normal power consumption. However, the extra nitrous oxide is a potent GHG molecule for molecule, it is 300 times more damaging than carbon dioxide. Which is where the rocket science helps out since Prof. Cantwell's spacecraft rocket thruster is a clean-burning nitrous oxide fuelled device.
"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 (1,649ºC), making it red hot, and it shoots out of the engine at almost 5,000 feet per second (1,524m/s), producing enough thrust to propel a rocket."
The exhaust gas from the decomposition reaction is oxygen-enriched air, and zero GHGs. This novel design has the potential for treating the world's wastewater cheaply while producing clean energy. The steps are straightforward - reduce aeration and oxygen levels to encourage nitrous oxide and methane gas production, using the methane to power the plant and a single rocket thruster about the size of a basketball is all that is needed to break down the nitrous oxide into clean, hot air.
Upgrades to existing plants would greatly reduce costs and GHG emissions, and, according to Criddle, "You even have the prospect of installing a wastewater facility where there is no energy source," he said. "This could be especially important in the Third World, where millions of people live with contaminated water."
It may also be possible to recover energy from nitrate-contaminated groundwater beneath fertilized agricultural fields that flow into rivers and the oceans, causing massive algal blooms that damage marine ecosystems.
Trevor Williams is a University of Victoria Mechanical Engineering PhD candidate specializing in renewable energy, power grid modeling and plug-in hybrid electric vehicles. He has a bachelors in Aeronautical Engineering, a Masters in Management Science and over 23 years international experience in the space industry, having worked on Earth observation and telecommunications satellites. He is the author of the Eco-Geek blog.
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