Performance of a Two-Stage Ammonia Removal Technology Based on Advanced Oxidation Processes: Effects of Operating Conditions

Air that smells of ammonia is more than just an unpleasant nuisance; it is a significant contributor to environmental harm. When released into the atmosphere, primarily from agricultural activities, this gas helps form fine particles that degrade air quality, acidifies soil and water, and causes nutrient overload in ecosystems. As environmental rules become stricter, the challenge is not just to remove this gas from waste streams but to do so without creating new problems, such as expensive wastewater or high energy costs. Scientists have long looked to advanced oxidation processes for a solution. These are methods that use powerful chemical reactions, often triggered by light, to break down pollutants. The goal is to transform harmful ammonia into stable, oxidized forms of nitrogen that can potentially be captured and reused, turning a waste product into a resource.

A team of researchers at the Technical University of Ostrava and the company Dekonta has been testing a specific two-stage system designed to tackle this problem. Their work focuses on a technology that first uses ultraviolet light to treat ammonia-laden air, and then moves the captured compounds into a liquid phase where they are treated with hydrogen peroxide and more light. The researchers wanted to understand exactly how the operating conditions of this system affect its ability to remove ammonia and what specific nitrogen compounds are left behind. They were particularly interested in whether they could control the process to produce useful, recoverable nitrogen species rather than a mix of unwanted byproducts.

The experiment began with air containing ammonia, which was generated by bubbling air through a solution of ammonium hydroxide. This air first entered the initial stage of the system, a stainless-steel reactor equipped with low-pressure mercury lamps that emit ultraviolet light at specific wavelengths. The researchers discovered that the presence of water vapor in this stage was crucial. When the air was humidified, the removal of ammonia improved significantly. This suggests that water vapor helps create the reactive species needed to break down the ammonia. Even without the ultraviolet light, some ammonia was removed simply by being absorbed into the water, but the light made the process much more effective. This first stage proved that controlled moisture is a key factor in preparing the ammonia for the next step.

The second stage of the technology is where the real transformation happens. The air, now carrying the products of the first stage, moves into a wet scrubber and then through a series of tubular reactors. Here, the system uses a solution of hydrogen peroxide, which is a strong oxidizing agent, combined with ultraviolet light. This combination generates hydroxyl radicals, which are highly reactive molecules that attack the ammonia-derived compounds in the water. The researchers found that how they added the hydrogen peroxide mattered greatly. Adding it more frequently kept a steady supply of the oxidant available, making the process more efficient. They also observed that the amount of ammonia entering the system changed the outcome. When the initial load of ammonia was higher, the system produced more oxidized nitrogen, but the balance shifted. Instead of converting everything into nitrate, a stable form of nitrogen, the process left behind more nitrite. This indicates that at higher ammonia levels, the system struggles to complete the final step of turning nitrite into nitrate.

The team also had to navigate a practical hurdle in their measurements. They initially tried to measure the remaining ammonia directly in the water using a standard chemical test, but they found that the hydrogen peroxide in the solution interfered with the results, making the data unreliable. To get around this, they measured the total amount of nitrogen in the water and tracked the specific amounts of nitrite and nitrate. By looking at the balance of these compounds, they could accurately determine how much ammonia had been converted. They discovered that simply letting the water sit longer in the reactors did not automatically improve the results. This finding highlights that the physical movement of the liquid and the availability of the oxidizing agent are just as important as the time the light shines on the mixture.

The study concludes that while the two-stage technology is effective, it requires careful tuning to work at its best. The researchers found that humidifying the air in the first stage boosts performance, and that the strategy for adding hydrogen peroxide in the second stage dictates the efficiency of the reaction. However, the system has a limit: when too much ammonia is introduced, the process favors the formation of nitrite over nitrate. The authors suggest that future improvements should focus on optimizing the flow and chemical conditions to push the conversion all the way to nitrate, ensuring that the maximum amount of nitrogen is recovered in a stable, useful form. This work provides a clear roadmap for refining a technology that could help clean the air while recovering valuable resources from waste.