Long-term Performance And Mechanism In The Surface Constructed Wetland For Treating Polluted River Water In Northern China

There were severe environmental problems especially water crisis including water shortages, water pollution and deterioration of water quality in north China. Faced with the above challenges, South to North Water Transfer Projectthe was designed to solve the water pollution problem and safeguard drinking water safety in northern China. Therefore, the surface wetland system was a potential and suitable technology for treating polluted river water to meet the requirement of water quality. In this study, the performance of the pilot-scale and full-scale surface constructed wetlands for treating Xinxue River water was evaluated, and influencing factor such as hydraulic retention time (HRT), on the efficiency of the constructed wetland system as well as mechanism of removing pollutants was also investigated, so that It was helpful for the widespread application in river water quality improvement.Results showed that the wetland systems could achieve an excellent removal performance, with the average COD and NH4-N removal efficiency (above 70%), and the effluent quality for COD and NH4-N was 10.72-19.34 mg/L and 0.18-0.90 mg/L, respectively, which met Grade-Ill of national surface water standards. However, the removal efficiency was lower for TN and TP (30-70%). Plants played an important role in the nutrient removal in constructed wetlands, and higher HRT was beneficial to pollutant removal. The k-C* model for COD and NH4-N removal each season was fitted well. The calculated k values for COD and NH4-N were 0.07-0.90 d-1 and 0.03-0.30d-1. In general the removal performance in spring and summer was better than in fall and winter.Total biomass in the planted microcosm units varied considerably among species over the course of the experimental period. The highest total biomasses (0.5459-1.6841kg/m2) were abtained in November and Iris pseudacorus was higher than Scirpus validusand followed by Typha orientalis and Phragmites australis. The maximum nutrient accumulation in plants was 8.46-30.98 gN/m2 and 0.46-2.13 gP/m2. Hence, the uptake of N and P by plants constituted 14.29-51.89% of the mass N removal and 10.76-34.17% the mass P removal, respectively. But Scirpus validus and Iris pseudacorus had a higher nutrient absorption capacity than Typha orientalis and Phragmites australis.The result of nitrogen mass balance revealed that plant uptake and sediment storage were the key factors limiting wetland nitrogen removal besides microbial processes. Plant uptake accounted for 8.40-34.33% of the total nitrogen input, while nitrogen removal by sediment storage and N2O emission contributed 20.59-34.47% and 0.60-1.94% respectively. However, the percentage of other nitrogen loss such as ammonia volatilization and N2 emission due to nitrification and denitrification was estimated to be 2.00-23.57%. It was also showed that ediment storage was the key factors limiting wetland phosphorous removal and to a certain extent plant uptake could also take effect. Plant uptake accounted for 4.81-22.33% of the total phosphorous input, while sediment storage and others accounted for 36.16-41.13% and 0.26-4.13% respectively.Based on the study above, the demonstrate project of surface constructed wetland was designed and built in Xinxue River. The results indicated that COD and NH4-N in effluent from wetland reached to class III of environmental quality standard for surface water, and the ecological function of lakeshore wetland was restored effectively.