Turning Wastewater into Fertilizer is Viable & could Make Agriculture More Sustainable: Study

According to environmental engineering researchers at Drexel University, wastewater draining from massive pools of sewage sludge has the potential to play a role in more sustainable agriculture. A new study of removing ammonia from wastewater and converting it into fertilizer suggests that it is not only technically feasible but also has the potential to reduce the environmental and energy footprint of fertilizer production – and even provide a revenue stream for utilities and water treatment facilities.

A Sustainable Nitrogen Source

Nitrogen production for fertilizer is an energy-intensive process that accounts for nearly 2% of global CO2 emissions. Researchers have looked into alternatives to the Haber-Bosch nitrogen production process in recent years, which has been the industry standard for over a century. One promising option, recently raised by some water utility providers, is to extract nitrogen from waste ammonia extracted 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, Ph.D., a professor in the College of Engineering who contributed to the study, which was recently published in the journal Science of the Total Environment. "This means that we are reusing existing nitrogen rather than expending energy and emitting greenhouse gases to harvest nitrogen from the atmosphere, which is a more sustainable practice for agriculture and could become a revenue source for utilities."

Municipal water treatment facilities have been challenged by the Clean Water Act of 1972 to meet water quality standards for water discharged into waterways. Ammonia is increasingly seen as a problem for aquatic environments, as high ammonia levels can cause an overgrowth of vegetation in streams and rivers, endangering fish species. Ammonia removal methods are generally time and space-consuming, and energy intensive.

Several North America and Europe facilities are investigating a process known as air-stripping. It removes ammonia by raising the temperature and pH of the water sufficiently to convert the chemical into a gas, which can then be collected as ammonium sulfate in concentrated form. However, deciding whether to invest in air-stripping requires a complex study of its technological and financial viability, known as a lifecycle analysis.

The Technion Israel Institute of Technology team, led by Gurian and Sabrina Spatari, Ph.D., conducts these analyses regularly to assess the full environmental and economic impact of various options for recycling and reusing waste or side-stream products as sustainable solutions. According to their analysis of this wastewater scenario, there is a complementary relationship that could lead to a more sustainable path for both farmers and water management authorities.

"Our analysis shows that implementing air-stripping technology at wastewater treatment plants producing ammonia sulphate fertiliser has a significant potential for environmental mitigation and economic benefit," they wrote. "In addition to producing marketable ammonia sulfate, the benefit of reducing ammonia load in the side-stream before it is recycled into the wastewater stream at the wastewater treatment plant provides an additional justification for using air-stripping."

The team conducted its lifecycle assessment and economic feasibility studies using data from Philadelphia's water treatment facility and several others in North America and Europe. They considered factors such as the cost of installing and maintaining an air-stripping system, the concentration of ammonia and the flow rate of the wastewater, the sources of energy used to power the collection and conversion process, and the production, transportation, and market price of fertiliser chemicals.

According to the life-cycle analysis, air-stripping emits five to ten times less greenhouse gas and uses five to fifteen times less energy than the Haber-Bosch nitrogen-producing process. Economically, the overall cost of producing fertiliser 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 make a profit. "Our research suggests that recovering ammonia can be cost-effective even at low concentrations."

They write, "concentration." "However, high ammonia concentrations are environmentally beneficial and can simultaneously support marginal production of ammonium sulphate with lower environmental impact, particularly for life cycle energy, greenhouse gas emissions, and several human and ecosystem health indicators, when compared to Haber-Bosch production."

Furthermore, the study suggests that water treatment facilities could save energy by air-stripping ammonia levels before the water reenters the waste treatment process. This is because it would reduce the time and processing required to treat the water and work well with softening processes that help slow chemical deposition on treatment plant infrastructure.

While the team acknowledges that air-stripping would produce less fertiliser than the industrial Haber-Bosch process, the ability to collect and reuse any amount of resources helps to improve the sustainability of commercial agriculture and keeps them from becoming water pollutants. "This suggests that air-stripping for ammonium sulphate recovery could be a small but significant step toward recovering and reusing the massive amount of nitrogen we use to sustain global agriculture," Spatari said.

"Furthermore, it presents a chemical production alternative that does not have the same negative environmental and human health effects as the current process. According to the findings of this study, water utility providers should consider investing in technologies that capture phosphorus and recycle it for agricultural use."

(Source: Drexel University)