Reynolds number-dependent permeability of wastewater sludge flocs Zeng Yang a,1 , Xiao-Feng Peng a,2 , Duu-Jong Lee b,3, * , Ay Su c,4 a Department of Thermal Engineering, Tsinghua University, Beijing 100084, China b Chemical Engineering Department, National Taiwan University, Taipei 10617, Taiwan c Department of Mechanical Engineering & Fuel Cells Research Center, Yuan Ze University, Chungli 320, Taiwan Received 24 July 2006; accepted 25 July 2006 Abstract Most theoretical models assume constant permeability of wastewater sludge floc. This work shows that, at creeping flow limit with small intrafloc Reynolds number, the permeability of floc can not only be affected by floc structure, but also by the external flow condition. The three- dimensional structure of flocs using the fluorescence in situ hybridization (FISH) and the confocal laser scanning microscope (CLSM) was firstly probed. Then, the volumetric grid models for sludge flocs were constructed. We noted that the floc permeability could keep unchanged, increased, or decreased at increased Reynolds number (Re). Flow redistribution among channels of various sizes contributes to the noted Re-dependent permeability of flocs. # 2007 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: Floc model; CLSM; Permeability; Flow distribution 1. Introduction Sludge flocs are regarded as highly porous fractal-like aggregates composed by many particles (Li and Ganczarczyk, 1989). Numerous structural models of flocs have been proposed to estimate the effects of advective flow on the intrafloc transport processes. Model considering the floc’s interior as a uniform porous medium is referred to as the ‘‘homogeneous model’’ (Chu et al., 2004a; Chung et al., 2004; Tsou et al., 2002; Wu et al., 2002; Wu and Lee, 1998, 1999, 2001). The model taking the floc as a porous medium of radially varying porosity (and permeability as well) (Chellam and Wiesner, 1993; Hsu et al., 2005a,b; Hsu and Hsieh, 2003, 2004; Veerapaneni and Wiesner, 1996) is named as ‘‘radially varying model’’. In some relevant works, the floc was regarded as of a self-similar, fractal-like structure (Li and Leung, 2005; Li and Logan, 1997a,b; Li and Yuan, 2002; Li et al., 2003). Solving the equations of motion with pre-described floc structure could derive the advective flow field. However, up to now no comparisons between the outputs of various floc models were made since there exists no common basis to compare with. Our group detailed the floc’s interior structure by employed the fluorescence in situ hybridization (FISH) technique and confocal laser scanning microscope (CLSM), and subsequently, established the three-dimensional floc model (Chu et al., 2004b, 2005; Chu and Lee, 2004a,b,c). Solving the Navior- Stokes equations based on a correlation between floc permeability and interior geometric parameters simulated the flow field in the floc interior. The permeability of sludge flocs could affect the efficiency of various processes, including those in the field of solid–liquid separation processes (Hwang et al., 2005; Skouras et al., 2004; Tan et al., 2004; Tao et al., 2005; Tung et al., 2004). We demonstrated in this work that the permeability of wastewater sludge floc can be affected not only by floc structure, but also by the external flow condition. The three- dimensional model for wastewater flocs was first detailed. Then the advective flow field was numerically simulated, from which the permeability of floc was estimated. The causes correspond- ing to the noted Reynolds number-dependent permeability of floc was attributable to the change in flow distributions through channels of different sizes could altered at increased Reynolds number. www.elsevier.com/locate/jcice Journal of the Chinese Institute of Chemical Engineers 38 (2007) 135–141 * Corresponding author. Tel.: +886 2 2362 5632; fax: +886 2 2362 3040. E-mail address: djlee@ntu.edu.tw (D.-J. Lee). 1 2 3 4 0368-1653/$ – see front matter # 2007 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jcice.2006.07.003