| The Zaohe River is one of major tributaries of the Weihe River. As the main drainage channel of Xi’an, the water quality of the Zaohe River is highly polluted. River pollution not only exacerbated the crisis of water resources, but also a serious threat to the sustainable development of economy and environment, thus solving the problem of river pollution is imminent. Although a variety of sewage treatment technology can be used to repair the pollution of rivers, but as a kind of environmental friendly and resource-saving ecological technology, constructed wetland has become a preferable choice to improve river pollution with the advantages of low cost, simple management and easy on-site restoration on the river.Because a single type of constructed wetland cannot provide the aerobic and anaerobic denitrification environment at the same time, more and more studies with hybrid constructed wetlands achieve effective treatment of wastewater. At present the studies of wetland plants and substrates are more focused on the selection of plants and substrates. The investigation that the effects of plants and substrates size grading in nutrients removal at the horizontal subsurface flow constructed wetlands treating the polluted river water are blank. In order to investigate the pollutants removal efficiencies of the surface(FWS)-horizontal subsurface flow(HF) hybrid constructed wetland(HCW) treating the highly polluted river and analyze the roles of the plants and substrates size grading in the second stage HFCW and the adaptability of the hybrid constructed wetlands to seasonal factors in the northwest region, two hybrid constructed wetlands which were composed of FWS+HF(with Phragmites) and FWS+HF(without plants) were constructed to provide the theory and application references for treating the highly polluted river with hybrid constructed wetlands. Two groups of constructed wetlands operated under the same hydraulic loading. During one year’s experiment, the research results showed that:⑴The average removal rates of TN, NH3-N, TP, COD, BOD5 and SS were 57.3%-63.1%, 65.7%-68.1%, 69.4%-82.7%, 85.3%-86.3%, 96.7%-96.9% and 97.9%-98.2%, respectively. Two groups of hybrid constructed wetlands have better purification effect for treating the highly polluted river. The removal efficiencies of various pollutants remain high levels, and the removal efficiencies of organic pollutants and SS are the most efficient and stable.⑵As the first stage of the HCW, FWS removed most of organic and SS which avoided the clogging of the HF and improved the removal of the nitrogen and phosphorus in the second stage wetland.⑶The removal efficiencies of SS, NH3-N, TN and TP in the planted HF were higher than the unplanted HF, but slightly less on COD and BOD5. The plants in the constructed wetlands have significant roles in promoting the removal of the phosphorus.⑷The removal of organic pollutants and suspended solids in the HCW were not affected by seasonal fluctuation, but the removal of nitrogen was influenced by seasonal fluctuation significantly.⑸The plants improved the nutrients removal ability of the HFCW by 8.9% for TN and 23.4% for TP, respectively.⑹Plants improved the nutrients removal throughout the wetland space. The combination roles of the substrates size grading and the plants kept the nutrients removal efficiencies of the internal space of the wetland much more uniformity.⑺The direct nutrients uptake ability of the plants was slightly higher than the indirectly promoting effect of the others in wetland, and the proportion of nitrogen and phosphorus removal by the uptake of the plants in the wetlands were 11.3% and 17.4% respectively. |
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