Methane and carbon dioxide emissions from constructed wetlands receiving anaerobically pretreated sewage

- Emissions increased from 267 mg CH4 m− 2 d− 1 (2nd yr) to 543 mg CH4 m− 2 d− 1 (6th yr).
- Methane emission factor increased from 4.6% (2nd yr) to 16.9% (6th yr).
- CH4 emissions correlate with wetlands age and the season (R2 = 0.89).
- CO2 emissions ranged mostly from 3500 to 5800 mg CO2 m− 2 d− 1.
- Reliable CH4 emissions are obtained in a few hours but CO2 emission requires 24 h.

The aim of this research was to determine methane and carbon dioxide emissions from a hybrid constructed wetland (CW) treating anaerobically pre-treated sewage. The CW was constituted of two horizontal flow (free water surface followed by a subsurface) units. A long-term study was carried out as both CW units were monitored for three campaigns in Period 1 (0.9-1.5 years after start-up), and four campaigns in Period 2 (4.5-5.8 years after start-up). The closed chamber method with collecting surfaces of 1810 cm2 was used. For this system, variability due to position in the transverse section of CW, plant presence or absence and recommended sampling period was determined.Overall methane emissions ranged from 96 to 966 mg CH4 m− 2 d− 1, depending on several factors as the operation time, the season of the year and the position in the system. Methane emissions increased from 267 ± 188 mg CH4 m− 2 d− 1 during the second year of operation to 543 ± 161 mg CH4 m− 2 d− 1 in the sixth year of operation. Methane emissions were related to the age of the CW and the season of the year, being high in spring and becoming lower from spring to winter. Total CO2 emissions ranged mostly from 3500 to 5800 mg CO2 m− 2 d− 1 during the sixth year of operation, while nitrous oxide emissions were below the detection limit of the method.

Evolution of total methane emissions from a hybrid CWs (FWS + HSSF) in operation since June 2005.125