(570j) Analysis of the Anaerobic Digestion Process Using Computational FLUID Dynamics

Anaerobic digestion is a complex multi-stage process involving physical, biochemical and fluid dynamic processes. It is a biological conversion process in which microorganisms transform the organic matter into a gaseous mixture mainly composed of methane and carbon dioxide and a digestate or slurry which is rich in ammonium and other nutrients through the concerted action of a close-knit community of bacteria (Wu 2012). The primary objective of anaerobic digestion is the stabilization of organic matter, with a concurrent reduction in odors, pathogen concentration, and mass of solid organic material. It includes a series of biochemical processes by different microorganisms to degrade organic matter under anaerobic conditions. 

One of the main variables in anaerobic digestion for providing an optimum performance is a proper mixing (Latha, Borman et al. 2009). It provides intimate contact between the feed sludge and active biomass, yielding uniformity of temperature and substrate concentration throughout the digester, and preventing both the formation of surface scum layers and the deposition of sludge on the bottom of the tank (Wu 2012). The most efficient mixing device in terms of power consumed per mixed volume is the mechanical mixer.

Computational fluid dynamics (CFD) is a numerical method that solves fluid motion and heat transfer as well as biochemical reactions. One of the CFD applications is the study of mixing. This technique is very powerful and spans a wide range of industrial and non-industrial application areas, such as aerodynamics of aircraft and vehicles, power plants, chemical process engineering and meteorology, among others.

In this work, the process was carried out experimentally in two steps: Pretreatment and anaerobic digestion, using coffee husk as raw material. The raw material was characterized by measuring moisture content (AOAC 928.09 method), klason lignin content (TAPPI 222 om-83 method), acid-soluble lignin content (TAPPI 250UM-85 method) holocellulose content (ASTM Standard D1104 method), cellulose content (TAPPI 203 os-74 method), ash content (TAPPI Standard T211 om-93 method), total nitrogen content (kjeldahl method), potash content (colorimetric method) and phosphorous content (colorimetric method). The ammonia pretreatment was implemented in order to remove part of the lignin and to enhance the hydrolysis of the biomass, preserving the most of the fermentable fraction. This process was performed at 51°C, with ammonia at 14.8% in a 1:10 solid to liquid ratio and a residence time of 27 h (Li, Merrettig-Bruns et al. 2014). After the pretreatment, the resulting stream was filtered and submitted to the anaerobic digestion process in a 50 L CSTR at 35°C during 20 days (Borja, Martín et al. 2005). The reactor used to carry out the biodigestion allows measuring temperature and concentrations inside of the reactor, at different distances in the radial and axial directions, being possible to construct a profile of these properties through the geometry of the reactor. Besides, the anaerobic digestion process was modeled and simulated creating a 3-D model of the reactor in the computational tool COMSOL Multiphysics. Then, temperature as well as concentration profiles are also obtained by using this tool in order to compare these results with those obtained through experimental stages.

All the obtained models and information are the basis for future design of more efficient anaerobic digestion processes.

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Latha, S., D. Borman, et al. (2009). CFD Multiphase Modelling for Evaluation of Gas Mixing in an Anaerobic Digester. 14th European Biosolids and Organic Conference and Exhibition. Leeds, UK, .

Li, Y., U. Merrettig-Bruns, et al. (2014). "Optimization of ammonia pretreatment of wheat straw for biogas production." Journal of Chemical Technology & Biotechnology: n/a-n/a.

Wu, B. (2012). "Integration of mixing, heat transfer, and biochemical reaction kinetics in anaerobic methane fermentation." Biotechnology and Bioengineering 109(11): 2864-2874.

Wu, B. (2012). "Large eddy simulation of mechanical mixing in anaerobic digesters." Biotechnology and Bioengineering 109(3): 804-812.