Washington University

UNDERSTANDING THE HYDRODYNAMCS AND PERFORMANCE OF ANAEROBIC DIGESTERS by Mehul S. Vesvikar ADVISOR: Professor Muthanna H. Al-Dahhan August 2006 St. Louis, Missouri Anaerobic digestion is an efficient way of treating animal wastes and biomass byproducts to reduce its pollution threat and obtain energy in the form of methane. The high failure rate of anaerobic digesters coupled with the lack of fundamental research prohibits the widespread use of anaerobic digestion. Assessing the role of mixing in performance of anaerobic digesters and its influence on digester design, scale and operation via experimental and modeling studies is the focus of this work. The energy consumption and ease of operation criteria led to the selection of gaslift digester mixed by gas recirculation. Performance studies in laboratory and pilot-scale digesters treating cow manure show that large-scale experimentation is required to obtain reliable information for design and scale-up of digesters. For the first time, Computer automated radioactive particle tracking (CARPT) and Computational Fluid Dynamics (CFD) are used to study the effects of operating variables, designs, internals, and scales on the mixing pattern and hydrodynamics of the anaerobic digesters. The comparison of dead zone volumes, circulation times and eddy diffusivities obtained from CARPT and CFD show that gaslift digester with draft tube diameter half of the reactor diameter and multiple point sparger provides better mixing than other digester configurations. CARPT and CFD data complemented the performance studies and conclude that the geometric similarity and equal power input per unit volume is not sufficient to obtain the same digester performance at two different scales. Further, successful development and implementation of multipleparticle tracking (MP-CARPT) in this work will overcome the limitations of singleparticle CARPT in future research on dense multiphase systems including anaerobic digesters. The knowledge gained from this dissertation will be useful for further investigations that can lead to efficient operation of anaerobic digesters by the potential end users. Dedicated to my dear Aaee (Mom) and Baba (Dad)