Chu C P, Lee D J
Chemical Engineering Department, National Taiwan University, Taipei 10617, Taiwan.
J Colloid Interface Sci. 2004 Sep 15;277(2):387-95. doi: 10.1016/j.jcis.2004.04.036.
The interior of sludge floc is highly heterogeneous, while the large pores in the floc control the advective flow. This work for the first time numerically details fluid flow and mass transfer processes in pores of activated sludge floc. The dimensionless permeabilities and mass dispersion coefficients were contoured against pore size ratio and the floc Reynolds number. With a pore size less than 20% of the floc size, the commonly adopted homogeneous model overestimates the floc permeability, and pore velocity is less than 2% of the bulk velocity. This is particularly true for flocs with low porosity. Although the convective flux is low, the dispersive mass transfer rate can be much higher than the diffusional rate, attributable to the strong Taylor dispersion effect. The three-dimensional pore structures in waste activated-sludge floc were identified using confocal laser scanning microscope (CLSM) images. Large pores were used to numerically estimate the permeability and dispersion coefficient for these pores. The permeability and the dispersion coefficient of the tortuous pores can be one order of magnitude lower than those for the equivalent straight pores. Besides the dispersion effect, the pore tortuosity appeared as the most important geometrical factor retarding the advective flow in the sludge pores. In addition, the small side pores connected to the large pore had only a mild effect on the flow process, and can be neglected in analysis.
污泥絮体内部高度不均一，而絮体中的大孔隙控制着平流。这项工作首次对活性污泥絮体孔隙中的流体流动和传质过程进行了数值详细描述。针对孔径比和絮体雷诺数绘制了无量纲渗透率和质量扩散系数的等值线图。当孔径小于絮体尺寸的20%时，常用的均匀模型会高估絮体渗透率，且孔隙流速小于总体流速的2%。对于低孔隙率的絮体尤其如此。尽管对流通量较低，但由于强烈的泰勒弥散效应，分散传质速率可能远高于扩散速率。利用共聚焦激光扫描显微镜（CLSM）图像识别了废弃活性污泥絮体中的三维孔隙结构。用大孔隙对这些孔隙的渗透率和扩散系数进行了数值估算。曲折孔隙的渗透率和扩散系数可能比等效直孔隙低一个数量级。除了弥散效应外，孔隙曲折度似乎是阻碍污泥孔隙中平流的最重要几何因素。此外，与大孔隙相连的小侧孔对流动过程影响较小，在分析中可忽略不计。
