Are different configurations of pilot-scale constructed wetlands carbon sources or carbon sinks?

Abstract

Constructed wetlands (CWs) are gaining recognition as important carbon sinks, subject to factors such as system design and vegetation. However, the effect of configuration on carbon emissions in CWs remains inadequately understood. We constructed three configurations of CWs, free-water surface flow (FWS), horizontal subsurface flow (HSSF), and vertical subsurface flow (VSSF), to assess contaminant removal performance and carbon emissions. Higher removal efficiencies for NH₄⁺-N, NO₃⁻-N and COD were observed in HSSF (72.06%, 60.90%, and 70.01%) and VSSF (75.18%, 48.94%, and 69.47%) compared to FWS (64.89%, 35.50%, and 58.83%). FWS exhibited the highest CH₄ emissions (1.59 mg/(m²·h)) and lowest CO₂ emissions (−176.26 mg/(m²·h)) due to a greater abundance of Methanobacterium and plant biomass. Higher N₂O emissions were observed in VSSF (0.33 mg/(m²·h)) compared to FWS (0.18 mg/(m²·h)) and HSSF (0.12 mg/(m²·h)). In general, the majority of carbon was buried in substrate (55.53%–64.50%), followed by plants (24.90%–40.84%) and wastewater (4.05%–14.08%). Carbon budget estimation showed that all CWs exhibited characteristics of carbon sinks. FWS exhibited the highest annual net carbon sink capacity at 4.78 kg CO₂-eq/(m²·yr), followed by VSSF and HSSF at 2.81 and 2.54 kg CO₂-eq/(m²·yr), respectively.

Graphical abstract

Keywords

Constructed wetlands / Carbon emission / Configuration / Pollutant removal / Seasonal variation

Highlight

	● This is the first study exploring carbon emissions in various CW configurations.
	● Carbon budget estimation supports the role of CWs as carbon sinks.
	● Substrate deposition accounted for the majority of carbon sink in CWs.
	● Electricity consumption accounts for the majority of total carbon emissions.
	● FWS CWs demonstrated the highest annual carbon sink, followed by VSSF and HSSF.

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