The clean nitrogen removal process based on solid carbon sources: Research progress and outlook

Nitrogen pollution is one of the main reasons for water eutrophication. The difficulty of nitrogen removal in low -carbon wastewater poses a huge potential threat to the ecological environment and human health. As a clean process for low-carbon wastewater treatment, the biological nitrogen removal process based on solid carbon sources (SCSs) has been widely concerned. To date, this process has become increasingly diverse, but lacks specific classifications, so its knowledge is confused. In order to better help readers understand the research progress and trend of this process, we classified it into solid-phase denitrification (SPD) process and solid-phase simultaneous nitrification and denitrification (SPSND) process from the perspective of target pollutants. The research progress of nitrogen removal performance, microbial community characteristics, SCSs metabolic characteristics and empirical models in recent years were reviewed and compared. Further, we searched 851 papers on the Web of Science and visualized using VOSviewer keywords, countries and source journals to emphasize the research hotspots of the process based on SCSs. As the result, blended SCSs have attracted more attention than synthetic and natural SCSs, and the SPSND process with better performance based on blended SCSs has achieved pilot-scale application. SCSs reconstructed microbial community structure of the SPD and SPSND systems, but the dominant denitrifying bacteria of the two were similar. The empirical models and SCSs metabolic patterns still need to obtain more complete information. Moreover, the future research trends of the process are as follows: 1) Needing clean and efficient strengthening technology. 2) Pollutants utilization and reduce carbon emissions. 3) The practical application of the process requires dealing with more complex pol-lutants. 4) Intelligent development. By summarizing the research progress of their performance and mechanism in detail, this review provides essential theoretical support for a cleaner, efficient and cost-effective treatment of the nitrogen removal process based on SCSs.