| Constructed wetlands (CWs) are widely used in urban and rural sewage treatment because of its low energy consumption and easy management. The efficiency of reoxygenation is the main factor affecting aerobic pollutant removal processes in CWs. Radial oxygen loss (ROL) of plant is one of the most important pathways for reoxygenation in CWs, providing oxygen for the rizosphere area to satisfy various physical, chemical and biological processes in CWs.In this work, the ROL variation was studied and the effects of seasonal variation, plant species and harvest on ROL were evaluated. The correlation between ROL and pollutant removal efficiency was investigated, to elucidate the microbial mechanism of ROL affecting pollutant removal in CWs. Differences in microbial abundance and community among wetlands with different plant species were studied to understand the influence of plant species. In addition, the performances of unharvested and harvested CWs were compared, as well as their microbial community indexes, to investigate mechanism on the effect of harvest. The main research conclusions are as follows:1. ROL variation has significant seasonal characteristics and could be affected by plant species and harvest. ROL rates were highest in summer followed by autumn and spring and lowest in winter. The highest ROL rate was 4.91±0.30 μmol/h/plant emerged in July and the lowest rate was 0.001±0.02 μmol/h/plant emerged in February. ROL rate is affected by plant physiological characteristics and positively related to the root porosity. The ROL rates were different between species. Phragmites australis showed much higher ROL rates than others whenever in summer or winter, while ROL rates of Typha orientalis were higher than Arundo donax in summer but lower in winter. The plant management-harvest had adverse impact on ROL no matter in summer or winter.2. Significantly positive correlation between pollutant removal efficiency and ROL were observed. The presence of plant could weaken the effects of seasonal variation on pollutant removal efficiency. Significant positive correlation between ROL and COD and NH4+ -N were obtained, with correlation coefficient of 0.786 and 0.758, respectively. In summer, both planted and unplanted wetlands could keep high pollutants removal efficiency with no significant difference (p>0.05). In winter, the pollutant removal efficiency and microbial abundance were significantly higher in planted CWs compared to the unplanted ones (p<0.05). The microbial abundance was higher on plant root surface than that of matrix. There were remarkable differences in microbial community between the two wetlands. Both of them had large proportion of Proteobacteria while there were more Cyanobacteria and photosynthetic bacteria in unplanted ones. In winter, the proportion of Proteobacteria in planted wetlands remained large but obviously decrease was observed in unplanted ones. Besides, significant decrease of Cyanobacteria and photosynthetic bacteria occurred in unplanted CWs. Thus, seasonal variation has more significant effect on microbial community of unplanted wetlands than that of planted ones.3. The removal efficiency varied among plant species and the differences were not significant in summer whereas significant differences were observed in winter. This was related to the variation of microbial abundance and community. In summer, both CWs kept high pollutants removal efficiency with no significant difference. However, the removal efficiency of T. orientalis wetlands was lower than that of P. australis and A. donax in winter. The abundance of ammonia oxygen bacteria (AOB) in wetland system was consistent with ROL changes. No significant difference in total bacteria abundance was observed in summer but highest total bacteria abundance was obtained in P. australis wetlands followed by A. donax and T. orientalis in winter. Both microbial species and composition in the wetlands had significant differences in the two seasons. The proportion of Proteobacteria was highest in P. australis wetlands followed by A. donax and lowest in T. orientalis wetlands in the two seasons. However, there were more Cyanobacteria in wetlands planted with T. orientalis in summer. Besides, the Cyanobacteria in T. orientalis wetlands reduced significantly in winter.4. Summer harvest could significantly improve the pollutant removal efficiency of CWs but winter harvest had negative effect. Both summer and winter harvest had negative effect on ROL. However, summer harvest could obviously improve the dissolved oxygen condition and reduce the shading effect of plant which is good for the aerobic and photosynethesis bacteria. Winter harvest could increase the effluent pollutant concentration and reduce the microbial abundance and activity which is bad for the pollutant removal efficiency. Therefore, based on the analysis of removal efficiency after harvest, we suggested that a summer harvest should be carried out but the winter harvest should be conducted when the temperature was below 4℃. |
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