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Under normal circumstances, in urban domestic wastewater, ammonia nitrogen accounts for approximately 40% to 60% of the total nitrogen content. For certain types of industrial wastewater, such as that from food processing and pharmaceutical industries, the proportion of ammonia nitrogen in total nitrogen can vary; it may be as high as 70%–80% in some cases, while it may be relatively lower in others. Ammonium nitrogen (NH₄⁺-N) refers to the nitrogen present in water in the form of free ammonia (NH₃) and ammonium ions (NH₄⁺). Total nitrogen (TN) includes ammonia nitrogen, organic nitrogen, as well as nitrate nitrogen (NO3⁻-N) and nitrite nitrogen (NO2⁻-N). Now, getting to the point: low ammonia nitrogen levels but high total nitrogen levels can be caused by the following reasons: 1. Nitrate nitrogen accumulates during the nitrification process; under aerobic conditions, ammonia nitrogen is converted into nitrate nitrogen (including nitrate and nitrite) by the action of nitrifying bacteria. Under normal conditions, the nitrate nitrogen produced is reduced to nitrogen gas and removed from the system during the subsequent denitrification process, under anaerobic conditions and through the action of denitrifying bacteria, thereby enabling the removal of total nitrogen. However, if problems arise in the denitrification process, such as inhibited activity of denitrifying bacteria, insufficient oxygen-deficient conditions, or inadequate supply of carbon sources, nitrate nitrogen cannot be properly reduced to nitrogen gas. This leads to a continuous accumulation of nitrate nitrogen in the water; although the ammonia nitrogen at the inlet can be effectively converted into nitrate nitrogen, resulting in low ammonia nitrogen levels in the effluent, the accumulated nitrate nitrogen cannot be removed, and as a result the total nitrogen concentration remains high. 2. Organic nitrogen is not effectively removed. During wastewater treatment, organic nitrogen must undergo a series of biochemical reactions before it can be converted into ammonia nitrogen, which can then be removed through nitrification and denitrification processes. If, during the treatment process, the microbial community responsible for breaking down organic nitrogen is not sufficient in number or lacks sufficient activity, or if the treatment conditions (such as temperature, pH value, dissolved oxygen, etc.) are not favorable for the decomposition and conversion of organic nitrogen, it may result in organic nitrogen not being fully converted into ammonia nitrogen. For example, too low a temperature may slow down the metabolic rate of microorganisms, resulting in incomplete decomposition of organic nitrogen ； An inappropriate pH value may affect enzyme activity, thereby inhibiting the decomposition of organic nitrogen by microorganisms. As a result, although ammonia nitrogen is effectively removed through nitrification and denitrification, resulting in low levels of ammonia nitrogen in the effluent, the inherent organic nitrogen in the wastewater is not completely converted or removed, which ultimately leads to high total nitrogen levels. 3. The internal recirculation ratio is too low. Internal recirculation refers to the recycling of the mixed liquid rich in nitrate nitrogen at the end of the aerobic tank to the front part of the anoxic tank, in order to supply nitrate nitrogen for denitrification reactions. The internal reflux ratio refers to the ratio of the amount of mixed liquid returning to the amount of feed water flow. If the internal reflux ratio is too low, it means there is insufficient nitrate nitrogen being recycled to the anoxic tank. In anaerobic environments, denitrifying bacteria use carbon sources to reduce nitrate nitrogen to nitrogen gas; a lack of sufficient nitrate nitrogen will limit the progress of the denitrification process. For example, under normal operation, an internal recirculation ratio of 200% is required to meet the denitrification process’s demand for nitrate nitrogen, but in practice the actual internal recirculation ratio is only 100% or even lower. This prevents denitrification from functioning properly, so that a large amount of nitrate nitrogen cannot be converted into nitrogen gas and released, resulting in low ammonia nitrogen levels in the effluent but still high total nitrogen levels. The best nitrogen removal efficiency is achieved when the internal recirculation ratio is maintained between 200% and 400%. 4. Insufficient carbon source: During denitrification, denitrifying bacteria require a carbon source as an energy source and electron donor to reduce nitrate nitrogen to nitrogen gas. If the available carbon source in the wastewater is insufficient, the metabolism and reactions of denitrifying bacteria will be restricted. During denitrification, nitrate nitrogen and nitrite nitrogen are converted into nitrogen gas (N₂) and released. If this process is insufficient, the nitrate nitrogen in the total nitrogen content cannot be effectively removed. Based on the operation data of the wastewater treatment plant, when the denitrification efficiency is below 80%, the removal efficiency of total nitrogen is significantly reduced. Common carbon sources that can be utilized by denitrifying bacteria include organic substances such as methanol, sodium acetate, glucose, etc. If the amount of organic carbon source present in the incoming water is insufficient, or if it is depleted during the treatment process without timely addition of additional carbon sources, denitrification cannot proceed adequately. For example, some industrial wastewater has a low carbon-to-nitrogen ratio (C/N), and the carbon sources it contains are not sufficient to support effective denitrification. Or, during the treatment process, due to unreasonable process design, the carbon source is over-consumed at the early stages, resulting in a shortage of carbon source when the denitrification stage is reached. In this case, even though the nitrification process can successfully convert ammonia nitrogen into nitrate nitrogen, the lack of a sufficient carbon source for denitrification prevents the adequate removal of nitrate nitrogen, resulting in low ammonia nitrogen levels in the effluent but high total nitrogen levels. 5. Improper control of dissolved oxygen: Dissolved oxygen plays a key role in the nitrification and denitrification processes in wastewater treatment. The nitrification reaction requires aerobic conditions, as well as a high concentration of dissolved oxygen to enable nitrifying bacteria to convert ammonia nitrogen into nitrate nitrogen. However, denitrification requires an anaerobic environment; if the dissolved oxygen concentration is too high, it will inhibit the activity of denitrifying bacteria and disrupt the denitrification process. For example, in the denitrification zone, if the dissolved oxygen level is too high due to uneven aeration, insufficient mixing, or improper process design, the normal metabolism of denitrifying bacteria as well as the activity of denitrificase will be inhibited. This prevents the denitrification process from proceeding effectively; even if the nitration step successfully converts ammonia nitrogen into nitrate nitrogen, the obstruction in denitrification prevents nitrate nitrogen from being fully converted into nitrogen gas and released, resulting in low levels of ammonia nitrogen in the effluent but high levels of total nitrogen. By adjusting process parameters, such as keeping dissolved oxygen (DO) below 0.5 mg/L, an environment more favorable for the growth of denitrifying bacteria is created. At the same time, by increasing the residence time in the hypoxic zone, sufficient time can be ensured for nitrate nitrogen to be converted into nitrogen gas. 6. Inappropriate sludge age: The sludge age refers to the average residence time of activated sludge within the entire system. Different microorganisms have varying growth and reproduction rates, as well as different requirements regarding sludge age. Nitrifying bacteria grow slowly and require a long sludge age to maintain their population and activity in the system. If the sludge age is too short, the nitrifying bacteria are removed from the system before they have had time to reproduce sufficiently, preventing the nitrification process from proceeding adequately and resulting in a reduced efficiency of converting ammonia nitrogen into nitrate nitrogen. On the other hand, if the sludge age is too long, it may lead to sludge aging and an imbalance in the microbial community structure; the activity and quantity of denitrifying bacteria may be affected, resulting in insufficient denitrification and an inability to effectively convert nitrate nitrogen into nitrogen gas, thereby leading to poor removal of total nitrogen. For example, in actual operation, if the sludge age is set too short in an attempt to achieve an excessively high sludge removal efficiency, incomplete nitrification may occur, resulting in insufficient conversion of ammonia nitrogen into nitrate nitrogen ； On the other hand, excessively prolonging the sludge age in order to reduce sludge treatment costs may lead to sludge aging, resulting in poor denitrification efficiency. 7. Defects in process design: Some wastewater treatment plants may lack an effective nitrogen removal step in their process due to an underestimation of the requirements for total nitrogen removal during the design phase. For example: 1. Insufficient volume of the anoxic zone: The anoxic zone is where denitrification reactions take place; if its volume is too small, it cannot provide sufficient space and time for denitrifying bacteria to carry out their functions, resulting in inadequate denitrification and poor removal of total nitrogen. 2. Inconsistent mixing in the anoxic zone: Inconsistent mixing leads to inadequate mixing of the wastewater within the anoxic zone, and some areas may not receive sufficient carbon sources and nitrate nitrogen, which affects the denitrification process. To address the issue of total nitrogen removal caused by design flaws, it is possible to increase or modify the anoxic zone and add hydrolysis-acidification pretreatment in order to improve the conversion rate of organic matter, thereby providing more carbon sources to support the denitrification process.