Efficient nitrogen removal by coupling with methane metabolism in three landfill leachate wastewater treatment...

High concentration of ammonium-nitrogen (NH₄⁺-N) is a widley known characteristic of landfill leachate, and therefore, a thorough understanding of the active nitrogen-related microorganisms is essential for designing an efficient landfill leachate treatment system. Here, we analyze the microbial nitrogen cycling and metabolic interactions at three full-scale landfill leachate treatment plants. Specifically, using metagenomics and metatranscriptomics techniques, we elucidate the mechanisms underlying high NH₄⁺-N removal rates achieved through distinct N removal strategies (nitrification-denitrification system at Jilong (JL), denitrification-ammonia oxidation-partial nitrification system at Xinfeng (XF), and simultaneous nitrification-denitrification-ammonia oxidation system at Wenshan (WS)), all three plants achieved efficient NH₄⁺-N removal of 96 %, 97.96 % and 97.33 % for JL, XF, and WS, respectively. Highly active methane (CH₄)-associated microorganisms, particularly CH₄-catabolizing bacteria, exhibited strong interactions with denitrifying bacteria in all three treatment plants, promoting effective nitrogen removal in the landfill treatment systems. In addition, enhanced anammox activity in WS treatment plant in 2023 and 2015 was compared using nitrogen-15 isotope labeling and in situ transcriptome analysis. By comparing microbial metabolic activity, strong interactions were observed between anammox bacteria and other nitrogen-transforming microbes. Meanwhile, detailed physicochemical parameter analysis revealed that optimal Fe³ ⁺ concentrations significantly promote the enrichment and enhanced activity of anammox bacteria. Collectively, this study highlights strategies for microbial-mediated high-efficiency ammonia nitrogen removal in full-scale landfill leachate and explains the underlying mechanisms that may lead to increased abundance and activity of anammox bacteria.