Wastewater treatment cleans water using physical, chemical, and biological methods to remove pollutants like organic matter, solids, and nutrients before it is returned to the environment. Nitrogen, often entering treatment plants as ammonia, is a significant pollutant requiring specific processing. Nitrification is the necessary biological process that transforms this nitrogen, preparing it for final removal from the water. This step is fundamental in modern wastewater management to protect public waterways.
Why Ammonia Must Be Removed
Ammonia is a widespread pollutant in wastewater, originating primarily from human waste and the breakdown of organic material. Discharging water with high concentrations of ammonia is harmful to aquatic ecosystems. Ammonia, particularly its un-ionized form, is directly toxic to fish and other aquatic life, causing gill damage and potentially leading to fish kills.
The process of converting ammonia also places a massive demand on a receiving body of water’s oxygen levels. Ammonia exerts what is known as Nitrogenous Biochemical Oxygen Demand (NBOD). This means that when ammonia is naturally oxidized in a river or lake, it consumes dissolved oxygen (DO) from the water.
The consumption of oxygen can lead to hypoxia, a state of severely depleted oxygen, which creates “dead zones” where most aquatic organisms cannot survive. An estimated 4.6 milligrams of oxygen are required to fully oxidize just one milligram of ammonia nitrogen. This substantial oxygen debt must be removed within the treatment plant to prevent ecological damage downstream.
The Biological Steps of Nitrification
Nitrification is a two-step biological oxidation process performed by specialized groups of autotrophic bacteria. These bacteria obtain energy by converting inorganic nitrogen compounds and use carbon dioxide or bicarbonate as their carbon source. The entire conversion process occurs under aerobic conditions, requiring a constant supply of oxygen.
The first stage is known as nitritation, where Ammonia-Oxidizing Bacteria (AOB) convert ammonia or ammonium into an intermediate compound called nitrite (\(NO_2^-\)). The most commonly studied AOB in wastewater systems is Nitrosomonas. This initial conversion is the slower, rate-limiting step for the overall nitrification process.
The second stage is called nitratation, which is carried out by Nitrite-Oxidizing Bacteria (NOB). These bacteria rapidly oxidize nitrite further into nitrate (\(NO_3^-\)). An example of an NOB is Nitrobacter, though other species like Nitrospira are also recognized as important contributors in treatment plants.
The action of these two bacterial groups ensures the complete transformation of toxic ammonia into nitrate. The final nitrate compound is not toxic to aquatic life, but it is a nutrient that can still contribute to algal growth. The presence of nitrate sets up the next necessary step in nutrient removal, denitrification, which fully removes nitrogen from the system.
Key Environmental Controls for the Process
Operators must carefully manage several environmental factors to maintain a healthy and efficient nitrifying bacterial population. Nitrifiers are slow-growing compared to other bacteria in the wastewater, which means they require a long Mean Cell Residence Time (MCRT), or sludge age. This MCRT is the average time a microbe stays in the aeration basin, and it must be high enough to allow the nitrifiers to reproduce sufficiently.
Dissolved Oxygen (DO) concentration is critical, as nitrifiers are obligate aerobes. While an optimal DO level is around 3.0 mg/L, the process can be sustained if the DO is kept above 2.0 mg/L in the aeration tank. Levels below this threshold significantly slow down the bacterial activity and can halt nitrification altogether.
Temperature influences the rate of nitrification because the bacterial growth rate decreases significantly in colder conditions. The optimal temperature range is around 28 to 32 degrees Celsius, but nitrification can continue down to approximately 7 degrees Celsius before activity drops off sharply. Treatment plants need to increase their MCRT during winter months to compensate for this slower biological activity.
The process of nitrification generates acidity, which consumes the wastewater’s natural buffering capacity, known as alkalinity. Approximately 7.14 milligrams of alkalinity, measured as calcium carbonate, are consumed for every milligram of ammonia nitrogen oxidized. This consumption can cause the water’s pH to drop. Since nitrifying bacteria thrive in a near-neutral to slightly alkaline environment (pH 7.2 to 8.0), operators must sometimes add supplemental alkalinity to prevent process failure.