Study On Enhancing Removal Of Nitrogen And Heavy Metals From Rainfall Runoff By Adding Crop Stalks Into Constructed Wetland

Runoff is an important pollution source that influences water quality. Constructed wetland technology is an effective means to control runoff pollution, however, carbon shortage affects the removal of pollutants. Therefore, this research has selected crop straw, which is cheap and environmental friendly, to be the carbon source to improve removal efficiency of pollutants in wetlands. This study includes runoff water quality monitoring, carbon source selection and its impact on removal efficiency of pollutants. The main results are as follows:(1)The study is to monitor rainfall runoff water quality. SS, COD, TN and TP have a first flush effect. The concentrations of pollutants SS, COD and TN rise rabidly at first, and then decrease stably after reaching a peak. However TP concentration falls more slowly than the other pollutants. With the increase of sunny days, rainfall runoff pollution is also increased; the pollution of private plots is the most serious, followed by road. Due to the high concentrations of pollutants such as SS, the rainfall runoff of four kinds of underlying surface (courtyard, roof, roads, plots) must be treated before recycling.(2) The study is to select carbon source. Crop straw after alkali treatment release lots of carbon and nitrogen at first, and then release slowly. But alkali treatment increase the costs. C:N ratio of wheat straw is high and its release rate is more stable, so wheat straw after simple treatment is suitable to be carbon source.(3) The study is to determine a proper way to add straw. Compared with the control group, single-layer adding has a weaker reinforcing effect on pollutants removal and three-layer adding causes high TN and NH4+-N concentrations. Double-layer adding can enhance nitrogen and phosphorus removal, and the concentration of effluent organic matter does not rise.(4) The study is to determine the adding weight of wheat straw. When the inlet simulates rainfall with low C/N ratio, constructed wetlands can remove more nitrogen and heavy metals with the weight increasing from 0.1 to 0.35kg/m3.But when the adding weight is from 0.35kg/m3 to 0.5kg/m3, pollutants removal has no significant increase. So the appropriate adding weight is 0.35kg/m3 and the better adding way is double-layer adding. When the water simulates sulfuric acid rainfall and the adding weight is 0.5kg/m3, the removal rates of COD, the removal rates of NH4+-N and TP are more than 85%, the removal rates of NO3--N and SO42-are 75%, the removal rates of Cd、Zn and Pb are more than 95%, the removal rate of Cu is 90.4%. So the better adding weight and way are confirmed, which is 0.5kg/m3 and double adding.(5) The study is also to quantify the wetland microorganisms. With the adding weight increasing, the number of various bacteria ascends. The spatial distribution number of bacteria, ammonifiers and nitrobacteria is as follows:up>low>middle, however, the spatial distribution number of denitrifying bacteria and sulfate-reducing bacteria is low>up>middle. The pollutants’ removal has a positive correlation with various bacterial number in the constructed wetland.