Friday, August 22, 2014

UC Davis biodigester hungers for food scraps


Rep. John Garamendi, UC Davis Chancellor Linda Katehi, CleanWorld CEO Michele Wong, UCD engineering professor Ruihong Zhang and others shovel food scraps into the biodigester in a symbolic opening ceremony Tuesday morning. The digester is currently running at 20 percent capacity. Sue Cockrell/Enterprise photo

From page A1 | April 23, 2014 |

A pungent pile of rotting fruits and veggies welcomed Earth Day to UC Davis.

West of campus, between Interstate 80 and the California National Primate Research Center, the four stocky white cylinders of  the UC Davis Renewable Energy Anaerobic Digester hunger for leftovers, ready to munch through 50 tons of them daily.

Combining food waste, agriculture byproducts and food processing scraps, the digester will turn about 18,250 tons of organic trash into electricity and fertilizer each year. That’s all of UCD’s organic waste plus more than 7,000 tons trucked in from local businesses.

The digester is owned and operated by CleanWorld, a Sacramento-based green technology company whose CEO is a UCD graduate.

Graduate students, state representatives, professors and administrators attended the opening Tuesday morning. Among other speeches, Rep. John Garamendi spoke to the importance of partnership between state, private industry and public research, and lead scientist and UCD engineering professor Ruihong Zhang discussed her research.

“It is exciting to see the technologies developed in my laboratories … solving real-world problems,” she said, speaking to about 200 attendees.

After the presentations, the six speakers, including CleanWorld CEO Michele Wong, used gold shovels to toss food scraps into the clean silver rotors of the doda, where the waste is first processed.

Built on top of the retired UCD landfill, the digester will generate about 5.6 million kilowatt-hours, or about 4 percent of the campus’ electricity needs. The biogas end product and methane gas from decomposing buried trash will spin power-generating microturbines.

Currently, the digester is accepting just 20 percent of its total capacity, but Wong hopes it will achieve full capacity by the end of May.

This digester is the third for CleanWorld, a green technology company that has opened two other facilities in the greater Sacramento area. It cost $8.5 million to build, funded by CleanWorld and grants from the California Energy Commission and the U.S. Department of Energy.

In a cost-neutral switch, the university will pay CleanWorld the same to process the waste and provide electricity as it did to send its excess to landfills and purchase energy from the Western Area Power Administration.

The digester technology was developed over the past 10 years by Zhang. Her pilot digester is still open for research and demonstrations at the UCD wastewater treatment center.

“I think of her as the Steve Jobs of anaerobic digestion,” said Tracy Saville, CleanWorld’s vice president of marketing. “Ruihong (Zhang) is truly the leading researcher in this field in the world.”

In anaerobic digestion, microbes breakdown organic compounds without the need for oxygen. Zhang’s digesters are substantially more efficient than others on the market, able to process material that is about 50 percent solid waste.

“Our process is designed to optimize the environment for the bacteria to grow,” Zhang said. “It takes all kind of biology, augmentation, engineering to make this technology work.”

Operating at temperatures around 125-135 degrees Fahrenheit, Zhang’s technology also uses a three-step system of digestion — the first of its kind.

Previous digesters process waste in one or two steps, and most eat through byproducts that are just 20 to 30 percent solid, requiring outside water sources to achieve optimal ratios. Zhang’s technology requires only the water squeezed out during the digestion, and it powers itself, using less than 10 percent of the total electricity generated.

Even the pipes transporting the gas to the turbines are specially designed to collect condensation, which is recycled back into the system, said CleanWorld operator Jeremy Bunch.

The liquid and solid byproducts of electricity generation can be turned into high-quality compost or fertilizers for 145 acres of farmland. CleanWorld and UCD representatives are talking with local farmers in the hopes of creating a market for the waste’s waste.

With 100 million tons of organic waste going to U.S. landfills each year, and an increase in demand for diesel alternatives like the digester’s natural gas, biodigester technology takes advantage of both ends of the system: waste processing and electricity and gas generation.

“In California alone, there’s 5.5 million (tons) going to landfill,” Saville said. “Literally, you could build 500 biodigesters in California at scale, and maybe grab 30 percent of that.”

— Reach Elizabeth Case at or 530-747-8052. Follow her on Twitter at @elizabeth_case



Elizabeth Case

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