Investigating CH4 and N2O emissions from eco-engineering wastewater treatment processes using constructed wetland microcosms

Methane (CH4) and nitrous oxide (N2O) are important greenhouse gases, because of their contribution to the global greenhouse effect. The present study assessed emissions of N2O and CH4 from constructed wetland microcosms, planted with Phragmites australis and Zizania latifolia, when treating wastewater under different biological oxygen demand (BOD) concentration conditions. The removal rate was 95% for BOD and more than 80% for COD in all three pollutant concentrations, both plants'removal rates of pollutants were at almost the same level, and both were found to resist BOD concentrations as high as 200mgL-1. When BOD concentrations fell below 200mgL-1, the soil plant units reached an average of 80-92% T-N and T-P removal rates; however, as the concentrations increased to 200mgmgL-1 or when during the initial phases of winter, the removal rates for T-N and T-P decreased to less than 70%. With NH3-N removal, the influences of BOD concentrations and air temperature were more obvious. When BOD concentrations increased to 100mgL-1 after October, an obvious decrease in NH3-N removal was detected; almost no nitrification occurred beginning in December at BOD concentrations of 200mgmgL-1. N2O and CH4 emissions showed obvious seasonal changes; higher emissions were observed with higher BOD concentrations, especially among Z. latifolia units. The enumeration of methane-oxidizing bacteria and methane-producing bacteria was also conducted to investigate their roles in impacting methane emissions and their relationships with plant species. The pollutant purification potentials of P. australis and Z. latifolia plant units during wastewater treatment of different pollutant concentrations occurred at almost the same levels. The nutrient outflow and methane flux were consistently higher with Z. latifolia units and higher concentrations of BOD. The more reductive status and higher biomass of methanogens may be the reason for the lower nitrification and higher CH4 emissions observed with Z. latifolia units and higher concentration systems. The Z. latifolia root system is shallow, and the activity of methanotrophs is primarily confined to the upper portion of the soil. However, the root system of P. australis is deeper and can oxidize methane to a greater depth. This latter structure is more favorable as it is better for reducing methane emissions from P. australis soil plant systems.