Abstract
This work aims to investigate the methane emissions from integrated vertical-flow constructed wetlands (IVCWs) when ethanol is added as an external carbon source. In this study, a gradient of ethanol (0, 2, 4, 8, 16 and 32 mmol/L) was added as the carbon source in an IVCW planted with Cyperus alternifolius L. The results showed that the methane emission flux at an ethanol concentration of 32 mmol/L was 32.34 g CH4 m−2 day−1 less than that of the control experiment (0 mmol/L) and that the methane emission flux at an ethanol concentration of 16 mmol/L was 5.53 g CH4 m−2 day−1 less than that at 0 mmol/L. In addition, variations in the water quality driven by the different ethanol concentrations were found, with a redox potential range of −64 mV to +30 mV, a pH range of 6.6–6.9, a chemical oxygen demand (COD) removal rate range of 41% to 78%, and an ammonia nitrogen removal rate range of 59% to 82% after the ethanol addition. With the average CH4-C/TOC (%) value of 35% driven by ethanol, it will be beneficial to understand that CH4-C/TOC can be considered an ecological indicator of anthropogenic methanogenesis from treatment wetlands when driven by carbon sources or carbon loading. It can be concluded that adding ethanol as an external carbon source can not only meet the water quality demand of the IVCW treatment system but also stimulate and increase the average CH4 emissions from IVCWs by 23% compared with the control experiment. This finding indicates that an external carbon source can stimulate more CH4 emissions from IVCWs and shows the importance of carbon sources during sewage treatment processes when considering greenhouse emissions from treated wetlands.
Highlights
Constructed wetlands (CWs) are an engineered form of wetland that utilizes the natural environment—substrates, microorganisms, and plants—as a form of water treatment, mimicking a naturally occurring wetland [1] and are designed to take advantage of naturally occurring processes involving wetland plants, soil matrices and microorganisms for the wastewater treatment [2]
To evaluate ethanol as a carbon source and its effect on the methane emissions in constructed wetlands, the objectives of this study were (1) to analyze the rate of the methane emissions from six different concentrations of ethanol added into integrated vertical-flow constructed wetlands (IVCWs); (2) to investigate the effects of ethanol in the methane flux and the carbon usage rate and assess the indication of CH4 -C/total organic carbon (TOC), and at the same time, (3) to explore the dynamic kinetic features of the methane emissions in an IVCW driven by different ethanol concentrations, which may have an impact on the methane emissions
A gradient of ethanol as a carbon source had a significant impact on the methane emissions in the IVCW in this study
Summary
Constructed wetlands (CWs) are an engineered form of wetland that utilizes the natural environment—substrates, microorganisms, and plants—as a form of water treatment, mimicking a naturally occurring wetland [1] and are designed to take advantage of naturally occurring processes involving wetland plants, soil matrices and microorganisms for the wastewater treatment [2]. The integrated vertical-flow constructed wetland (IVCW) is a special wastewater treatment system that has a nice wastewater treatment effect and functions of landscape design and viewing. IVCW consists of a down-flow tank and an up-flow tank This U-shaped flow structure gives rise to the metagenic “aerobic-anoxic-anaerobic-anoxic-aerobic” multifunctional layers [3]. The structure of the influent tank allows it to function as an aerobiotic zone, whereas the effluent tank is an anaerobic zone [4]
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