New anaerobic digester technology improves the atmosphere

A new type of high-rate anaerobic digester system created by a UC Davis professor was given its first commercial application recently at a packaging plant in Sacramento. The digester allows organic solid waste to be recycled more efficiently.

Ruihong Zhang, a professor in the department of biological and agricultural engineering, created the process in the lab at UC Davis. In 2006, a research and demonstration facility was built on South Campus to test and fine-tune the process to make it ready for public and commercial use. Plans are being made to use the facility as a model for a larger facility to be built on the site of the current UC Davis landfill.

“I think it’s a very unique opportunity that we can take technology that has been developed right here on campus by Dr. Zhang and apply that to our own facilities,” said Sid England, Assistant Vice Chancellor of the UC Davis Environmental Stewardship and Sustainability program.

The planned project would convert agricultural and landscape waste created on campus, as well as leftover food scraps from the dining halls, into a fuel mixture made of 90 percent methane that could be used to fuel Unitrans buses. A mixture made of methane and carbon dioxide could also be converted into electricity for use on the campus grid, in order to be part of the West Village zero net energy project.

“Dr. Zhang’s technologies allow material that is up to 50 percent solid with no additional water added,” said Michele Wong, CEO of Clean World Partners, which has licensed the technology from the University of California and runs the new plant in Sacramento.

“Other digesters require that you have material that is much lower in solid content (10 to 20 percent), so you must add water, [which] increases the capital costs,” Wong said.

Initially foreseen as being a potentially valuable means of large-scale energy generation, anaerobic digester processing plants now are most highly valued for their ability to reduce landfill use. Reducing the amount of organic, or carbon-based, solid waste in landfills also reduces the amount of greenhouse gases released into the atmosphere, since methane, which is created when organic waste decomposes, is a highly potent greenhouse gas.

Widespread use of more inexpensive anaerobic digester processes such as Zhang’s can reduce a significant portion of the smog in California air basins. The smog is caused by volatile organic compounds (VOCs) created by the decomposition of dairy cow manure and other organic wastes.

“If we use digesters to treat manure, those VOCs won’t be formed and emitted, because the bacteria in the digesters will convert the manure into methane and carbon dioxide,” Zhang said.

Zhang’s husband, Zhongli Pan, is an adjunct professor of food engineering at UC Davis and works in the same department as Zhang. Their son, Philip, is a senior environmental engineering student at UC San Diego.

“She [Zhang] always believes that new technologies and discoveries will be the key to provide our society [with] a better living environment and make the energy supply sustainable,” Pan said.

Zhang travels frequently to China to help further advance the work in her field.

Zhang is listed as the sole inventor on two patents involving the high-rate anaerobic digester process. She is also a co-inventor of three other patents related to the process. Clean World Partners has licensed all five patents from the University of California but is currently only using two of the licenses. Zhang will receive a percentage of the royalties generated, per university policy.

BRIAN RILEY can be reached at science@theaggie.org.