Wastewater is water affected by human use, carrying contaminants from domestic, industrial, and agricultural sources. A major pollutant is ammonia, which exists in a chemical balance with its ionized form, ammonium ($\text{NH}_4^+$), depending on the water’s $\text{pH}$ and temperature. At the typical $\text{pH}$ of municipal wastewater, ammonium ions are generally the dominant form. This nitrogen compound must be removed before the treated water can be safely returned to the environment.
Why Ammonia Must Be Removed
Releasing untreated wastewater containing high concentrations of ammonia severely disrupts aquatic ecosystems. Ammonia is directly toxic to fish and other aquatic organisms, causing gill damage, inhibiting growth, and leading to death. This toxicity drives the strict regulatory limits placed on ammonia discharge by environmental protection agencies.
Ammonia also contributes to eutrophication, where excessive nutrients stimulate the overgrowth of algae and aquatic plants. When these organisms die and decompose, biological processes consume large amounts of dissolved oxygen from the water. This increased demand for oxygen, known as biochemical oxygen demand, can quickly deplete the oxygen supply, leading to fish kills and creating “dead zones.”
Biological Nitrogen Removal: The Microbial Approach
The most common method for treating nitrogen compounds in wastewater relies on a two-step biological process carried out by specialized microorganisms. This approach, known as nitrification-denitrification, efficiently converts the nitrogen content into harmless nitrogen gas. This technique is effective for treating large volumes of municipal wastewater and is favored due to its environmentally benign end product.
The first step, nitrification, is an aerobic process requiring a sufficient supply of dissolved oxygen. During this phase, autotrophic bacteria sequentially oxidize the ammonium ions. Nitrosomonas first converts the ammonium ($\text{NH}_4^+$) into nitrite ($\text{NO}_2^-$), and then Nitrobacter oxidizes the nitrite into nitrate ($\text{NO}_3^-$).
Nitrifying bacteria are sensitive to environmental conditions, including temperature, often necessitating additional insulation or heating in colder climates to maintain efficiency. Although nitrification transforms the toxic ammonia, the resulting nitrate is still a nutrient that contributes to eutrophication. Therefore, a second step is required to complete the nitrogen removal.
The second step, denitrification, occurs in an anoxic environment, meaning no dissolved oxygen is present. Heterotrophic bacteria, such as Pseudomonas species, use the nitrate produced in the first step as an alternative to oxygen for respiration. The bacteria reduce the nitrate ($\text{NO}_3^-$) through a series of reactions, ultimately converting it into nitrogen gas ($\text{N}_2$).
The nitrogen gas is released into the atmosphere, which is approximately 80% nitrogen, completing the removal process. Denitrification requires a source of organic carbon, which acts as an electron donor for the bacteria. In wastewater streams with a low carbon-to-nitrogen ratio, an external source of carbon, such as methanol or acetic acid, may need to be supplied to ensure efficiency.
Physical and Chemical Removal Techniques
When biological treatment is not feasible, or when dealing with highly concentrated industrial waste streams, physical and chemical methods offer alternatives. These techniques do not rely on microbial action and are often used for pretreatment or polishing the final effluent.
Air stripping relies on converting the ammonium ion ($\text{NH}_4^+$) back into gaseous ammonia ($\text{NH}_3$) for physical removal. This conversion is achieved by increasing the wastewater $\text{pH}$, typically above 10, which shifts the chemical balance toward the volatile ammonia gas. The high-$\text{pH}$ water is then passed through a stripping tower where a large volume of air is blown through it, transferring the ammonia gas from the liquid to the air.
Ion exchange is a chemical and physical process that uses specialized solid resins to selectively remove ammonium ions from the wastewater. As the water flows over the resin beads, the ammonium ions ($\text{NH}_4^+$) are captured and exchanged for a less harmful ion, such as sodium. Once the resin is saturated with ammonium, it must be regenerated using a strong salt solution, making this method cost-effective for medium-scale applications.
Breakpoint chlorination is a chemical oxidation technique used to polish the effluent and remove residual ammonia. Chlorine is added to the water until a specific dosage, known as the “breakpoint,” is reached. The chlorine chemically reacts with the ammonia, oxidizing it and converting it primarily into harmless nitrogen gas ($\text{N}_2$), although some nitrate may also be formed. This method is simple to implement, as chlorine is often already used for disinfection, but careful management of the dosage is required to avoid unwanted byproducts.
Managing Treatment Byproducts
The method of ammonia removal dictates the form and complexity of the resulting byproducts that must be managed. The biological nitrification-denitrification process is advantageous because its primary end product is inert nitrogen gas ($\text{N}_2$), which is released into the atmosphere. The only major residual is the excess microbial sludge, a common byproduct of nearly all biological wastewater treatment.
In contrast, non-biological methods often result in concentrated liquid or solid wastes requiring further processing. For example, regenerating resins used in ion exchange produces a highly concentrated ammonia brine. This brine is a secondary waste stream that cannot be discharged directly and requires a separate treatment or disposal plan.
Air stripping, when coupled with a scrubbing tower, can recover gaseous ammonia in a liquid form, such as concentrated ammonium sulfate solution. This concentrated product, which can be up to 35% ammonium sulfate, may be a viable resource for use as a liquid fertilizer if local logistics allow. However, if the ammonia is not recovered, chemical additives used in air stripping, such as lime for $\text{pH}$ adjustment, contribute to the generation of chemical sludge that must be disposed of.