| With the development of our economy and society, the increasing water demand and waste of water resources and pollution makes the water resources in China become more prominent. At present, the activated sludge sewage treatment method is widely used around the world, but the method is costly and operationally complex, and requires a lot of professional and technical personnel, thus to use less in rural areas and small towns. As an efficient, without energy consumption, easy management and environmentally-friendly water treatment technology, constructed wetland has been widely used in various fields. However, the stability of the constructed wetland in the actual operation of the process of contaminant removal is essential, and many factors, such as type of wetlands, building parameters, operational factors and environmental conditions, have a significant impact on its performance. Therefore, it is necessary to investigate the long-term operational stability of constructed wetland and the factors that affect its performance.This study concentrated on the removal efficiency, stability, and impact factors of the long-term running artificial wetlands for pollutant removal. The main contents and results are as follows:(1) The effects of constructed factors (wetland aspect ratio, filler type, plant type), operating factors (influent loading, hydraulic retention time) and environmental factors (temperature) on the performance of a pilot-scale constructed wetlands system with an area of about150m2were systematically studied. The results show that the larger the wetland aspect ratio (aspect ratio=2), cinder fill, and Cyperus alternifolius are more conducive to the removal of pollutants. The amount of pollutant removal increased with increase in influent loading. The removal efficiency decreased at high hydraulic loading. However, the removal efficiency was unstable at low hydraulic loading. In terms of the removal efficiency and removal amount, the appropriate influent loadings are5～30,10～55,2.5～10,5～12,0.4～1.2g/m2d for TOC, COD, AN, TN, and TP, respectively. The appropriate hydraulic retention time (HRT) is about5d. The high temperature is more conducive to the removal of pollutants. The statistical analysis showed that type of fillers caused a significant impact on the removal of phosphorus. Whereas, constructed factors caused no significant effect on the removal of other contaminants. The influent loading showed significant positive correlation with the removal amount of pollutants. However, HRT showed negative correlation with the removal amount of pollutants. Both influent loading and HRT caused no significant impact on the removal efficiency of pollutants. The partial correlation analysis showed that the effect of influent loading on the removal amount of pollutants is greater than that of the HRT. Temperature showed a significant positive correlation with TP removal.(2) Based on the28-month monitoring data, it is found that that the gravel and cinder constructed wetlands can be effective and stable removal of organic matter. Furthermore, pollutant removal gradually increased with increasing years of running. The limited dissolved oxygen within the constructed wetland caused limited and stable removal of ammonia nitrogen and TN in the gravel and cinder wetlands. Gravel wetlands were ineffective for phosphorus removal, showing fluctuation of the P removal rate within a certain range. The cinder showed high removal of phosphorus, but the P removal rate decreased with increasing in the running time. Cinder wetland showed slightly higher organic matter and TN removals than that of gravel wetland. For P, however, it was significantly higher than the gravel wetland. No significant difference was found for AN removal in cinder and gravel wetland. It can be concluded that cinder can be used as preferred filler in artificial wetlands.(3) Effect of the subsurface constructed wetland evolution into free surface flow constructed wetland on removal of organic matter, nitrogen, and phosphorous in wastewater was studied. The experimental results indicated that the performance of FFSCW derived from the evolution of SSCW due to clogging for COD, TOC, total nitrogen (TN), and total phosphorus (TP) removal was higher than that of SSCW with the same substrate and plant. The laboratory culture experiments were adopted to evaluate the effect of the constructed wetland evolution on the organic matter mineralization, nitrification/denitrification, removal of nitrogen and phosphorus. It was shown that, after evolution of SSCW into FFSCW, the mineralization rate for organic matter (as TOC) was1.82mg-h"1, and it was1.49mg-h’1for SSCW. The removal efficiency for NO3-was96.8%, and it was58.1%for SSCW. The abiotic denitrification removal efficiency was40%, and it was28.2%for SSCW. In addition, the maximum equilibrium adsorption capacity of the substrate after evolution for phosphorus (as P) was160mg·kg-1, and it was140mg·kg-1for SSCW substrate. The organic coverage of the substrate is beneficial to phosphorus removal. The nitrification ability decreased after evolution. |
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