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

Abstract 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 cm 2 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 CH 4  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 CH 4  m − 2  d − 1 during the second year of operation to 543 ± 161 mg CH 4  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 CO 2 emissions ranged mostly from 3500 to 5800 mg CO 2  m − 2  d − 1 during the sixth year of operation, while nitrous oxide emissions were below the detection limit of the method.