Ammonia emissions from surface flow and subsurface flow constructed wetlands treating dairy wastewater
Received for publication January 14, 2008. Agricultural wastewater treatment is important for maintaining water quality, and constructed wetlands (CW) can be an effective treatment option. However, some of the N that is removed during treatment can be volatilized to the atmosphere as ammonia (NH3). This removal pathway is not preferred because it negatively impacts air quality. The objective of this study was to assess NH3 volatilization from surface flow (SF) and subsurface flow (SSF) CWs. Six CWs (3 SF and 3 SSF; 6.6 m2 each) were loaded with dairy wastewater (300 mg L–1 total ammoniacal nitrogen, TAN = NH3–N + NH4+–N) in Nova Scotia, Canada. From June through September 2006, volatilization of NH3 during 12 or 24 h periods was measured using steady-state chambers. No differences (p > 0.1) between daytime and nighttime fluxes were observed, presumably due in part to the constant airflow inside the chambers. Changes in emission rates and variability within and between wetland types coincided with changes in the vegetative canopy (Typha latifolia L.) and temperature. In SSF wetlands, the headspace depth also appeared to affect emissions. Overall, NH3 emissions from SF wetlands were significantly higher than from SSF wetlands. The maximum flux densities were 974 and 289 mg NH3–N m–2 d–1 for SF and SSF wetlands, respectively. Both wetland types had similar TAN mass removal. On average, volatilization contributed 9 to 44% of TAN removal in SF and 1 to 18% in SSF wetlands. Results suggest volatilization plays a larger role in N removal from SF wetlands.