Wastewater-sludge Two-phase Model And Liquid-gas Two-phase Model Of Carrousel Oxidation Ditch

The terminal treatment efficiency and the effluent water quality of oxidation ditches are relative with its hydrodynamic characteristics. The foundation of wastewater-sludge or liquid-gas two-phase model and the simulation of gas distribution of oxidation ditches have an importantly referential value for the design and optimization of oxidation ditches, which are also a crucial and difficult issue in the term of oxidation ditch modeling. Utilizing Particle Image Velocimetry (PIV) technology and Computational Fluid Dynamics (CFD) numerical calculation, combined with field tests of a full-scale oxidation ditch, this paper discussed the rules of flow field, sludge distribution and gas distribution in a Carrousle oxidation ditch, improving the understanding on sludge-wastewater two-phase flow mechanism in oxidation ditches, proposing a model setting-up method for the liquid-gas two-phase flow in oxidation ditches, and providing a basis for the simulation of dissolved oxygen distribution in oxidation ditches. It has great significances on optimization of oxidation ditches in the term of hydrodynamics.This paper used PIV technology used to study the settling of activated sludge flocs under turbulent flow conditions. Experimental results showed that the sludge settling velocity is tightly related with particle diameter of the sludge flocs and turbulence intensity of the flow field. A larger particle diameter led to a higher settling velocity while the higher turbulence intensity led to lower settling velocity. Through modifying Vesilind equation, a function of sludge settling velocity under turbulent condition was yielded. Settling velocity shows a power-type relation to sludge particle diameter and an exponential-type relation with turbulence intensity and sludge concentration. This function can reflect the influence of particle diameter, turbulence intensity and sludge concentration on sludge settling velocity.CFD numerical calculation was combined with field experimental test to study flow field of oxidation ditches. A sludge-wastewater two-phase turbulence model of oxidation ditches was set up by coupling sludge settling model under turbulent condition with a turbulence mixture model, separating the vertical movement of sludge and wastewater. By computing through the comprehensive CFD software, FLUENT 6.1, the flow field of mixture flow and the sludge distribution is simulated and analyzed. Parameters and sections were reasonably selected for calibration and then this model was modified according to the measured velocity in the oxidation ditch of a wastewater treatment plant by Generalized Likelihood Uncertainty Estimation (GLUE) to reduce the modeling errors.A liquid-gas two-phase model of oxidation ditches was build, which treated wastewater and sludge as a liquid phase and took into consideration the free movement of liquid-gas interface. This model can reflect the distribution of gas-phase volume fraction in the oxidation ditches. Compared with the simulation result of wastewater-sludge two-phase model, this model is shown to better model the velocities of upper layer and surface layer of oxidation ditches.