Biogas production is all in the mixing

Green acres

Engineers at Washington University in St. Louis, using an impressive array of imaging and tracking technologies, have determined the importance of mixing in anaerobic digesters, reactors that use bacteria to breakdown organic matter in the absence of oxygen.

They are studying ways to take “the smell of money,” as farmers long have termed manure, and produce biogas with it. The major end product of anaerobic digestion is methane, which can be converted to methanol or, when partially oxidized, to synthesis gas, a mix of hydrogen and carbon monoxide.

The goal is two-fold; one is to have farms that grow their own energy by using readily available farm waste to power the farm, the other is to eliminate the environmental threat of methane, a greenhouse gas considered 22 times worse than carbon dioxide.

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 U.S. Department of Energy in 2001 to research anaerobic digestion. Since 2004, he and various collaborators have published no fewer than 16 papers on their anaerobic digester studies, the most recent a paper in Biotechnology and Bioengineering 100 (1): 38-48, 2008.

“Each year livestock operations produce 1.8 billion tons of cattle manure,” al-Dahhan said. “”If it sits in fields, the methane from the manure is released into the atmosphere, or it can cause ground water contamination, dust, ammonia leaching, not to mention bad odors. Treating manure by anaerobic digestion gets rid of the environmental threats and produces bioenergy at the same time. That has been our vision.”