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. A new study suggests that it’s not only viable, but also could help to reduce the environmental and energy footprint of fertilizer production. The study was made by focusing on the process of removing ammonia from wastewater and converting it into fertilizer, which might even provide a revenue stream for utilities and water treatment facilities.
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, recently raised by some water utility providers, is gleaning nitrogen from the waste ammonia pulled from water during treatment.
“Recovering nitrogen from wastewater would be a desirable alternative to the Haber-Bosch process because it creates a ‘circular nitrogen economy,’” said Patrick Gurian, PhD, a professor in the College of Engineering who helped lead the research, which was recently published in the journal Science of the Total Environment. “This means we are reusing existing nitrogen rather than expending energy and generating greenhouse gas to harvest nitrogen from the atmosphere, which is a more sustainable practice for agriculture and could become a source of revenue for utilities.”
Ammonia is a concern as elevated levels can result in overgrowth of vegetation in streams and rivers which can endanger fish species. But the options for removing ammonia are generally time and space consuming as well as energy-intensive.
One option being explored is air-stripping. It works by removing ammonia by raising the temperature and pH of the water just enough to convert the chemical into a gas, which can then be collected in concentrated form as ammonium sulfate.
The research team, led by Gurian and Sabrina Spatari, PhD, from Technion Israel Institute of Technology, analyzed a wastewater scenario that suggests there is a complementary relationship that could result in a more sustainable path for both farmers and water management authorities.
“Our analysis identifies a significant potential for environmental mitigation and economic benefit from implementing air-stripping technology at wastewater treatment plants for producing ammonia sulfate fertilizer,” they wrote. “In addition to ammonia sulfate production as a marketable product, the benefit of reducing the ammonia load in the side-stream before it is recycled into the wastewater stream at the wastewater treatment plant provides an additional justification for adopting air-stripping.”
The team 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, according to Drexel University.
Findings show that air-stripping emits about 5 to 10 times less GHG than the Haber-Bosch process and uses about 5 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.
“Our study suggests that recovering ammonia can be cost-effective even at low concentration,” they write. “Although high ammonia concentration is environmentally favorable, and can simultaneously support marginal production of ammonium sulfate with lower environmental impact, particularly for life cycle energy, greenhouse gas emissions, and several human and ecosystem health indicators, compared to the Haber-Bosch production.”
While the team acknowledges that air-stripping would churn out fertilizer in smaller amounts than the industrial Haber-Bosch process, being able to collect and reuse any quantity of resources helps to improve the sustainability of commercial agriculture and prevents them from becoming water pollutants.