Key Points:
- Researchers are evaluating a sustainable alternative for the production of nitrogen for fertilizer, called air-stripping.
- A lifecycle analysis shows the process is both feasible and cost-effective.
- The new process uses about uses about 5 to 15 times less energy than the current standard.
The wastewater draining from massive pools of sewage sludge has the potential to play a role in more sustainable agriculture, according to environmental engineering researchers at Drexel University.
The production of nitrogen for fertilizer is an energy-intensive process and accounts for nearly 2% of global carbon dioxide emissions. In the last several years, researchers have explored alternatives to the Haber-Bosch nitrogen production process, which has been the standard for more than a century.
One promising possibility is air-stripping, a process that removes ammonia by raising the temperature and pH of the water enough to convert the chemical into a gas, which can then be collected in concentrated form as ammonium sulfate.
To get a better idea of the technological and financial viability of converting to air-stripping, researchers used data from Philadelphia’s water treatment facility and several others across North America and Europe. They looked at factors ranging from the cost of installing and maintaining an air-stripping system, to the concentration of ammonia and flow rate of the wastewater; to the sources of energy used to drive the collection and conversion process; to the production and transportation cost and market price of the fertilizer chemicals.
The study, published in Science of the Total Environment, show that air-stripping emits about five to 10 times less greenhouse gas than the Haber-Bosch nitrogen-producing process and uses about five to 15 times less energy.
From an economic perspective, the overall cost of producing fertilizer chemicals from wastewater is low enough that the producer could sell them at a price more than 12 times lower than Haber-Bosch-produced chemicals and still break even.
In addition, the study suggests that water treatment facilities may enjoy energy savings by air-stripping the ammonia to reduce levels before the water it reenters the waste treatment process. This is because it would cut the time and processing needed to treat the water and fits in well with softening processes that help to slow chemical deposition on the treatment plant infrastructure.
“This indicates that air-stripping for recovery of ammonium sulfate could be a small part—but an important step—toward recovering and reusing the massive amount of nitrogen we use to sustain global agriculture,” said study author Sabrina Spatari from Technion Israel Institute of Technology. “And, significantly it presents an alternative for chemical production that does not have the same level of deleterious environmental and human health effects as the current process.
Information provided by Drexel University.