Carbon and nitrogen dynamics and greenhouse gases emissions in constructed wetlands: a review

Abstract

The removal efficiency of carbon (C) and nitrogen (N) in constructed wetlands (CWs) is very inconsistent and frequently does not reveal whether the removal processes are due to physical attenuation or whether the different species have been transformed to other reactive forms. Previous re- search on nutrient removal in CWs did not consider the dy- namics of pollution swapping (the increase of one pollutant as a result of a measure introduced to reduce a different pollu- tant) driven by transformational processes within and around the system. This paper aims to address this knowledge gap by reviewing the biogeochemical dynamics and fate of C and N inCWsand their potential impact on the environment, and by presenting novel ways in which these knowledge gaps may be eliminated. Nutrient removal in CWs varies with the type ofCW, vegetation, climate, season, geographical region, and management practices. Horizontal flow CWs tend to have good nitrate (NO− 3 ) removal, as they provide good conditions for denitrification, but cannot remove ammonium (NH+ 4 ) due to limited ability to nitrify NH+ 4 . Vertical flow CWs have good NH+ 4 removal, but their denitrification ability is low. Surface flow CWs decrease nitrous oxide (N2O) emissions but increase methane (CH4) emissions; subsurface flow CWs increase N2O and carbon dioxide (CO2) emissions, but de- crease CH4 emissions. Mixed species of vegetation perform better than monocultures in increasing C and N removal and decreasing greenhouse gas (GHG) emissions, but empirical evidence is still scarce. Lower hydraulic loadings with higher hydraulic retention times enhance nutrient removal, but more empirical empirical evidence is required to determine an optimum de- sign. A conceptual model highlighting the current state of knowledge is presented and experimental work that should be undertaken to address knowledge gaps across CWs, veg- etation and wastewater types, hydraulic loading rates and regimes, and retention times, is suggested. We recommend that further research on process-based C and N removal and on the balancing of end products into reactive and benign forms is critical to the assessment of the environmental per- formance of CWs.