Study On The Surface Flow Wetland Under Forest And Its Purification Efficiency For Water In Nanfei River

Chaohu Lake, one of the five largest freshwater lakes in China, has been suffering high pollutants emission for a long term, which results in serious deterioration of water quality, frequent outbreaks of cyanobacteria, even degradation of Chaohu aquatic ecosystem. These environmental events have severe impacts on regionally social and economic development and thus Chaohu Lake has been listed as one of China’s three key lakes for governance.Since Nanfei River is one of important rivers flowing into Chaohu Lake, it is significant for water quality improvement of Chaohu Lake to abate water pollution of Nanfei River. In this study, we regarded surface flow constructed wetlands in Binhu national forest park west to Nanfei River as research objects, and aimed to study the purification effects of different treatments (different hydraulic loading, different water depth, with or without hydrophyte) on polluted water and to obtain effective operation parameters of surface flow constructed wetlands for sustainable running. The results showed that:(1) The nitrogen was mainly adsorbed in soil within the first four minutes; the phosphorus was adsorbed in soil mainly in 1500 minutes. (2) the contents of nitrogen adsorbed in soil were 992.08 mg/kg and 1508.93 mg/kg before and after the flood respectively, with an increase of 52.01%; the phosphorus adsorption capacity in soil were 256.43 mg/kg and 534.75 mg/kg before and after the flood respectively, with an increase of 108.59%; (3) the increase of hydraulic loading were related to the decrease of pollutants removal efficiency in surface flow constructed wetlands. The highest removal efficiency was observed with 50cm/d hydraulic loading and the removal efficiency of total nitrogen(TN), ammonia(NH4+), nitrate(NO3-) and CODMn were 16.84%,7.88%,33.95% and 11.96%, respectively. (4) The highest removal efficiency was captured when the water depth was 40cm with Alternanthera philoxeroides planted. And the removal efficiency of total nitrogen, ammonia, nitrate and CODMn were 28.77%,23.13%,70.03% and 7.71%, respectively. (5) the average of dissolved oxygen (DO) in ditches increased with the increasing of hydraulic loading and the DO were 1.14 mg/L,1.62 mg/L and 2.29 mg/L from low to high. (6) carbon/nitrogen ratio (C/N) showed significant correlation with the removal efficiency of TN and TN was removed most when the C/N reached around 0.744-0.889.With the integrated consideration of pollutant removal efficiency, engineering amount and cost of constructed wetlands, Alternanthera philoxeroides with 40cm water depth,50cm/d hydraulic loading was regarded as the optimal treatment by maintaining HRT for 19.2h and controlling C/N within 0.744-0.889.