Nitrogen removal and associated greenhouse gas production in laboratory-scale treatment wetlands

Abstract

Treatment wetlands (TWs) are designed to treat domestic wastewater and water polluted from non-point sources such as agricultural runoff. Because many recent design improvements have increased aerobic removal pathways, nearly complete removal of biological oxygen demand (BOD) and oxidation of ammonium to nitrate in domestic wastewater is possible. These improvements have come at the expense of reducing the TW capacity to remove nitrate. Nitrate is also a main pollutant of concern in many non-point pollution sources. Organic carbon (OC) is a limiting factor for microbial nitrate removal and in wetlands can be supplied externally or provided by plants. Nitrate removal has the potential to release greenhouse gases (GHG) resulting in TWs being capable of acting as a net source or sink for GHG. Increasing our understanding of nitrogen removal and GHG production in TWs is the overarching goal of this project. A multi-year controlled environment greenhouse study measured water quality within 15-day incubations over annual cycles of temperature as well as greenhouse gas production at the seasonal extremes of the annual cycle. The experiment consisted of microcosms planted with either Carex utriculata, Deschampsia cespitosa, Phragmites australis, Schoenoplectus acutus, Typha latifolia or left unplanted. The fully factorial experiment also included three levels of OC addition, ranging from zero to two times the stoichiometric equivalent required for complete nitrogen removal. Nitrogen removal was affected by all experimental factors; plant species, OC addition, and temperature, with plant species mediating the effects of carbon and temperature in some treatments. The three highest performing species, C. utriculata, P. australis and S. acutus, removed nitrogen at an annual rate exceeding 166 g m -2 yr -1, without OC; only C. utriculata showed less N removal in winter. Incubation time series analysis indicated greater total and seasonal removal capacity for these plant treatments. Total GHG emission was dominated by summer CO 2 emission and varied by plant treatment and carbon load. CO 2 emission correlated negatively with OC addition in the high performing species attributable to plant biomass that decreased with OC addition. N 2O production significantly increased with the addition of organic carbon and did not vary significantly by season.