According to environmental engineering researchers at Drexel University, wastewater discharged from huge pools of sewage sludge has the potential to play a role in more sustainable agriculture. A new study, which looks at the process of removing ammonia from wastewater and turning it into fertilizer, suggests that it is not only technically feasible, but can also help reduce the environmental and energy footprint of fertilizer production—and may even provide a revenue stream for utilities and water treatment facilities.
Sustainable nitrogen source
Producing nitrogen for fertilizer is an energy-intensive process and accounts for approximately 2% of global carbon dioxide emissions. In the past several years, researchers have discovered 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 the recovery of nitrogen from ammonia waste that is drawn from the water during treatment.
said Patrick Gurian, PhD, a professor in the College of Engineering who helped lead the research, which was recently published in the journal macroenvironmental science. “This means that we reuse existing nitrogen rather than expend energy and generate greenhouse gases to harvest nitrogen from the atmosphere, which is a more sustainable practice for agriculture and can become a source of income for utilities.”
A cleaner way to clean
Under the Clean Water Act of 1972, municipal water treatment facilities were challenged to meet water quality standards for water discharged into waterways. Ammonia is increasingly seen as a concern for aquatic environments as elevated levels of ammonia can lead to an overgrowth of vegetation in streams and rivers which can endanger fish species. Typically, ammonia removal options take time, space, and can consume a lot of energy.
One option that is being explored by many facilities in North America and Europe is a process called air stripping. It removes ammonia by raising the temperature and pH of the water enough to turn the chemical into a gas, which can then be collected in a concentrated form as ammonium sulfate.
But making an investment decision to switch to air stripping requires a complex study – called a life cycle analysis – of its technological and financial feasibility.
Explore the option
The team, led by Gurian and Sabrina Spatari, Ph.D., of the Technion Israel Institute of Technology, regularly performs these analyzes to assess the full environmental and economic impact of various options for recycling and reusing waste or by-products as sustainable solutions. Their analysis of this wastewater scenario suggests a complementary relationship that could lead to a more sustainable path for both farmers and water management authorities.
“Our analysis identifies significant environmental mitigation potential and economic benefit from implementing air separation technology in wastewater treatment plants for the production of ammonia sulfate fertilizer,” they wrote. “In addition to producing ammonia sulfate as a marketable product, the benefit of reducing the ammonia load in the side stream before it is recycled into the wastewater treatment plant’s wastewater stream provides additional justification for adopting air separation.”
Using data from the water treatment facility in Philadelphia and several other facilities across North America and Europe, the team conducted a life cycle assessment and feasibility studies. They considered factors ranging from the cost of installing and maintaining an air separation system, to the ammonia concentration and wastewater flow rate; to the energy sources used to drive the collection and conversion process; For production cost, transportation and market price of fertilizer chemicals.
The results of the life cycle analysis show that air stripping emits about five to 10 times less greenhouse gases than the Haber-Bosch nitrogen production process and uses about five to 15 times less energy.
From an economic perspective, the total cost of producing chemical fertilizers from wastewater is low enough that a producer can sell them at 12 times less than the chemicals produced by Haber-Bosch and still break even.
“Our study indicates that ammonia recovery can be cost-effective even at low levels
Focus, “they write.” Although a high ammonia concentration is environmentally favorable, it can simultaneously support marginal production of ammonium sulfate with less impact on the environment, especially for life cycle energy, greenhouse gas emissions, and many human health and ecosystem indicators, compared to Haber-Bosch production.”
In addition, the study suggests that water treatment facilities may enjoy energy savings by stripping ammonia from the air to reduce levels before returning the water to the waste treatment process. This is because it will shorten the time and processing required for water treatment and is well suited to softening processes that help slow chemical deposition on treatment plant infrastructure.
While the team acknowledges that stripping the air would produce less fertilizer than an industrial Haber-Bosch process, the ability to collect and reuse any amount of resources helps improve the sustainability of commercial farming and prevents them from becoming water pollutants.
“This suggests that separating air to recover ammonium sulfate could be a small part – but an important step – towards recovering and reusing the vast amount of nitrogen we use to sustain global agriculture,” Spatari said. “It remarkably presents an alternative to chemical production that does not have the same level of adverse effects on the environment and human health as the current process. This research suggests that water service providers could also consider investing in technologies that would capture phosphorus and recycle it for agricultural use.”