Methane Emissions from Constructed Wetlands Treating Agricultural Wastewaters

Abstract

Methane emissions were measured during mid-summer in four pilot-scale constructed wetlands that had treated dairy farm wastewaters for a period of 2 yr. Measurements were made at up and downstream sites in wetlands receiving low and high wastewater loadings (∼26 and 45 mm d -1 ), both in the presence of wetland vegetation (Schoenoplectus validus). An automated flux chamber (enclosure area 0.25 m 2 ) and gas circulation system, and associated sampling and chromatographic analysis system, were used to make measurements directly in the field. Median emissions ranged between 48 and 482 mg CH 4 m 2 d -1 , without discernible diurnal patterns. Upstream sites, closest to wastewater inflows, generally showed significantly higher (P < 0.05) emissions than downstream sites in the same wetland. Unvegetated sites tended to show higher emission rates than corresponding vegetated sites, with highest rates recorded at the highest loaded unvegetated site. Redox potentials in the surface 100 mm of the substratum at upstream sites, with and without vegetation, showed consistently more oxidized conditions in the presence of plants. This suggests that plant root-zone oxidation was acting to suppress methanogenesis and/or enhance methane oxidation in the vegetated wetlands. Emissions from the vegetated constructed wetlands were comparable with those reported for natural wetlands and inorganically fertilized rice paddies. Methane emissions were estimated to account for around 2 to 4% of wastewater C loadings to the vegetated wetlands and 7 to 8% of loadings to the unvegetated systems during the period of measurement.