Researchers from China have shown that Myriophyllum elatinoides, or Christmas tree plants in China, can remove nearly all nitrogen from swine wastewater, thereby reducing the emission of greenhouse gases such as nitrous oxide and nitric oxide. Their findings have been published in Environmental Science and Pollution Research.
China is one of the world’s largest producers of pork, and swine wastewater treatment is becoming an urgent issue in the country. In recent years, farmers are turning to constructed wetlands that use the natural functions of vegetation, soil and organisms to treat different water streams. Constructed wetlands are an effective and inexpensive technology for removing nitrogen from swine wastewater.
However, the treatment process produces high amounts of N2O and NO. This is because only the conversion of nitrogen into N2O, NO and nitrogen gas are considered as permanent removal of the substance.
To investigate the role played by different plants in converting nitrogen to N2, researchers from the Institute of Subtropical Agriculture, Chinese Academy of Sciences, measured the NO and N2O emissions from wetland environments that contained the plants Myriophyllum elatinoides (Christmas trees), Alternanthera philoxeroides and Eichhornia crassipes, comparing their results to a control that did not have vegetation.
They found that the maximum emissions of NO and N2O occurred in the microcosm with E. crassipes, which was significantly higher than those with M. elatinoides, A. philoxeroides and the control.
“This result reflects strong nitrification and denitrification occurred in E. crassipes,” says Zhang Shunan, a doctoral researcher at ISA.
“During the whole incubation period, total nitrogen removal rates of 96.4%, 74.2%, 97.2% and 47.3% were observed for the M. elatinoides, A. philoxeroides, E. crassipes, and (the control) microcosms respectively, but no significant difference occurred in M. elatinoides and E. crassipes.”
In addition, the emissions of N2O from the E. crassipes microcosm accounted for 2.10% of initial total nitrogen load and 2.17% of the total amount of total nitrogen removal, compared with less than 1% for the other microcosms.
These findings indicate that wetlands with Christmas trees may be most optimal for swine wastewater treatment, based on its higher removal of N and lower emissions of NO and N2O.
The article can be found at Zhang et al. (2015) Emissions of NO and N2O in wetland microcosms for swine wastewater treatment.