| South to North Water Transfer Project was designed to solve the water pollution problem and protect the safety of drinking water in northern China. Nansi Lake is one of the most important lakes as water channel and storing lake in this project, but a series of problems hampered the construction of the project seriously, such as water pollution, reduction of the wetland area, and the weak regulation capability. In this paper, the pollutants removal efficiencies of the pilot-scale and full-scale biological island grid systems were evaluated. The microbial mechanism of removing pollutants and the optimization of the technical parameters were also investigated. This study provides technical information and engineering support for designing and running of the biological island grid technology in the Xiaosha River, an important inflow river of Nansi Lake.Results showed the biological island grid system had an excellent long-term removal performance. The NH4-N removal efficiency was about95%generally, with an almost zero effluent concentration of NH4-N. Over75%COD was removed, and the COD effluent concentration was lower than20.0mg/L, this water quality could reach standard Ⅲ of the groundwater. The removal efficiency was lower for TN and TP (about70%), the effluent concentration of TN and TP was1.33～3.18mg/L and0.36～0.85mg/L respectively, which were higher than Standard III of the groundwater. The possible reason should be the small size of experiment system and shortage of the microbe species and amount. In addition, the influent concentration of TN and TP were also too high. The results also suggested that this technology was less effective on the TP removal and more research and improvement is needed.The experiment of process optimization showed that the filler length, plant density, coverage of the island grid and hydraulic retention time (HRT) as well as other factors affected the water purification ability. This study suggested a group conditions to obtain a better pollutants removal efficiency:the filler length accounted for2/3of the water depth,32Oenanthe javanica per m2, coverage of island grid was about30%,6days of HRT for engineering application. Iris pseudacorus and Oenanthe javanica should be harvested in early November. Actually, the harvest time of different plants may be adjusted due to their tolerance to low temperature.Analysis of DGGE map showed that the diversity index was1.93-2.65, providing abundant microbial information. The result showed that the microbial community structures were similar between different plants systems, and the seasonal variation affected them significantly.46.2%of the microorganisms in the system belonged to Proteobacteria, followed by uncultured bacteria. There were lots of nitrogen-fixing and nitrate-degradation bacteria among them, which had important meaning to the nitrogen removal from the polluted water.Based on the study above, the demonstrate project of biological island grid was built in Xiaosha River. The project had effective purification efficiency; the effluent water quality could reach the Standard III of the groundwater. Meanwhile, the long-time run of this project brought remarkable social, ornamental and economic benefits. |
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