CFD simulation of mixing in anaerobic digesters Mitsuharu Terashima a, * , Rajeev Goel b , Kazuya Komatsu a , Hidenari Yasui c , Hiroshi Takahashi d , Y.Y. Li e , Tatsuya Noike f a Kurita Water Industries Ltd., Gochoyama 1-1, Kawada, Nogi-Machi, Shimotsuga-gun, Tochigi 329-0105, Japan b Hydromantis, Inc., 1 James Street South, Suite #1601, Hamilton, ON, Canada L8P 4R5 c Faculty of Environmental Engineering, The University of Kitakyushu, 1-1, Hibikino, Wakamatsu-ku, Kitakyushu 808-0135, Japan d Graduate School of Environmental Studies, Tohoku University, Aza-Aoba, Aramaki, Aoba-ku, Sendai, Miyagi 980-8579, Japan e Department of Civil Engineering, Tohoku University, Aza-Aoba, Aramaki, Aoba-ku, Sendai, Miyagi 980-8579, Japan f Advanced Research Institute, Nihon University, 4-2-1, Kudan-kita, Chiyoda-ku, Tokyo 102-0073, Japan article info Article history: Received 2 October 2007 Received in revised form 18 July 2008 Accepted 19 July 2008 Available online 9 December 2008 Keywords: Anaerobic digestion Mixing CFD Rheology abstract A three-dimensional CFD model incorporating the rheological properties of sludge was developed and applied to quantify mixing in a full-scale anaerobic digester. The results of the model were found to be in good agreement with experimental tracer response curve. In order to predict the dynamics of mixing, a new parameter, UI (uniformity index) was defined. The visual patterns of tracer mixing in simulation were well reflected in the dynamic variation in the value of UI. The developed model and methods were applied to determine the required time for complete mixing in a full-scale digester at different solid con- centrations. This information on mixing time is considered to be useful in optimizing the feeding cycles for better digester performance. Ó 2008 Published by Elsevier Ltd. 1. Introduction Anaerobic digestion is a common process for stabilizing and reducing the excess biological sludge from wastewater treatment plants. Performance of anaerobic digestion mainly depends on feed characteristics, feeding patterns, pH, temperature, redox potential, hydraulic/solid retention time and mixing characteristics. Mixing in anaerobic digesters is important to transfer substrates to micro- organisms, uniform the pH and temperature, dilute inhibitory sub- stance and prevent stratification and short-circuiting (US EPA, 1987; Bello-Mendoza and Sharratt, 1999). Although, the impor- tance of mixing is clear from many studies (Elnekave et al., 2006; Borole et al., 2006; Karim et al., 2005), its effect on the performance of anaerobic digesters is still confusing. While some studies have reported that digesters performance deteriorate at higher mixing (Go ´ mez et al., 2006; Stroot et al., 2001), many others have sug- gested that mixing improves performance (Elnekave et al., 2006; Borole et al., 2006; Karim et al., 2005; Kalia and Singh, 2001; Brade and Noone, 1981a, b). A careful examination of this conflicting lit- erature reveals that the studies reporting performance improve- ments mainly attribute it to minimization of non-uniformity, dead space and short-circuiting in digester. On the other hand, studies reporting performance deterioration have suggested dis- turbance to spatial juxtaposition of microorganisms due to exces- sive shear at high mixing intensities (Stafford, 1982; McMahon et al., 2001). Both of these observations highlight the importance of achieving an appropriate level of mixing in anaerobic digesters. Tracer studies have been traditionally used to characterize dead space and short-circuiting in the digesters (Zoltek and Gram, 1975; Rundle and Whyley, 1981; Leighton and Forster, 1996; Bello-Men- doza and Sharratt, 1999). Although, tracer study is a valuable experimental method for flow characterization, the approach is re- source intensive and sometime may not be applicable at full-scale plant due to various operational constraints. CFD-based analytical techniques for flow characterization is one of the promising alter- natives to overcome these experimental difficulties. Lately, CFD modeling is finding many applications in water and wastewater treatment field (Knatz, 2005; Nisipeanu and Harwood, 2002; Ta, 2001). For example, CFD models for solid–liquid separation in set- tlers, retention time optimization in chlorine contact tank (Greene et al., 2002, 2004), optimization of rapid mixer (Park et al., 2003), flocculator (Haarhoff and Van Der Walt, 2001) and dissolved air floatation (Kwon et al., 2006) have been used with good success. CFD modeling has also been used for characterizing flow pat- tern inside anaerobic digesters (Karim et al., 2004; Vesvikar and Al-Dahhan, 2005). Vesvikar and Al-Dahhan (2005) demonstrated that this approach could be used to optimize the tank configura- tion. One of the drawbacks of the available CFD models is that they 0960-8524/$ - see front matter Ó 2008 Published by Elsevier Ltd. doi:10.1016/j.biortech.2008.07.069 * Corresponding author. Tel./fax: +81 280 331577. E-mail addresses: mitsuharu_terashima@hotmail.com, mitshuharu_terashima@ kurita.co.jp (M. Terashima). Bioresource Technology 100 (2009) 2228–2233 Contents lists available at ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/locate/biortech