Ammonia is one of the most commonly produced chemicals globally. Efficiently removing it from wastewater streams could allow it to serve as a source of revenue in many applications.
Ammonia is currently the second most commonly produced chemical in the world (after sulfuric acid), with around 200 million metric tons (m.t.) produced in 2018. Nearly 85% of all ammonia produced is used for fertilizers (1). Although nitrogen comprises almost 80% of the earth’s atmosphere, because it is inert, it is chemically and biologically unavailable for use in this form. Therefore, ammonia is used in fertilizers to provide the nitrogen source required to increase crop yields.
Ammonia and ammonia byproducts have a multitude of applications. For example, ammonia is often employed as a refrigerant in cooling systems. Compression converts it into a liquid with relative ease. When converted back into a gas, it absorbs heat. The textile industry uses ammonia in the dyeing of wool, cotton, and silk, as well as in the production of nylon. Household floor cleaners and detergents commonly use ammonia. Chemical process industries (CPI) facilities use ammonia for pH control and to manage NOx. NOx, when combined with ammonia under the correct conditions, produces nitrogen and water. Ammonia is also an important ingredient in fermentation processes — it serves as a source of nitrogen to encourage microorganism growth and adjust the pH.
Although common in nature and widely used industrially, ammonia can damage human health and ecosystems. In many countries, including the U.S., authorities classify ammonia as an extremely hazardous substance. Facilities that produce, store, or use it in significant quantities must follow strict reporting procedures. In the atmosphere, gaseous ammonia reacts with other pollutants to form tiny particles of ammonium salts that degrade air quality and, by affecting breathing, harm human health (2).
A 2018 study mapped atmospheric ammonia levels with unprecedented precision around the globe (3). Satellite recordings identified 248 nitrogen emission hot spots (defined as areas with diameters of less than 50 km). Eighty-three of those hot spots arise from agricultural activity that involves large numbers of cows, pigs, and chickens. Ammonia emissions from feedlots come primarily from livestock waste. One hot spot found over Eckley-Yuma, CO, coincides with two large cattle feedlots. A fainter hot spot exists over a complex of geothermal power plants in California.
Industrial emissions are responsible for 158 hot spots, primarily locations that produce ammonia-based fertilizers (4). As a general conclusion, it appears that emissions generated from human activities dominate natural ones. Addressing the management and control of ammonia is an ongoing issue.
While ammonia is present throughout the solar system, found on most planets, one of the primary naturally occurring sources of ammonia on earth is the decay of organic matter. Ammonia forms as amino acids that degrade during acidogenesis. It also forms as part of the excreta cycle of humans and animals, as the kidneys secrete ammonia to neutralize excess acid. Consequently, it is a commonly encountered water pollutant.
In an age where sustainability, resource recovery, and the circular economy are essential social themes, this article describes methods to remove, recover, and recycle ammonia from wastewater — particularly wastewater streams from anaerobic digestion processes and landfill leachate. One promising method, thermal ammonia stripping, could use waste heat to remove ammonia from wastewater streams. The article also describes an example of a successful implementation of thermal ammonia stripping in Hong Kong.
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