Integrated environmental influences quantification of pilot-scale constructed wetlands based on modified ecological footprint assessment

Abstract

Constructed wetlands (CWs) are widely used for non-point source pollution control and water environmental quality improvement. Though it is effective in water quality improvement under most conditions, the overall impacts on the ecological environment in terms of greenhouse gases (GHGs) emissions is a growing concern. Besides, large area requirement has limited further applications of the technology in urban areas. A novel assessment tool of integrating grey water footprint into the ecological footprint framework is established for the assessment of pilot-scale CWs. Findings are compared with a natural riparian wetland adjacent to the researched CWs which were monitored simultaneously. Results demonstrated the CWs had relatively good water quality polishing performance, especially for nitrogen removal. Nonetheless, a large amount of CO₂ and some CH₄ and N₂O emissions were recorded. Meanwhile, a substantial amount of CO₂ was also sequestrated by wetland plants via photosynthesis. The strong reducing environment of the CWs inhibited CO₂ and N₂O generation to a great extent. Calculation of all gaseous emissions and sequestration in CO₂ equivalents demonstrated that CWs are an efficient carbon sink. By contrast, the natural wetland was a carbon source because of the high emission of CO₂ and N₂O under its weak reducing environment conditions and low gross primary production. The carbon footprints of the constructed and natural wetlands were -24.24 and 12.99 gha respectively. Modified ecological footprint values were determined by integrating the carbon footprint, water footprint and build-up lands footprint, and a value of -24.36 gha was obtained for the CWs and 12.99 gha for the natural wetlands. The results indicated that the CWs had substantial beneficial impacts on the ecological environment. On account of the multifunctional service values provided by the CWs, a typical paradigm for water pollution remediation and carbon sequestration was presented for ecological and environmental governance, especially for riparian areas.

Keywords: Carbon footprint; Greenhouse gases emission; Grey water footprint; Wastewater treatment; Wetlands.