Research On Hydraulic Behaviors And Mathmatic Model Of Vertical Flow Constructed Wetlands

Constructed wetlands (CWs) for wastewater treatment with the advantages of simpleness of equipment, low cost of construction, operation and maintance have been thrived at 1970's. Especially vertical subsurface flow constructed wetlands (VFCWs) with intermittent feeding are increasingly used during the last several years due to their good efficiency regarding the removal of nitrogen. However, there is a lack of understanding in detail about the transformation and elimination processes of pollutants in CWs. Most of their design and operation are still based on'rules of thumb', lead to a quite different removal effect or unsuccesful use and result in discouraging the development and application of CWs.Aims to VFCW with intermittent feeding, some studies were conducteded. Firstly, to investigate the hydraulic behaviors on use of modified residence time distribution (RTD) theory and hydraulic efficiency and to discuss the potential effects hydraulic condition has on polltants removal. Secondly, to determine the flow pattern and the regularity of wetland pollutant reduction by reproducing the experimental RTD and first-order removal kinetics models utilizing reaction theory and to present first order removal rate constants. Thirdly, to simulate dynamic hydraulic behaviors of pilot-scale VFCW employed the two-dimensional hydraulic simulation program HYDROUS-2D. Fourthly, to develop a simplified biological model based on HYDROUS-2D and activated sludge models (ASMs) and to be calibrated with a pilot-scale vertical flow constructed wetland.In the case of study on hydraulic behaviour and the effects of hydraulic conditions on contaminant removal efficiency of VFCWs with intermittent feeding, the influences of plants and feeding strategies on hydraulic behaviors of VFCW are discussed and the impacts on polltants removal efficiency with all sorts of hydraulic operations including hydraulic loading, residence time, flow rate, feeding interval and water distribution are explored. Results indicate that plants have less influence on effluent rate and RTD, but is somewhat helpful to improve hydraulic efficiency. Higher flow rate under the same daily hydraulic loading and feeding interval or less feeding interval but the same daily hydraulic loading and the flow rate can increase effective volume ratio, improve hydraulic performance and advance wetland removal efficiency of chemical oxygen demand (CODcr), annonium-nitrogen (NH4-N), total nitrogen (TN). Hydraulic efficiency compared between continuous and intermittent feeding has no distinctively difference when the same flow rate was employed. Otherwise intermittent feeding can obtain higher flow rate under the same hydraulic loading to improve hydraulic perfermace or can obtain lower daily hydraulic loading under the same flow rate compared with continuous feeding to guarantee enough residence time which increases pollutans removal. Uniformly water distribution can promote the upper areas of VFCW are untilized to increase hydraulic loading in case of the pollutants removal performance.In the case of the study on flow pattern and first-order removal kinetics models, the tanks in series (TIS) model, TIS with delay model and shiffted lognormal distribution model were employed to reproducing the experimental RTD. All proved capable of fitting the RTD curves and the flow pattern of VFCW can be presumed as flow characteries of three continuous stired tank reactors (CSTR). Furthermore, first-order removal kinetics models were conducted to simulate the reduction of CODcr, NH4-N, TN and the first order removal rate constants, background concentrations and temperature correction coefficient of VFCW with intermittent feeding are all presented.In the case of the study on the two-dimensional dynamic hydraulic simulation, effluent rate, cumulatived effluent and tracer experiments of VFCW with intermittent feeding are simulated using simulation program HYDROUS-2D for flow and transport in saturated and unsaturated zones. Hydraulic parameters and soil parameters of VFCW with the main substrate consists of quartz sand are calibrated. The simulated results show a good match with measured data. Moreover, the valuable experience with the in-depth study on hydraulic numerical model of CWs are gained and the platform to propose a biological model of VFCW is provided.In the case of the study on mechanistic model of VFCW, a simple biological model learned from CW2D model, ASMs and a two-step nitrification (ASM3₂N) model for the process nitrification and denitrification on both nitrite and nitrate is proposed based on the two-dimensional dynamic hydraulic model, which can describe the multi-component reactive transport. By comparing with the measured data, the model is available to simulate the effect of COD, NH4-N and TP removal and dynamic changes of dissolved oxygen (DO), heterotrophic organisms, autotrophic organisms (nitrosomonas and nitrobacter), ornanic carbon compounds, NH4-N, nitrite-N (N02-N), nitrate-N (N03-N), inorganic phosphorus (IP) in VFCW.All the studies above enhance knowledge of hydraulic behaviors of VFCW with intermittent feeding to offer a support of theories and technologies and practical experiences on optimal design and optimal operation of VFCW, to increase understanding and insight of process and mechanisms in CW, and to prompote the development of mathematical models of CW which advance the widespread use of the ecological tenology.