Comparative Analysis of Greenhouse Gas Emission from Three Types of Constructed Wetlands

Abstract

Copyright: © 2015 Tao W. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Wastewater treatment processes can produce anthropogenic greenhouse gases, including carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O). Wastewater treatment in the U.S. emitted 17.8 Tg CO2equivalent (CO2-eq) in 2012, accounting for 0.27% of the U.S. total greenhouse gas emission and 14.3% of the total greenhouse gas emission from waste management and treatment activities (U.S. EPA, 2014). Constructed wetlands are green infrastructure for treatment of various types of wastewater. As ecologically engineered treatment systems, constructed wetlands mimic the appearance of natural wetlands (Figure 1). Constructed wetlands utilize natural processes involving macrophytes, soils or other porous media, and the associated microbial assemblages for water quality improvement. There are mainly three types of constructed wetlands [1]. Free water surface (FWS) wetlands typically consist of cells with aquatic plants, relatively impermeable rooting substrate, and shallow water. Treatment in FWS constructed wetlands occurs as water flows slowly above ground through the leaves and stems of aquatic plants. Horizontal subsurface flow (HSSF) wetlands contain beds of porous media that may have been planted with aquatic plants. Wastewater flows horizontally beneath the surface of the medium beds. Vertical flow (VF) wetlands contain beds of media that may have been planted with aquatic plants. Water is distributed over the ground surface or from the base, and flows downward or upward through the medium beds. When designing a constructed wetland treatment system, wetland type is first selected in regards to several factors such as land availability, local climate, and wastewater characteristics. Sustainability has become an important consideration in wastewater treatment. Correspondingly, greenhouse gas emission from constructed wetlands has been measured in full-scale constructed wetlands in the last decade. Mander et al. [2] compared the three types of constructed wetlands in terms of CO2-C, CH4-C, and N2O-Nemission rates separately. Actually, the same mass emission rates of CO2, CH4, and N2O contribute very differently to global warming. To compare the overall effect of greenhouse gas emissions amongst different types of constructed wetlands, it is necessary to calculate CO2-eq emissions [3,4]. With inclusion of climate-carbon feedbacks in response to emissions of non-CO2 gases, the 100-year global warming potential (GWP100) is 1 for CO2, 34 for CH4, and 268 for N2O (IPCC, 2013). CO2-eq emission rate in the three types of constructed wetlands were then obtained by multiplying the emission rate of a greenhouse gas reviewed by (2) by its GWP100 as given in Equations 1-3: CO2-eq of CO2, mg/m /h=(CO2, mg C/m 2/h) × (44/12) × 1 (1) CO2-eq of CH4, mg/m /h=(CH4, mg C/m 2/h) × (16/12) × 34 (2) CO2-eq of N2O, mg/m /h=(N2O, mg N/m 2/h) × (44/28) × 268 (3) Based on emission rates of individual greenhouse gases, Mander et al., [2] concluded that CO2 emission was significantly lower in FWS constructed wetlands than in HSSF and VF constructed wetlands, CH4emission was significantly lower in VF constructed wetlands than HSSF constructed wetlands, and that there were no significant differences in N2O emission in various wetland types. Based on the CO2-eq emission rates (Table 1), we found that: 1. CO2-eq emission rate in constructed wetlands is in the order of HSSF>VF>FWS, either in terms of total or individual greenhouse gases except for CH4 that FWS>VF; 2. CO2-eq emitted from constructed wetlands is in the order of CO2>CH4>N2O for all the three types of constructed wetlands. 3. CO2 accounted for 76% of the total CO2-eq emitted from VF constructed wetlands; 4. Both CO2 and CH4 are the primary greenhouse gases emitted from FWS constructed wetlands; and 5. CO2 and CH4 accounted for 51% and 41% of total CO2-eq emitted, respectively, from HSSF constructed wetlands.