Study On Mechanism And Improvement Of Nitrogen Removal In Vertical Subsurface Flow Wetland

According to the "environmental statistics bulletin 2004" issued by State Environmental Protection Administration of China, the domestic wastewater had surpassed the industrial wastewater both in discharge load and pollutant load, and become the largest pollution source. Considering the difficulty to control and collect the rural domestic wastewater, the tough task to tackle with rural domestic wastewater is confronted.As a new wastewater treatment technology, the constructed wetlands has high treatment capacity, strong adaptability to load fluctuation, simple process and instruments, convenient maintenance, low investment, and ecological benefits. A great appliance prospect can be predicted in the countryside and rural area where the municipal sewage network cannot reach.This paper reviewed and analysed previous work and studied the nitrogen removal mechanism in vertical subsurface constructed wetlands by a set of pilot experiment equipment simulating the treatment of domestic wastewater in a vertical subsurface constructed wetland. Meanwhile, several measures were researched to improve the removal efficiency of NH₄⁺-N and TN.The study indicated the nitrogen was most potentially removed by the nitrification and denitrification process. Compared with the half-saturated operation(the height of the saturated zone was 60cm and the unsaturated zone was 60cm), higher oxygen transfer rate was observed in the unsaturated operation, thus higher NH₄⁺-N removal efficiency was expectable. However, during unsaturated operation, most organic matters were degraded when the NH₄⁺-N have almost been nitrified, so that the successive denitrification seldom occurred due to the lack of carbon sources. The experiments with five kinds of substrates indicated that, to promote the NH₄⁺-N removal efficiency, the substrate should have neutral effluent, long hydraulic detention time, large specific surface area and be less inclined to be clogged. In this experiment, the NH₄⁺-N removal efficiency with intermittent inflow at every one hour was not better than that with continuous inflow. When the height of wetland were consistent, higher NH₄⁺-N removal efficiency was expected as hydraulic load decrease; when the hydraulic load was consistent, higher NH₄⁺-N removal efficiency was accompanied with larger height of wetland.In a tow-step vertical subsurface flow constructed wetland, feeding a portion of wastewater into the second unit still cannot achieve high TN removal rate due to the lack of carbon sources. To some extent, increase the feeding amount of wastewater in the second unit can increase the TN removal rate. In the second unit, as substrate, medium size blast furnace slag did better than coarse sands in TN removal. In a one-step vertical subsurface flow constructed wetland, when the reflux rate is 2, the TN removal rate achieved 68% and the TN in the effluent was about 10mg/L. In addition, the TN removal effect could be better by increasing reflux rate.