Tao T, Peng X F, Lee D J, Hsu J P
Thermal Engineering Department, Tsinghua University, Beijing, 100084, China.
J Colloid Interface Sci. 2006 Jun 15;298(2):860-8. doi: 10.1016/j.jcis.2006.01.002. Epub 2006 Feb 21.
This study examined shape changes in two typical wastewater flocs subjected to cyclic freezing and thawing and the associated force exerted by the ice front. While freezing, the engulfing ice front gradually pulled the floc apart. Subsequent thawing only partially restored the floc's shape. By the Maxwell model, used to interpret gross shape deformations, both flocs were visco-elastic objects exhibiting time-varying rheological characteristics which were more viscous than elastic. Detailed observations of floc 1 deformation demonstrated a two-stage force-displacement relationship. Following 1 cycle of freezing and thawing, the interior structure of the floc deteriorated and the force required to elongating a unit length of floc decreased by 60%. The original floc 2 had a dense "core" and loose "tail"; the core was more resistant to deformation under normal stress than the loose tail. Although both flocs had similar shapes and sizes and were acquired from the same activated sludge stream at a wastewater treatment plant, their rheological behaviors differed substantially. A comprehensive theoretical model for freezing and thawing processes should incorporate these rheological characteristics as they corresponded to observed structural changes and reduction in bound water content in sludge following a cyclic treatment of freezing and thawing.
本研究考察了两种典型的废水絮凝体在循环冻融作用下的形状变化以及冰前沿施加的相关作用力。在冷冻过程中，逐渐推进的冰前沿会将絮凝体逐渐拉开。随后的解冻过程只能部分恢复絮凝体的形状。通过用于解释总体形状变形的麦克斯韦模型可知，两种絮凝体均为粘弹性物体，呈现出随时间变化的流变特性，且粘性大于弹性。对絮凝体1变形的详细观察表明，其力 - 位移关系呈两阶段变化。经过1个冻融循环后，絮凝体的内部结构恶化，使絮凝体伸长单位长度所需的力降低了60%。原始的絮凝体2有一个致密的“核心”和松散的“尾部”；在法向应力下，核心比松散的尾部更抗变形。尽管两种絮凝体形状和大小相似，且取自污水处理厂同一活性污泥流，但它们的流变行为却有很大差异。一个全面的冻融过程理论模型应纳入这些流变特性，因为它们与观察到的结构变化以及污泥经循环冻融处理后结合水含量的降低相对应。
