Ammonia, specifically Total Ammoniacal Nitrogen (TAN), is a strictly regulated environmental concern in wastewater management, particularly in the final discharge, known as effluent. TAN is the combined measure of un-ionized ammonia (\(\text{NH}_3\)) and the ammonium ion (\(\text{NH}_4^+\)), which are common in sewage and industrial waste streams. High levels of this nitrogen compound released from treatment facilities can severely compromise the health of receiving rivers and lakes. The presence of ammonia in treated water indicates a breakdown in the biological processes designed to remove it, presenting a substantial operational challenge for treatment plants.
Initial Nitrogen Load and Sources in Influent
The fundamental cause of high effluent ammonia begins with the sheer volume of nitrogenous compounds entering the treatment facility, known as the influent load. Domestic sewage is a primary source, where nitrogen originates from the breakdown of proteins and urea in human and animal waste. This organic nitrogen rapidly converts to ammonia and ammonium as it travels through the sewer system, establishing a baseline concentration often ranging from 20 to 40 milligrams per liter.
This base load is amplified by industrial contributions, which introduce highly concentrated streams of nitrogen. Industries like food processing, fertilizer manufacturing, chemical production, and petroleum refining often discharge process wastewater containing high concentrations of ammonia. When this elevated nitrogen load enters the system, it can overwhelm the capacity of the plant’s biological treatment units, leading to high effluent ammonia levels.
Breakdown in Biological Nitrification
Ammonia removal relies on nitrification, a two-step aerobic biological process that converts toxic ammonia into less harmful nitrate. This process is mediated by slow-growing, autotrophic bacteria. The first step involves ammonia-oxidizing bacteria (Nitrosomonas), which convert ammonia (\(\text{NH}_3\)) to nitrite (\(\text{NO}_2^-\)). Nitrite-oxidizing bacteria (Nitrobacter) then complete the conversion by oxidizing nitrite to nitrate (\(\text{NO}_3^-\)).
Failure often occurs if the hydraulic retention time (HRT) is too short, causing bacterial washout where nitrifiers are flushed out faster than they can reproduce. Nitrifying bacteria are also highly sensitive to inhibition by toxic chemicals, such as heavy metals (copper and zinc) or specific organic compounds from industrial spills. Furthermore, high concentrations of the substrate itself, like free ammonia or nitrite, can be self-inhibitory, reducing the active biomass. Insufficient or inhibited microbial populations allow ammonia to pass through the plant untreated, resulting in elevated effluent concentrations.
Limiting Operational and Environmental Factors
The failure of nitrification is frequently traced to operational and environmental factors that inhibit bacterial activity. The process requires a constant supply of Dissolved Oxygen (DO) because it is an aerobic reaction, consuming approximately 4.57 grams of oxygen for every gram of ammonia nitrogen oxidized. If the DO concentration drops below the optimal threshold of about 3 milligrams per liter, the reaction rate slows considerably. If DO falls to 0.5 milligrams per liter, the nitrification process can be completely inhibited.
Temperature is another highly influential factor, as nitrifying bacteria are extremely sensitive. Optimal activity occurs between 28 and 32 degrees Celsius. Below this range, the metabolic rate decreases sharply, and at temperatures below 10 degrees Celsius, the nitrification rate can be less than half of its maximum. This temperature sensitivity often leads to a significant buildup of ammonia, particularly during winter months.
The chemical environment must also be precisely controlled, as nitrifying bacteria thrive within a near-neutral to slightly alkaline pH range, with an optimum near 8.0. A deviation outside this optimal band, such as a drop below 6.5, severely inhibits the bacteria. This pH drop is often caused by insufficient alkalinity, as nitrification consumes about 7.1 grams of calcium carbonate for every gram of ammonia nitrogen oxidized. If alkalinity is not replenished, the pH buffer is lost, halting the reaction.
Finally, sudden, high-volume flow events, known as hydraulic overload, dramatically reduce the time the wastewater spends in the reactor. This prevents the slow-acting bacteria from completing the necessary oxidation reaction before the water is discharged.
Ecological Impact of Untreated Ammonia Discharge
The discharge of effluent containing high levels of ammonia poses significant threats to the receiving aquatic environment. The most pressing consequence is the direct toxicity of un-ionized ammonia (\(\text{NH}_3\)) to aquatic life, which increases significantly with higher water temperatures and pH levels. This toxic form crosses the gill membranes of fish and invertebrates, causing internal chemical imbalances that damage tissues and organs, often resulting in death even at low concentrations.
Ammonia also contributes to the depletion of oxygen in the receiving water body through nitrogenous biochemical oxygen demand (NBOD). Once discharged, the ammonia continues to nitrify, consuming large amounts of dissolved oxygen needed by aquatic organisms. This oxygen consumption can lead to hypoxic conditions, causing fish kills and reducing biodiversity. Furthermore, ammonia acts as a nutrient, contributing to eutrophication, where excessive nutrient loading stimulates the rapid overgrowth of algae.