Study On Treatment Mechanism Of Saline Wastewater By Halophytes Constructed Wetlands

Saline effluents refer to the wastewater containing high concentration of soluble solid inorganic salts, which are generated from seawater substitute, printing and dyeing factory, paper mill, petroleum chemical industry, pesticide production and food processing industrial. The discharge of saline effluents will pollute the soil, surface water and underground water, and has a great harm to the environment. The saline effluents treatment methods are mostly based on membrane and osmosis desalination technology. However, there are many problems, such as its high-cost and membrane fouling. The application of biotechnology, especially constructed wetlands, has become the focus of recent research. The salt tolerant mechanism of plants and microorganisms has been gradually discovered, which provides a theoretical basis for the application of constructed wetland in the removal of saline wastewater pollutants. Because of the friendly environment, low operation cost and good treatment effect, it is very important to study the constructed wetlands in the treatment of saline wastewater.In this paper, based on field investigation of plants and nitrogen cycling bacteria in natural and constructed wetlands, we selected two typical halophytes-Phragmites australis and Suaeda salsa, and set the two halophytes constructed wetland systems, studying the removal effect and its mechanisms of pollutants in saline wastewater. The main conclusions are as follows:(1) Investigation of natural saline wetland in Yellow River Delta found that the absolute abundance of anaerobic ammonia oxidation bacteria (anammox) in soil and water accounted for the most in four selected genes of nitrogen cycle. Besides, we found that the constructed wetlands have more advantages in the removal of nitrogen pollutants than that of natural wetland. There was no significant difference between bacterial community structure of inshore and offshore soil in Yellow River Delta area. However, there were significant differences in Euryarchaeota and Woesearchaeota (archea). The main differences lay in halobacteria(Euryarchaeota) and AR15-18 (Woesearchaeota). Specially, a large part of Woesearchaeota is unknown and worth further studying. Compared with natural saline and non-saline wetlands, nitrogen transformation rates in large-scale constructed wetlands was more higher, for which the uniformed sandy loam made the upper and lower layers maintain similar microbial quantity.(2) The treatment of saline wastewater by the constructed wetlands with halophytes has achieved good results. This study figure out that the relative COD removal rates (calculated as the ratio of the tested group to that of control group) in Suaeda salsa and Phragmites australis constructed wetlands reached the optimal and were 83.44% and 88.54% under the salinity of around 0.7%, respectively. There was no significant difference in relative nitrogen removal rates between different salinity in Phragmites australis constructed wetlands. However, in Suaeda salsa constructed wetlands, the maximum relative nitrogen removal rates in the salinity about 1.4% was 135.69%, while. For the two systems, the relative total phosphorus removal effect reached the maximum at 1.4% salinity, for Suaeda salsa the maximum was 100.06%, while for Phragmites australis were 174.43% and 355.19%. During the whole experimental period, the changes of the salinity in the sewage were not obvious. In particular, in the salinity of 1.4% (Suaeda salsa) and 0.7% (Phragmites australis), the salinity in those adding anammox increased. However, the phenomenon has not been replicated, and need further research.(3) From the study of material flow and distribution, we have addressed the destination of pollutants in constructed wetlands. The main route of the wetland organic carbon was biological aerobic catabolism, followed by the plant fixation, drainage and sediment accumulation; In Suaeda salsa wetlands, nitrogen transformation in those inoculated with anammox was larger than those without inoculation, In the Phragmites australis with anammox, the competitiveness of those with high sale was weaker than the that of salt-free ones. Most of the nitrogen flew in the water rather than absorbed by other microbial processes; Anammox may have inhibitory effect on phosphorus degradation bacteria.(4) From the study of salt tolerance of halophytes, we found out the influence of salt on saline effluents treatment performance. Salinity did greater damage on Suaeda salsa than Phragmites australis. Therefore, as the salinity reached 1.4%, even 2.1%, the integrity of Suaeda salsa was better than Phragmites australis, leading to the higher removal efficiency in Suaeda salsa. Different salt resistance mechanism resulted in the differences of damage between the two plants, which influenced the treatment effect of saline wastewater.(5) From the test of the nitrogen cycling microorganism abundances, we discovered the microbial mechanisms of nitrogen removal. Anammox are more suitable and resistant to high salinity than ammonia oxidizing bacteria, which may contribute to the treatment of saline wastewater. In high salinity (above 0.7% are high salinity in this study), compared with the ammonia oxidizing bacteria and anammox, nitrite oxidizing bacteria (Nitrospira) are more sensitive to salinity, resulting in large inhibition on nitrification in systems. Anammox have a strong resistance to salinity, and the CANON technology is an effective way to remove the pollutants.In this paper, halophytes and anammox were applied to constructed wetlands in treating saline effluents. It can not only effectively improve the pollutant removal effect, but save costs and achieve advanced saline effluents treatment in larger area. Besides, it’s of vital important to increase the added value of the wetlands and the social benefits. The research results provided basic theory and practice for enhanced CANON process in treating saline effluents by constructed wetlands, and reference for the optimization of the wetlands, which had important significance for purification of saline effluents and protection of water environment.