| On the basic of the structure and the purification mechanism of wetland, constructed wetlands(CW) was defined as a wetlands specially constructed for the purpose of pollution control and waste management. Generally, constructed wetlands are divided into three types: surface flow constructed wetlands, subsurface flow constructed wetlands and vertical flow wetlands. The constructed wetlands have the advantages of low construction investment and operation cost, easy management, stable operation, better removal efficiency, great compatibility to loading changing and salient profit of ecological suroundings,etc.This experiment was designed in order to describe the removal efficiency of subsurface flow constructed wetland when it was used to deal with the urban polluted rivers. Through the analysis of two polluted rivers treatment effect, Laoduanpu river and Qianbang river in Shanghai, results showed that (1) Because of high water pollutant load, effluent concentration of pollutants also remains high, the main pollutant concentrations were higher than the main pollutant concentrations were higher than the surface water quality standard.Ⅴ. but the water pollutants including organic pollutants, total phosphorus, ammonia nitrogen, suspended solids and other major pollutants have been higher removal (conversion) efficiency. in Laoduanpu river and Qianbang river, the CODCr removal rate were respectively 42% and 89%, TP removal rates were 60%, 61%, the effect of the removal of pollutants can meet the design goals; (2) Subsurface Constructed wetlands can combine many other technologies,such as pretreatment technology, and ecological reconstruction technology, to provide a better base of water quality and the environment for the recovery of urban river ecosystem function and the natural landscape; (3) In the testing process, some indicators of pollutants show a larger value of change, the maximum of the CODCr in the Laoduanpu river come to the head of 184 mg / l, resulting in decline of the average removal rate. After analyzing the causes,the continuously pump make the hydraulic load of wetlands beyond the design capacity,and this research attempt to test and solve the control problem of the hydraulic retention time(HRT) through another experiment.To solve the contradictions between smaller capacity and higher processing requirements of urban little scale wetland (50—100m2), the experiment set a shorter time frame of 2-8h, the result shows that: (1) Controling HRT is a practicably way of solving the contradiction; (2) In 8h HRT, the messure value of DO often was zero, we consider that the small-scale CW should adopt a new shorter HRT; (3) With the growth of HRT, the removal of pollutants results show a different extent, by comprasion, the pollutant can be divided into two types, high sensitivity and low sensitivity to respond to HRT changing, the former including COD,BOD,SS and TN,the latter including Chla and TP, This article holds that the HRT between 6-8 hours, intermittent water distribution methods can be enhace the capacity of constructed wetlands of pollutant removal; (4) Under short HRT, some of pollutant will be very difficult to remove, such as the removal efficiency of TP was only 27.2% in 8h HRT, so we must develop other technologies to solve the problem, such as the selection of substrates, wetlands engineering structures betterment, in order to further improve the system operating efficiency.
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