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Studies Show Importance of Mixing in Biogas Production

Engineers at Washington University in St. Louis have determined the importance of mixing in large anaerobic digesters that produce biogas.

Muthana al-Dahhan, Ph.D., Washington University professor of energy, environmental and chemical engineering, and his doctoral students Khursheed Karim, Rajnesh Varma, Mehuld Vesvikar and Rebecca Hoffman have determined that mixing is the most crucial step in the success of large, commercial anaerobic digesters that can react 1,500 gallons of manure. In addition to graduate students, numerous undergraduates have contributed to the research.

Al-Dahhan received a $2.2 million grant from the US Department of Energy in 2001 to research anaerobic digestion. There were then, as now, 100 anaerobic digesters in operation in the United States, but a high percentage—76%—regularly failed.

Al-Dahhan and his colleagues at Washington University, Oak Ridge National Laboratory and the Iowa Energy Center based in Ames, Iowa, studied the configuration, design, hydrodynamics and mixing parameters of reactors, first on a laboratory scale, in reactors that held less than four liters of manure. They then went to Oak Ridge Laboratory to a pilot plant and tested a reactor that held 100 liters.

As size increased, we found mixing plays a very important role in successful operations. Intensity of mixing also is important. We found that if intensity of mixing is reduced, failure often is a consequence.

—Muthana al-Dahhan

Anaerobic digestion of manure is opaque. To understand the hydrodynamics of anaerobic digestion al-Dahhan and colleague used a multiple-particle, computer-automated radioactive particle tracking system, computational fluid dynamics and gamma ray computed tomography to see where and under what conditions biochemical stagnant or dead zones occurred. They also analyzed mixing systems, hydrodynamics, shear effect and reactor configuration.



So, you put it in a vacuum cement mixer. Whatever techniques required we should do. I am amazed that it took them this long and a $2.2m grant to find this out, but that is just me. I still think gasification is a better way to go on a large scale. We can make enough SNG from biomass to not have to import any in the U.S.


There was an award-winning digester installed at a prison in Africa (Kenya?) about 5-10 years ago, for processing sewage, and the most notable feature to me was that it had three separate chambers connected by siphon pipes. The pressure of the methane in the first tank would eventually cause the fluids (and a significant fraction of the settled solids) to 'flush' into the second tank, giving it a stir and starting the digestion process anew.

One of the most brilliant pieces of simple-but-effective design I've seen in a while.

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