Changing the order and ratio of substrate filling reduced CH4 and N2O emissions from the aerated constructed wetlands

Constructed wetlands (CWs) have been used to enhance pollutant removal by filling several types of material as substrates. However, research on substrate filling order remains still limited, particularly regarding the effects of greenhouse gas (GHG) emissions. In this study, six CWs were constructed using zeolite and ferric-carbon microelectrolysis (Fe -C) fillers to evaluate the effect of changing the filling order and ratio on pollutant removal, GHGs emissions, and associated microbial structure. The results showed that the order of substrate filling significantly impacted pollutant removal performance on CWs. Specifically, CWs filled with zeolite in the top layer exhibited superior NH 4 + -N removal compared to those filled in the lower layer. Moreover, the highest NH 4 + -N removal (95.0 % +/- 1.9 %) was observed in CWs with a zeolite to Fe -C volume ratio of 8:2 (CW Ze-1 ). Moreover, zeolitefilled at the top had lower GHGs emissions, with the lowest CH 4 (0.22 +/- 0.10 mg m - 2 h -1 ) and N 2 O (167.03 +/- 61.40 mu g m - 2 h -1 ) fluxes in the CW Ze-1 . In addition, it is worth noting that N 2 O is the major contributor to integrated global warming potential (GWP) in the six CWs, accounting for 81.7 % -90.8 %. The upper layer of CWs filled with zeolite exhibited higher abundances of nirK , nirS and nosZ genes. The order in which the substrate was filled affected the microbial community structure and the upper layer of CWs filled with zeolite had higher relative abundance of nitrifying genera ( Nitrobacter , Nitrosomonas ) and denitrifying genera ( Zoogloea , Denitratisoma ). Additionally, N 2 O emission was reduced by approximately 41.2 % -64.4 % when the location of the aeration of the CWs was changed from the bottom to the middle. This study showed that both the order of filling the substrate and the aeration position significantly affected the GHGs emissions from CWs, and that CWs had lower GHGs emissions when zeolites were filled in the upper layer and the aeration position was in the middle.