Zhiyong Duan

The dissolved oxygen is a major factor influencing the water quality of a water body. The gas exchange rate determines the speed that the low soluble gases, including dissolved oxygen, transfer from atmosphere to water. The wind-driven gas exchange rate model is important for the water quality modeling in water bodies such as lakes and estuaries when the wind is the predominant factor. Although empirical formulae have been developed for wind-driven gas exchange rate, they cannot explain the mechanisms of wind-driven gas exchange rate. The empirical formulae are normally limited in their applicability due to the specific experimental conditions under which they were established. This study is to develop a mechanistic wind-driven gas exchange rate model for the general application ranges. This mechanistic wind-driven gas exchange rate model correlates the gas exchange rate with the hydrodynamic parameters based on the surface renewal mechanism and the two-film transfer mechanism. The gas exchange rate values predicted with this model show reasonable agreements with the experimental data in the wind-driven gas exchange systems. This model can be applied for water bodies with wind blowing over the water surface as the predominant factor influencing the exchange rate of the low soluble gases.

The reaeration rate determines the speed that the dissolved oxygen is restored to the saturation level. The reaeration rate is determined by the surface renewal rates from the friction interfaces in the water bodies, including the water/bed interface, the shear‐flow interface and the air/water interface. The formulae of reaeration rate for the air/water interface and the water/bed interface were developed in prior studies. However, no formula of the reaeration rate driven by the shear flows was developed. In this study, a mechanic model of the reaeration rate driven by the shear flows is developed to fill in the gap. The flow velocity profile in the shear flows and the Surface Renewal Theory are employed to derive the corresponding model. The predictions of the formulae for these three types of friction interfaces are compared for the same phase velocity to investigate the reasonability of the reaeration rate model for the shear‐flow interface. The predictions of the model for the shear‐flow interface are between those for the air/water interface and for the water/bed interface. The model in this study is also verified to have reasonable agreements with the experimental data. The model developed in this study can be applied for the prediction of the low soluble gases’ transfer rate between air and water in shear flows.

Constructed wetlands technology is an important type of wastewater treatment technologies. Compared with other wastewater treatment technologies, the aquatic plants are special components in constructed wetlands for water pollutants removal. However, the comprehensive functions of aquatic plants in constructed wetlands were little emphasized before. In this study, the comprehensive functions and effects of aquatic plants in constructed wetlands to the water pollutants removal are investigated. Multiple common used aquatic plants are compared with their biomass, nitrogen and phosphorus absorption capacity, and oxygen secretion by root systems. These are used to investigate the mechanisms of the functions of aquatic plants in water pollutants removal in constructed wetlands. In this study it is found different types of aquatic plants have different comparable advantages in biomass, nitrogen and phosphorus absorption capacity, and oxygen secretion by root systems. The aquatic plants in constructed wetlands provide dissolved oxygen to the environment around their roots. The aquatic plants themselves have absorption for the nitrogen and phosphorus to directly removal them from the wastewater. Their roots together with the fillers in the constructed wetlands establish the habitat environment for the microorganisms which degrade the water pollutants. The aquatic plants play important and special roles in constructed wetlands for wastewater treatment. These findings can be used for the improvement of the water pollutants removal efficiency in constructed wetlands. The improved constructed wetlands can be used for eutrophication control with higher efficiency.

The meteorologic station provides precipitation, air temperature, dew point, etc. for hydrology and water quality modeling in the watershed. However, the meteorologic stations are absent in some watersheds, which brings difficulty for the watershed modeling. In this study, the method of constructing the virtual meteorologic station in the watershed is developed. This can overcome the difficulty of the absence of meteorologic station and makes it possible for watershed modeling. The data from the precipitation stations inside the objective watershed and the weather stations close to the objective watershed are used to construct the virtual meteorologic station inside the objective watershed. The delineation tool in the better assessment science integrating point and nonpoint (BASINS) is used to delineate the objective watershed. The hydrological simulation program—FORTRAN (HSPF), incorporated in BASINS, is used to model the stream flows in the watershed. The National Elevation Data (NED) terrain dataset is used for watershed delineation. In this study it is found the modeled stream flows and water quality with the virtual meteorologic station in the watershed have reasonable agreements with the observations. The method of constructing the virtual meteorologic station developed in this study can be used for the modeling of hydrology and water quality in the watersheds where the meteorologic stations are absent.

A streamflow-driven gas-liquid transfer rate model was developed. This model was developed in terms of total surface renewal rate and gas diffusion coefficient. The total surface renewal rate was considered to be the sum of the surface renewal rates caused by the turbulence from the air-water interface and that from the water-bed interface. A general mixing length formula and a vertical fluctuation velocity formula was developed to formulate the surface renewal rate. Predictions of this model show good agreements with the measurements in previous studies and the predictions of Churchill's, O'Connor-Dobbins', and Owens-Gibbs' formulae. It was indicated in some previous studies that the existing streamflow-driven gas-liquid transfer rate formulae have limited application ranges. The model developed in this study can be applied for all the normal ranges of flow velocity and water depth in natural rivers, which generalizes and simplifies the applications.