Does duckweed ponds used for wastewater treatment emit or sequester greenhouse gases?

The reduction of greenhouse gases (GHG) emissions is important challenge in the wastewater treatment plants. In this way, the present study aimed to evaluate the GHG emissions and carbon dioxide fixation by duckweed ponds (DWP) applied to treat municipal wastewater in a polishing stage. Two pilot DWP (3000 L) were operated in a series with real wastewater receiving a flow rate of 200 L d^-1 and organic load rate of 39 g COD ha^-1 d^-1. Beyond the standard physicochemical parameters for wastewater monitoring, the gases emissions from pond surface were measures by using a static flux chamber with infrared probes installed inside to detect CO₂ and CH₄ concentration. Operating the DWP with a load of 18.1 kg TN ha^-1 d^-1 and 2 kg TP ha^-1 d^-1, across 425 days of monitoring, higher COD and nutrient removal efficiency was identified (79%, 93% and 84% for COD, TN and TP, respectively). The CO₂ emission rate ranged from 3048 to 6017 mg CO₂ m^-2 d^-1 and the fixation rate ranged from 19,592 to 42,052 mg CO₂ m^-2 d^-1. Methane emission was not detected (less than 0.1%). Moreover, low abundance of archaeal community was identified in both DWP. The results showed that in presented conditions, under low COD loading rate DWP could fix at least three times more CO₂ than it emits, highlighting the sustainability of this natural technology.