Enhanced nitrogen removal in modular moving bed constructed wetland at low temperature: Optimization of dissolved oxygen distribution and reconfiguration of core microbial symbiosis.

Low temperatures can significantly reduce nitrogen (N) removal efficiency of constructed wetlands (CWs), thus limiting the application of this technology in cold climates and cold areas. We developed modular moving bed constructed wetlands (MMB-CWs) by integrating biofilm method into CWs through specialized design and achieved satisfactory N removal under ambient condition. Evaluating the N removal performance of MMB-CWs at low temperature is crucial for promoting CWs in cold climates. This study investigated the N removal performances of MMB-CWs and the variations of core functional genera at low temperature. Results indicated that the MMB-CW with a 60 % substrate filling rate achieved the highest N removal efficiency of 68.6 %, exceeding horizontal subsurface flow CW by 19.5 % (p < 0.05). The incorporation of vertical baffles and partial substrate filling optimized the distribution and concentration of dissolved oxygen. Although microbial community in the MMB-CW experienced a decline in microbial richness and diversity, N-transforming genera became more concentrated. Proteobacteria increased significantly from 46.6 % to 69.0 % (p < 0.05) as temperature decreased, in which the denitrifying genera including unclassified_f__Comamonadaceae, Hydrogenophaga and Acinetobacter increased significantly (p < 0.05) and dominated the N removal process. The distribution of N-transforming functional genes suggested that denitrification was the primary pathway for N removal at low temperature, while anaerobic ammonium oxidation played a pivotal role as well. The findings reveal the mechanism by which the MMB-CW enhance N removal in low C/N wastewater at low temperature, providing strategy and theoretical support for improving the N removal performance of CWs in response to low temperature stress.