| 摘要: | Over 50% drinking water was supplied to the Taiwan’s public by sludge blanket clarifiers (Chen et al., 2003; Lin et al., 2004). The sludge blanket performs dual functions as a filter as well as a particle coagulator. Coagulation, the chemistry-based treatment stage, controls the characteristics of the generated sludge layer, whereas sedimentation, the hydrodynamic treatment stage, controls sludge layer stability. The existence of a sludge blanket in clarifiers is thereby essential to produce quality effluent water. In addition, the flow dynamics is an important parameter for the design of clarifiers.
The use of solids flux theory continues in many studies in the design and operation of sludge treatment processes (Takacs et al., 1991; Daigger, 1995; Wett, 2002). Ekama and Marais (2004) gave a survey on the development of one dimensional (1D) settler modeling. Although application of solids flux theory is good for studying the performance of clarifiers, it does not adequately describe the effect of hydrodynamic behavior in clarifiers (Narayanan et al., 2000). Computational fluid dynamics (CFD) has shown to be a powerful tool for resolving complex practical problems in engineering. (Hsu et al., 2007; Videla et al., 2008; Lin et al., 2008; Tao et al., 2008; Yang et al., 2007) Therefore some studies attempted to simulate full clarifiers via CFD (Deininger et al., 1998; Burger et al., 2005; Fan et al., 2007). Recently Wu et al. (2007) simulated flow pattern in a clarifier with porous medium as sludge blanket by 3D CFD. The first work to utilize a 3D, multiphase flow simulation for a clarifier is by Wu et al. (2008). Weiss et al. (2007) utilized non-Newtonian flow to model a circular secondary clarifier and showed that viscosity of sludge dominates the flow in clarifier. This study attempts to improve clarifier effluent quality by altering its geometric construction. The simulation is based on the sludge blanket clarifier at the Bansin Water Treatment Plant, Taiwan.
Bansin Water Treatment Plant (BWTP) is in Banchiao City, Taipei County, Taiwan. About every 20 minutes sludge blanket overturns somewhere and effluent solid flux increases. The turbidity of the clarified water is generally too high to produce quality clean water after sand filtering. In this work, four types of constructions of clarifiers are simulated by 3D, multiphase flow model to improve clarifier effluent quality. Type A is the conventional clarifier used in BWTP. Type B changes inlet pipe to a large one (1.6 folds). Type C changes reaction well angle from 1200 to 900. Type D changes reaction well angle from 1200 to 600. As shown in previous research (Yang et al., 2008) that, the velocity of suspension in the reaction well is one of the factors affecting the quality of water discharge, so the improved method of Type B is expected to slow the velocity of flow by enlarging diameter of inlet pipe. The improvement method of Type C is the angle of 900 of reaction cover. It is hoped that the backflow could be limited within the reaction well, so that the residence time of suspension in the reaction well can be increased. The improved method for Type D reverses the whole reaction cover, so the water quality may not deteriorate for reason of easy outflow of the suspension from the reaction well. |
