Nitrogen removal in the sewage treatment process is a significant challenge. The increase in nitrogen content in sewage leads to the eutrophication of water bodies and the deterioration of water quality in polluted environments. Therefore, converting nitrogen into non-polluting gases is a crucial and essential part of the sewage treatment process. Compared to physical, chemical, and physicochemical methods, biological denitrification is not only simple to operate and economically effective but also has less secondary pollution and saves energy. This paper summarizes the latest research progress on mainstream biological denitrification technology in WWTPS (wastewater treatment plants) and discusses its research background, methodology, and challenges. It is noted that the traditional biological nitrogen removal method is stable and widely used, but it has drawbacks such as high costs and long reaction times, especially in high-nitrogen-load wastewater treatment where its effectiveness is limited. The short-cut nitrification–denitrification process suits high-nitrogen-loading and a low C/N ratio wastewater as it reduces carbon source consumption. However, the problems of water quality fluctuation and unstable dissolved oxygen still need to be solved. The anaerobic ammonia oxidation process efficiently converts ammonia and nitrite to nitrogen using anaerobic ammonia-oxidizing bacteria, consuming less energy but facing limitations due to slow bacterial growth rates and stringent environmental conditions. The heterotrophic nitrification–aerobic denitrification process merges the traits of heterotrophic nitrifying bacteria and aerobic denitrifying bacteria, effectively reducing the ecological footprint and enhancing treatment efficiency. This approach is a pivotal focus for future research endeavors.

Municipal solid waste incineration fly ash (MSWIFA) is a promising carbon sequestration material. However, the leaching behavior of harmful elements during the carbonation process is a problem that must be considered. This paper screened four different MSWIFAs to explore their physicochemical properties, the carbonation efficiency, and the leaching behavior of harmful elements. All of the MSWIFAs exhibited a relatively high carbonation efficiency (17.2–27%). The carbonation of MSWIFA effectively inhibited the leaching of Pb, Ba, and Se in the fly ashes. However, the leaching concentration of Cd in the carbonized fly ash increased instead. The leaching concentration of Cr in MSWIFA was not significantly affected by the carbonation reaction. With the addition of 5% CaO, not only was the carbonation efficiency of MSWIFA significantly improved (the efficiency increased by 16.6%), but also the leaching of harmful elements was greatly reduced, achieving the purpose of stabilizing MSWIFA.