Greenhouse gas production and global efficiency of planted and artificially aerated constructed wetlands

Background/Question/Methods Constructed wetlands (CWs) are artificial ecosystems built to remove nutrients and organic carbon from wastewaters, and act as important buffers to receiving natural environment. The contribution of CWs to global warming via production of greenhouse gases (GHG) is receiving increasing attention given that high emission rates could mitigate their environmental benefits. The objectives of our study were to isolate the relative contribution of 3 macrophyte species (Phragmites australis, Typha angustifolia, Phalaris arundinacea) and artificial aeration on GHG production and nutrient removal across different seasons using an experimental mesocosm approach. From May to December 2006, we measured nitrous oxide (N₂O), carbon dioxide (CO₂) and methane (CH₄) fluxes using the headspace equilibration method in 14 CW mesocosms supplied with 60 L m^-2 d^-1 of wastewater. We also monitored nutrient removal efficiency (nitrogen species, phosphorus, organic carbon, oxygen demand, suspended solids), physico-chemical variables (pH, water temperature, redox potential) and hydraulic properties (hydraulic residence time and dispersion, proportion of dead volumes).

Results/Conclusions Fluxes of CH₄ were reduced in planted units, aerated units and during the winter likely as a function of greater oxygenation under these conditions. Fluxes of CO₂ were higher in planted units and during the summer, and this may be due to higher microbial respiration rates in these treatments. Fluxes of N₂O were higher in units planted with P. australis and during the summer, but the high variability in fluxes observed for this gas prevented the identification of any clear pattern. Globally, T. angustifolia had the lowest GHG production among the macrophyte species surveyed. Global warming potential (GWP) was higher in unplanted units, non-aerated units, and during the summer. Methane was the main contributor of GWP in non-aerated units, whereas CO₂ was the most important GHG in planted and aerated units. N₂O had a minor contribution to GWP in all treatments. We found that artificially aerated and planted units (especially those with T. angustifolia) had the overall best performance among the treatments tested in this study, with the lowest GWP, the highest removal efficiencies and the best hydraulic properties.