Abstract
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 research on nutrient removal in CWs did not consider the dynamics of pollution swapping (the increase of one pollutant as a result of a measure introduced to reduce a different pollutant) 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 in CWs and 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 of CW, vegetation, climate, season, geographical region, and management practices. Horizontal flow CWs tend to have good nitrate (NO3−) removal, as they provide good conditions for denitrification, but cannot remove ammonium (NH4+) due to limited ability to nitrify NH4+. Vertical flow CWs have good NH4+ 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 decrease 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 evidence is required to determine an optimum design. A conceptual model highlighting the current state of knowledge is presented and experimental work that should be undertaken to address knowledge gaps across CWs, vegetation 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 performance of CWs.
Highlights
Increasing anthropogenic loading of reactive nitrogen (Nr; all forms of nitrogen (N) except di-nitrogen gas, N2) along the N cascade in the environment raises many critical concerns for human health, drinking water quality (Gray, 2008), coastal and marine water degradation as well as algal blooms and hypoxia (Conley et al, 2009; Rabalais et al, 2010)
While the biogeochemistry of wetlands in general has been discussed in the literature (Whalen, 2005; Reddy and Delaune, 2008), less is known about the delivery pathways of the transformation products of carbon (C) and N from Constructed wetlands (CWs) treating
Many investigations target single contaminant remediation and disregard the reality of mixed contaminants entering and leaving CWs. They do not consider the dynamics of pollution swapping driven by transformational processes within and around the system
Summary
Increasing anthropogenic loading of reactive nitrogen (Nr; all forms of nitrogen (N) except di-nitrogen gas, N2) along the N cascade in the environment raises many critical concerns for human health, drinking water quality (Gray, 2008), coastal and marine water degradation as well as algal blooms and hypoxia (Conley et al, 2009; Rabalais et al, 2010). They do not consider the dynamics of pollution swapping (the increase in one pollutant as a result of a measure introduced to reduce a different pollutant) driven by transformational processes within and around the system This means that potential negative impacts that CWs may have on the environment, such as greenhouse gas (GHG) emissions (IPCC, 2013; Clair et al, 2002; Mander et al, 2008; Mitsch and Gosselink, 2000) or enhancement of pollution swapping (Reay, 2004), are not accounted for in analyses. SF – surface flow; HSSF – horizontal subsurface flow; VSSF – vertical subsurface flow; ∗ mg N m−2 h−1
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