Ammonia, a compound of nitrogen and hydrogen, is a common component in wastewater. Its presence in high concentrations in treated wastewater, known as effluent, poses a significant environmental challenge. Effective removal of ammonia is crucial to prevent adverse impacts on aquatic ecosystems and to comply with environmental regulations. Understanding the various factors that lead to elevated ammonia levels in wastewater effluent is essential for optimizing treatment processes and safeguarding water quality.
Sources of Ammonia in Incoming Wastewater
Ammonia enters wastewater treatment plants from several origins, with the initial concentration in the influent directly influencing the treatment load. Domestic sewage is a primary source, containing ammonia from the breakdown of organic nitrogen-containing materials like proteins and urea in human waste, as well as from cleaning products. Human activities such as bathing, laundry, and food preparation also contribute to this ammonia load.
Industrial discharges are another significant source. Industries involved in chemical manufacturing, producing fertilizers, explosives, and synthetic fibers, often release ammonia into their wastewater. Food processing facilities, including meat packing and dairy plants, generate ammonia. Additionally, the pharmaceutical industry and petroleum refining processes can contribute ammonia. Agricultural runoff, particularly from fields treated with synthetic fertilizers and animal waste, can also carry ammonia into wastewater systems.
Biological Treatment Deficiencies
The removal of ammonia in wastewater treatment largely relies on a biological process called nitrification, where specialized bacteria convert ammonia into less harmful forms of nitrogen. Nitrification is a two-step process: ammonia is first oxidized to nitrite by ammonia-oxidizing bacteria (AOB) like Nitrosomonas, and then nitrite is oxidized to nitrate by nitrite-oxidizing bacteria (NOB) like Nitrobacter. Deficiencies in this biological mechanism can lead to high ammonia concentrations in the treated effluent.
An insufficient population of nitrifying bacteria is a common issue. These bacteria have a relatively slow growth rate compared to other microorganisms in wastewater, making them susceptible to washout if the sludge retention time (SRT) is too short. A low SRT means that the bacteria are removed before they can grow and establish sufficient numbers to process the ammonia load.
Inhibitory substances can severely hinder the activity of nitrifying bacteria. Heavy metals, organic chemicals like phenols and cyanides, and high concentrations of free ammonia or nitrous acid can be toxic, interfering with the enzymes these bacteria need for nitrification. Nitrification also consumes alkalinity (the water’s capacity to neutralize acids). Approximately 7.14 milligrams of alkalinity (as CaCO3) are consumed for every milligram of ammonium ions oxidized. If there is insufficient alkalinity, the pH can drop significantly, as hydrogen ions are produced during the oxidation of ammonium. This reduction in pH, particularly below 6.7, decreases the activity of nitrifying bacteria, as they operate optimally between 7.0 and 8.5.
Operational and Environmental Challenges
Environmental conditions and operational issues within a wastewater treatment plant can impair the efficiency of biological treatment processes, particularly nitrification. Dissolved oxygen (DO) levels are critical, as nitrification is an aerobic process requiring adequate oxygen. Inadequate DO levels, especially below 0.5 mg/L, severely inhibits bacterial activity, leading to incomplete ammonia conversion. A range of 2.0 to 3.0 mg/L is generally considered sufficient for significant nitrification.
Temperature fluctuations significantly affect nitrifying bacteria, which are highly sensitive. Their metabolic activity and growth rates decrease substantially at lower temperatures, often requiring longer sludge retention times in colder conditions. Optimal temperatures for nitrification are 20°C to 30°C; activity reduces significantly below 16°C.
Hydraulic overload, with high flow rates, can reduce the hydraulic retention time (HRT), meaning wastewater spends less time in the treatment reactors. This results in insufficient contact time for the biological processes to remove ammonia, and can also lead to the washout of biomass from the reactors. A lack of essential macronutrients, such as phosphorus, or micronutrients can limit the growth and performance of the bacterial community.
Sudden influxes of high organic matter (organic shock loads) or toxic substances (toxic shock loads) can overwhelm the system, consuming oxygen needed for nitrification or directly inhibiting nitrifying bacteria, hindering ammonia conversion.
Chemical Factors and System Overload
Beyond biological and operational challenges, specific chemical conditions and inherent system limitations can contribute to high ammonia in effluent. Certain industrial chemicals, distinct from those inhibiting bacterial growth, can act as direct toxicants to the biological system, impacting microbial activities including ammonia removal. Extreme pH conditions can also lead to ammonia stripping, where ammonium ions convert to ammonia gas within the plant, especially at higher temperatures and pH above 9. If not properly managed, this conversion can lead to re-dissolution downstream or air quality concerns.
Even with optimized operations, a wastewater treatment plant’s design capacity may be insufficient for the incoming ammonia load. This “system overload” can occur due to factors like population growth in the service area, increased industrial activity discharging ammonia-rich wastewater, or a sustained change in influent characteristics exceeding design capabilities. The plant cannot process the ammonia load, leading to elevated effluent levels.