Carbon and nitrogen dynamics and greenhouse gas emissions in constructed wetlands treating wastewater: a review
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 n...
Main Authors: | , , , , , , |
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Format: | Article |
Language: | English |
Published: |
Copernicus Publications
2016-01-01
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Series: | Hydrology and Earth System Sciences |
Online Access: | http://www.hydrol-earth-syst-sci.net/20/109/2016/hess-20-109-2016.pdf |
Summary: | 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
<i>pollution swapping</i> (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 (NO<sub>3</sub><sup>−</sup>) removal, as they provide good conditions for denitrification, but cannot remove ammonium (NH<sub>4</sub><sup>+</sup>) due to limited ability to nitrify NH<sub>4</sub><sup>+</sup>. Vertical flow CWs have good NH<sub>4</sub><sup>+</sup> removal, but their denitrification ability is low. Surface flow CWs decrease nitrous oxide (N<sub>2</sub>O) emissions but increase methane (CH<sub>4</sub>) emissions; subsurface flow CWs increase N<sub>2</sub>O and
carbon dioxide (CO<sub>2</sub>) emissions, but decrease CH<sub>4</sub> 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. |
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ISSN: | 1027-5606 1607-7938 |