Research On Ecological Water Quality Models For Seasonal Rivers In Northern China

Northern China owns a large population, developed industry and agriculture, and has a great demand of water supply. However, the local annual rainfall is not enough and varies dramatically throughout different months and years. The water shortage phenomenon in dry seasons is serious, and the per capita water resources are deficient. Since the 1990’s of last century, with the rapid development of social economy and the acceleration of the process of urbanization, pollutant emissions in Northern China has increased, leading to the destruction of the river ecological system, the decline of the self purification ability, the serious pollution of water quality. It’s urgent that engineering measures be adopted in order to improve the self purification capacity of rivers, to achieve sustainable improvements in river water quality.The river water is a complex ecological system, whose purification process is controlled by microbiology, chemistry, atmospheric physics, fluid mechanics and other factors. All of these factors interact with each other and influence each other. Carrying out many field experiment needs to invest a lot of enormous human, material and financial resources, while studying rivers by the establishment of water quality model, which can describe degradation, transport and transformation of pollutants in water bodies quantitatively or qualitatively and provide the basis for management and monitoring, based on the principle of the reactors, saves the cost. The development and application of water quality models were summarized, the advantages and disadvantages of several commonly used surface water quality models were compared, and the WASP model developed by the United States Environmental Protection Agency (EPA) was chosen in this approach, according to the specific characteristics of seasonal rivers of Northern China. Based on the principle of the reactor, a mathematic model was established, aiming to simulate and predict the water quality of rivers in Northern China. The original and modified WASP model was used to evaluate the water quality and the influence factors of removal efficiency when aeration and constructed wetland were employed, respectively. Thus the foundation of quantization, optimization, decision of the river ecological reactor has been laid.Aeration is one of the main technical methods to remediate polluted rivers, and mathematical models are main ways to predict and assess the environmental pollution. WASP model was first used to study the effect of aeration on improving water quality of a small river over a time span in this approach. The initial results showed that the simulation values were consistent with practical monitoring data, which could be useful for the management and control of polluted water. Furthermore, simulations under different aeration conditions showed that aeration technology could significantly lower the level of chemical oxygen demand(COD), ammonia-nitrogen(NH3-N) of river water, thus improving water quality. With the improvement of dissolved oxygen(DO), the water quality could be further improved; However, the increase of removal rate of pollutants would slow down. Also, there were remarkable differences among various months throughout the whole year, and aeration from May to September was better. Considering economic costs and environmental benefits, river aeration with a 4mg/L DO standard in summer is the best condition.The main advantages of using constructed wetlands(CWs) are flexibility in sizing and site selection, low construction and maintenance cost, low energy requirement and good performance in sewage treatment. Nowadays, process-based model tools for free water surface constructed wetlands(FWS CWs) are lacking. In this paper, WASP model was modified based on the characteristics of wetlands, and the influence of aquatic plants and hydrodynamic condition on water quality were taken into consideration. A typical northern river FWS CW in Shandong Province was selected to prove that the WASP simulated results were in accordance with the observed data and the modified WASP model was able to predict and simulate water quality of FWS CW systems. The approach has provided a guide to the layout of wetland vegetation. The simulation results showed:the FWS CWs could have obvious removal effect of pollutants in the four seasons, but the removal ability of ammonia nitrogen was weakened in winter due to the low temperature. Results demonstrated that both the increase of the vegetation density and vegetation diameter in the FWS CW could help reduce the concentrations of NH3-N of the effluent, but the removal efficiency for COD was hardly influenced. In addition, the comprehensive management strategy and design of constructed wetland should be combined with the economic costs, plant growth environment, control goals and other factors.To summarize, in this research, the river was regarded as a series of reactors, WASP water quality model was selected as the basic tool, and the application scope of the original model was expanded. The water quality can be simulated, predicted, and analyzed with this method, which is significant in theory. In the light of the present situation of rivers in Northern China, the river water quality model, integrated with technique methods such as river aeration or constructed wetland, has been established. By assuming different conditions of various engineering methods, changing the parameters of the model, the impact of factors on water quality was forecasted, which shows advantages of model methods. The research findings would help guide the construction of river ecological restoration engineering projects and water pollution control and management, and provide a scientific basis for decision-making.