Research trends and future directions on nitrification and denitrification processes in biological nitrogen removal

Biological nitrogen removal technologies, such as nitrification and denitrification, are commonly employed to reduce nitrogen degradation in wastewater. Bibliometric analysis was conducted to assess the qualitative and quantitative aspects of 5257 relevant publications from the Scopus database to understand the trends between 2012 and 2022. Our analysis showed that efficiency of nitrogen removal in actual applications depends on the types of wastewater, bioreactor configurations, microorganism communities, assessment methods as well as, environmental parameters such as dissolved oxygen and quantifying the nitrogen elements. In addition, several innovative nitrification and denitrification processes, have been discovered, including anaerobic ammonium oxidation (anammox) and complete ammonia oxidation (comammox). However, to enhance the nitrification and denitrification processes extensive studies should be conducted on: (i) the impact of operating parameters and surrounding circumstances; (ii) minimising and recovering the nitrous oxide (N2O) emission; and (iii) applying the next-generation sequencing method to study the diversity of microbial community. In a nutshell, this study reviews the effectiveness of the nitrification and denitrification processes for nitrogen removal in wastewater system in addition to identifying potential future perspectives.