In many industrial wastewater treatment systems, Ammonia is one of the most difficult parameters to control during operation. There are many cases where the effluent COD has met the requirements, but Ammonia still tends to increase or exceed standards, forcing businesses to increase aeration time, add chemicals, or adjust operating modes. However, the root cause does not always stem from the pollution load. In many cases, an inefficient nitrification process is the reason why effluent Ammonia increases even though COD is compliant. Therefore, to treat Ammonia stably, businesses need to clearly understand the operating mechanism of nitrifying bacteria as well as the necessary conditions for this microbial group to develop and maintain long-term efficiency.
Nitrifying bacteria are a group of microorganisms capable of converting Ammonia (NH₄⁺) into Nitrate (NO₃⁻) through a biological oxidation process in an aerobic environment. This is the first and also the most important step in the Nitrogen treatment cycle of a wastewater treatment system.
The nitrification process occurs in two consecutive stages:
Nitrate is then transferred to the anoxic tank where the denitrifying bacterial group continues to convert it into Nitrogen gas (N₂) which escapes from the wastewater. Therefore, nitrification is only one part of the Nitrogen removal process and needs to be combined with the denitrification process to achieve Total Nitrogen treatment efficiency.
Compared to the group of microorganisms that decompose organic matter, nitrifying bacteria have a much slower growth rate and are sensitive to environmental changes. When operating conditions are unsuitable, the density of this microbial group will drop rapidly, leading to a decline in Ammonia treatment efficiency.
NH₄⁺ + 1,5O₂ → NO₂⁻ + 2H⁺ + H₂O
This process is carried out by the Nitrosomonas bacteria group. This is the stage that consumes the most oxygen and simultaneously generates H⁺ ions, reducing both the pH and the alkalinity of the system.
NO₂⁻ + 0,5O₂ → NO₃⁻
This stage is handled by Nitrobacter or Nitrospira, helping to convert Nitrite into the less toxic Nitrate before it continues to be treated in the anoxic tank through the denitrification process.
From the two reactions above, it can be seen that the nitrification process both consumes dissolved oxygen and generates H⁺ ions, causing the pH to gradually decrease during operation. Theoretically, to oxidize 1 mg of Ammonia-N, the system needs about 4.57 mg of oxygen and consumes about 7.14 mg of alkalinity (converted as CaCO₃). This is the reason why businesses should not only monitor Ammonia but also simultaneously control DO, pH, and alkalinity to ensure the nitrification process occurs stably.
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Nitrification microbiological process
Nitrification efficiency depends not only on the number of microorganisms but is also influenced by various operating conditions. In reality, there are many cases where businesses add microorganisms but Ammonia does not decrease because factors such as DO, pH, alkalinity, or sludge age have not been appropriately maintained. Therefore, before adjusting the system, businesses need to simultaneously evaluate the conditions below.
Dissolved oxygen (DO) is the most important condition for nitrifying bacteria to operate. In many wastewater treatment systems, DO in the aerobic tank is usually maintained at about 2–4 mg/L. If DO remains low for a prolonged period, Ammonia can increase even if COD is being treated stably. In this case, businesses should check the air blower, air distribution system, and the high influent wastewater load. When Ammonia increases abnormally, DO is always the first parameter that needs to be checked before adjusting other factors or adding microorganisms.
During the nitrification process, microorganisms consume alkalinity, causing the pH to gradually decrease. If the alkalinity is insufficient to maintain buffering capacity, the pH will drop to a level that inhibits microbial activity. Therefore, when Ammonia is difficult to treat, businesses should check both pH and alkalinity simultaneously instead of just adjusting one parameter.
During the nitrification process, microorganisms consume alkalinity, causing the pH to gradually decrease. If the alkalinity is insufficient to maintain buffering capacity, the pH will drop to a level that inhibits microbial activity. Therefore, when Ammonia is difficult to treat, businesses should check both pH and alkalinity simultaneously instead of just adjusting one parameter. (Note: This section duplicates the content from section 2.2 in the original Vietnamese text).
Due to their slow growth rate, nitrifying bacteria need to be maintained in the system long enough to form a stable population. If too much sludge is wasted or sludge is washed out with the effluent water, the amount of nitrifying bacteria will significantly decrease, and the Ammonia treatment efficiency will drop accordingly.
In many industrial wastewater treatment systems, sludge age is usually maintained for 8–12 days or more to create favorable conditions for this microbial group to develop. The specific value can vary depending on the technology and operating conditions of each system.
Fluctuations in flow rate or influent Ammonia concentration also directly affect the adaptability of nitrifying bacteria. If the load increases suddenly, the microbial system needs time to rebalance; during this period, effluent Ammonia may increase even if the equipment is still operating normally.
Therefore, the equalization tank plays an important role in stabilizing the flow rate and pollution load before the wastewater enters the biological structures.
Reference Operating Conditions Table
| Factor | Reference | ValueImpact when not ensured |
| DO | 2–4 mg/L | Nitrification rate decreases, Ammonia increases |
| pH | 7.0–8.5 | Microorganisms are inhibited |
| Temperature | 20–35°C | Slow Ammonia conversion |
| Alkalinity | Enough to stabilize pH | pH drops, nitrification becomes inefficient |
| Sludge Age | ≥ 8–12 days | Microorganisms are washed out, density decreases |
| Influent Load | Limit large fluctuations | Susceptible to shock loading, Ammonia increases |
Operating note: DO, pH, alkalinity, temperature, and sludge age do not operate independently but are closely related to one another. Just one factor changing outside the appropriate operating range can cause nitrification efficiency to decline, even if the remaining parameters are stable. Therefore, businesses should make an overall assessment instead of just focusing on a single indicator.
Nitrification efficiency usually does not decline suddenly but occurs in stages. If a business waits until Ammonia exceeds the standard to check the system, they may have missed the most favorable time to resolve it. Therefore, tracking the fluctuation trends of indicators and operating data will help detect abnormalities early before they affect effluent water quality.
Some common signs include:
The above signs are only for initial reference. To determine the correct cause, businesses need to combine wastewater analysis results with the actual operating data of the system.
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Nitrification in wastewater treatment
When effluent Ammonia tends to increase, businesses should not add microorganisms immediately but need to correctly determine the cause of the nitrification decline. First, it is advisable to check DO, pH, alkalinity, sludge age, and influent wastewater load to evaluate the system's operating conditions.
After determining the cause, businesses need to prioritize stabilizing operating parameters, limiting load fluctuations, and maintaining the equalization tank to operate effectively. The addition of microorganisms should only be carried out when the microbial population has genuinely declined or after incidents such as shock loading or prolonged system shutdowns.
Besides, tracking the trends of Ammonia, Nitrite, DO, and pH over multiple operating cycles will help evaluate the effectiveness of adjustments and early detect abnormal signs before effluent water quality is affected.
In case the system experiences shock loading or the nitrifying bacteria population is severely degraded, businesses can consider adding specialized microbial preparations after stabilizing the operating conditions. Adding microorganisms when DO, pH, or alkalinity are not yet suitable usually does not bring the desired results.
During operation, many businesses face difficulties in treating Ammonia not because of the technology but due to inappropriate system adjustments. Some common mistakes include:
Correctly assessing the cause before intervening will help businesses shorten treatment time, limit incurred costs, and maintain a more stable system in the long run.
Nitrifying bacteria are the determining factor for Ammonia treatment efficiency in many industrial wastewater treatment systems. However, for this microbial group to operate stably, businesses need to simultaneously control DO, pH, alkalinity, temperature, sludge age, and influent wastewater load instead of just adding microorganisms when an incident occurs.
Besides adding microorganisms when necessary, businesses should establish a process for periodically monitoring parameters such as DO, pH, alkalinity, Nitrite, and sludge age to detect signs of declining nitrification early. Well-controlled operating conditions will help the Ammonia treatment system be more stable, reduce operating costs, and limit the risk of effluent wastewater exceeding standards.
Read "How Does High Ammonia Affect the Environment and the Wastewater Treatment System?" to better understand the causes and practical Ammonia control solutions.
Normally, DO in the aerobic tank should be maintained at about 2–4 mg/L to ensure the nitrification process occurs stably.
Not always necessary. You should first check DO, pH, alkalinity, temperature, and sludge age to accurately determine the cause before adding microorganisms.
Because the two groups of microorganisms that treat COD and Ammonia have different growth characteristics. If nitrification conditions are not suitable, Ammonia can still increase even if COD has been treated effectively.
