Nitrate \({(\text{NO}}_{3})\) is found in surface and groundwater; in developing countries, people primarily intake water without any treatment. However, drinking water is detrimental to human beings, especially infants. Ammonium Oxidizing Bacteria (AOB) oxidize ammonium to Nitrite (NO2) and then Nitrite Oxidizing Bacteria (NOB) to NO3. Conversion of NO2 into NO3 required 0.5 mg/L of oxygen, and this caused an increase in the 25% running cost of the total oxygen supply in the reactor. Therefore, it is essential to restrict the process of NO2 accumulation in the reactor using different control strategies. NO2 accumulation in highly concentrated reactors is common. Little or no attention has been drawn to the accumulation of NO2 in low-feed concentrated reactors (approximately 50 mg TAN/L or less) without any chemical inhibitors. In this review, the accumulation of NO2 in low-concentrated reactors has been focused on. It was found that controlled strategies are mandatory for accumulating the NO2 in the reactors without using any chemical inhibitors. Furthermore, NO2 accumulation can be achieved by regulating pH, dissolved oxygen (DO), sludge retention time (SRT), real-time aeration control, anoxic conditions, and chemical inhibitors. On the other hand, the addition of chemicals (such as NH2OH, copper (II) and arsenic (III), zinc oxide nanoparticles (ZnO-NPs), chromium (III), and Cr (IV)) for NOB suppression necessitated a costly supplementary treatment method to remove hazardous chemicals from treated water.
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