Impact of external carbon source addition on methane emissions from a vertical subsurface‐flow constructed wetland

In this study, different concentrations of urea (0, 12.1, 30, 45, 61 and 80 mmol·L−1) were added separately, as external carbon sources, to a two‐stage vertical subsurface‐flow constructed wetland (VSSF CW) where Cyperus alternifolius L. was planted, with the aim of understanding methane (CH4) emissions driven by urea. Results indicate that the average CH4 emissions from a two‐stage VSSF CW were 6.88, 7.11, 6.22, 7.45, 5.06 and 2.80 mol·m−2·day−1, corresponding to urea concentrations of 0, 12.1, 30, 45, 61 and 80 mmol·L−1 added in the VSSF CW, respectively. Urea as a carbon source had an average of 31.57% of influent total organic carbon (TOC). It was transformed into CH4‐C, of which CH4‐C/TOCinfluent may be be considered as an important component when anthropogenic methanogenesis from treatment wetlands was driven by carbon sources or carbon loading. Methane emissions were at their lowest when the C/N ratio was 5.89, at a urea concentration of 80 mmol·L−1. Principal component analysis (PCA) indicates that CH4 correlated positively with temperature and redox conditions (Eh). Methane emissions driven by urea in the two‐stage VSSF CW were found to be in accordance with the second‐rate dynamics kinetic model (kinetic constant = 22.94 mg CH4·h−1, R2 = 0.99), which can be considered as a high level of CH4 emissions. It indicates that external carbon sources can influence CH4 emissions from two‐stage VSSF CW significantly. © 2019 Society of Chemical Industry and John Wiley & Sons, Ltd.