Ammonia (\(\text{NH}_3\)) and its ionic form, ammonium (\(\text{NH}_4^+\)), are nitrogenous compounds that pose a significant challenge in municipal water treatment. Their presence in raw wastewater is unavoidable, requiring complex and energy-intensive processes for removal before discharge. Managing this nitrogen load is paramount for environmental compliance and public health protection. This article explores the primary origin of this pollutant, its environmental significance, and the biological methods used to neutralize it.
The Chemical Pathway of Ammonia Generation
The primary source of ammonia at a municipal wastewater treatment plant is the organic nitrogen compounds in domestic sewage. Specifically, the breakdown of urea from urine and proteins from feces provides the bulk of the nitrogen load. While industrial and agricultural runoff may contribute, human waste is the dominant factor in municipal systems.
This conversion occurs through ammonification, a natural process that begins almost immediately within the sewer collection system. Heterotrophic microorganisms, such as bacteria and fungi, rapidly hydrolyze complex organic nitrogen molecules. These microbes secrete enzymes that break down proteins and urea, releasing amino acids that are then deaminated.
This process converts organic nitrogen into ammonia or ammonium. Since urea is quickly hydrolyzed by the urease enzyme, a significant portion of the nitrogen is already in the form of ammonium by the time it reaches the plant inlet. The treatment plant is primarily tasked with removing an already-formed pollutant rather than preventing its creation.
Environmental Impact and Regulatory Concern
Ammonia removal is a mandatory step in wastewater treatment because of its impact on receiving water bodies. When released into rivers or streams, the compound is toxic to aquatic life, particularly fish and invertebrates. This toxicity is dependent on the form the nitrogen takes in the water.
Ammonia exists in equilibrium between its less toxic, ionized form (ammonium) and its highly toxic, unionized form (ammonia gas). The balance between these two forms is heavily influenced by the water’s pH and temperature. Higher temperatures and elevated pH levels strongly shift the equilibrium toward the formation of the unionized ammonia.
Even ammonium presents a problem because it consumes dissolved oxygen as it naturally converts to nitrate. This process can lead to oxygen depletion, which is harmful to aquatic ecosystems. Consequently, regulatory permits strictly enforce limits on the concentration of total nitrogen discharged from treatment facilities.
Managing Ammonia Through Biological Treatment
Wastewater facilities manage the generated ammonia through a sophisticated two-step biological process known as nitrification-denitrification. This process relies entirely on cultivating specific types of bacteria in controlled environments. The initial step, nitrification, must occur under aerobic, or oxygen-rich, conditions.
In the first stage of nitrification, ammonia-oxidizing bacteria (AOB), such as Nitrosomonas, convert ammonium into an intermediate compound called nitrite. The second stage immediately follows, where nitrite-oxidizing bacteria (NOB), such as Nitrobacter, transform the nitrite into nitrate. This entire conversion is an oxidation process that requires a substantial amount of dissolved oxygen.
The next step is denitrification, which is necessary to remove the nitrogen from the water entirely. This reduction process must take place in an anoxic environment, meaning one where molecular oxygen is intentionally kept at extremely low levels. Here, facultative heterotrophic bacteria utilize the nitrate as a substitute for oxygen to metabolize an organic carbon source.
During this metabolism, the bacteria break the chemical bonds of the nitrate molecule, converting the nitrogen compound into harmless nitrogen gas. This gas then naturally bubbles out of the wastewater and is released into the atmosphere, completing the necessary removal of the nitrogen load. Maintaining the specific oxygen levels, temperature, and pH for these two distinct groups of slow-growing bacteria is the central engineering challenge in meeting stringent discharge standards.