Challenges and Operational Strategies to Achieve Partial Nitrification in Biological Wastewater Treatment: A Review

Partial nitritation is a promising technology for wastewater treatment systems and, in symbiosis with other nitrogen removal approaches (i.e., Anammox bacteria), is attractive for reducing costs compared to conventional technologies. However, the intrinsic problems related to the different characteristics of the effluent induce unstable process conditions, including the subsequent accumulation of nitrate, which also reduces the partial yield of nitritation. Several studies highlight the persistent obstacles in preventing nitrate accumulation by nitrite-oxidizing bacteria, identified as the main challenge in the partial nitritation process. Consequently, this study conducted a comprehensive literature review, exploring various strategies to overcome these bottlenecks. Addressing the suppression of ammonia-oxidizing bacteria and the inhibition of nitrite-oxidizing bacteria involved consideration of operational strategy. Notably, pH emerged as an essential factor affecting microbial activity and process stability, influencing the efficiency of biochemical reactions. In addition, other interferents, such as organic compounds and metals, can influence the health and activity of microorganisms, affecting the overall effectiveness of the nitrogen removal process. The systematic control of various environmental and operational variables is essential for the stability of the process, demonstrating that a single strategy does not define the control of partial nitritation in wastewater. To date, maintaining dissolved oxygen in the range of 0.4 to 1 mg O₂ L⁻¹ and temperatures between 25 and 35 °C remains the most viable strategy for promoting stable partial nitritation. Finally, it is imperative to carry out further studies to develop control strategies and technologies, guaranteeing the efficiency of large-scale nitrogen removal systems and maintaining environmental safety standards.