The concept of treating municipal wastewater using wetland ecosystems evolved from the fact that wetlands include both aerobic and anaerobic environments, which enhances N removal by combining several biogeochemical processes. When constructed treatment wetlands (CTWs) are built for the purpose of removing nitrogen from wastewater but are operated with a permanently flooded hydrological regime, they may generate appropriate conditions for the production of greenhouse gases such as nitrous oxide (N2O) and methane (CH4). We studied the effect of three different drying and flooding hydrological regimes (2, 7, and 14 day-dry periods before re-flooding again) on the fluxes of methane and nitrous oxide in mesocosms from a permanently flooded aridland CTW located in Phoenix, AZ, USA.
We found that methane and nitrous oxide fluxes remained constant during the study period in the control mesocosms, however, both gases responded similarly across all experimental drying and re-flooding hydrological regime treatments. High methane fluxes were observed immediately after the re-flooding event (0-5 and 5-15 minutes) from all treatment mesocosms followed by smaller fluxes as mesocosms continued to be flooded. Conversely, low methane fluxes were observed during the drying periods across all treatment mesocosms. Nitrous oxide fluxes were consistently higher during the drying periods compared to the immediate re-flooding event and flooded periods. Smaller nitrous oxide fluxes and occasional fluxes indicating consumption were observed during the flooded periods of the study from all treatment mesocosms. These mesocosm results show that applying a drying and re-flooding hydrological regimes to existing permanently flooded constructed wetlands can potentially trigger unexpected greenhouse gas emissions during dry and wet periods.