Biodigester transforms food waste into fertilizer, energy

UC Davis biodigester

The UC Davis Renewable Energy Anaerobic Digester, which officially opened Earth Day, April 22, 2014, converts organic waste from campus and other sources into clean energy for the campus electrical grid. The anaerobic digestion technology used in the facility was invented by UC Davis professor Ruihong Zhang and licensed to Sacramento-based CleanWorld. (UC Davis)

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It’s a blistering hot day in Davis, California, and the sun beats down on four massive silos that are all connected through various networks of pipes. Workers on ladders are drilling new panels into one of these tanks, disturbing whatever silence that would otherwise give the illusion of solitude. There is very little shade on the dry grassland that was once an active landfill, but Abdolhossein Edalati has found the one spot with coverage.

He examines the contents of a series of glass jars. One is full of little brown pellets. The other contains a dark brown liquid and is labeled “Raw digestate – non-hazardous.”

UC Davis graduate students Tyler Barzee, left, and Abdolhossein Edalati, center, and the inventor of the concept behind the UC Davis Renewable Energy Anaerobic Biodigester (READ), professor Ruihong Zhang, Ph.D., describe how the facility converts food waste into energy and fertilizer. (Justin Rex/Texas Tech University)

“We’re trying to produce sustainable fertilizers,”  explains Edalati, a graduate student in the University of California, Davis, Department of Biological and Agricultural Engineering.

Edalati said that each kind of fertilizer has its benefits and its drawbacks. The liquid form can be applied through drip irrigation, a farming method in which the plant roots are slowly watered via small tubes; however, it contains very little nitrogen – in many cases, not even 1 percent. The pellets, on the other hand, contain up to 5 percent nitrogen but cannot be applied through the ease of drip. 

Plants need nitrogen to thrive. They use it to make chlorophyll, a compound that aids them in photosynthesis.

“Soils need organic matter returned to them to support the soil microbiology that helps in crop cultivation,” Edalati says. The fertilizer he is creating helps provide that.

These fertilizers are the byproduct of the Renewable Energy Anaerobic Biodigester (READ), which make up the four giant silos that tower in the sun behind Edalati. The vision behind READ is to break down organic waste and produce a recycled product that can be used in farming and agriculture. It can hold up to 50 tons of organic waste like food and manure.

Ruihong Zhang, Ph.D.
Ruihong Zhang, Ph.D., in 2014 when the UC Davis Renewable Energy Anaerobic Biodigester (READ) facility opened. (UC Regents)

The concept for READ was invented by Ruihong Zhang, Ph.D., of the UC Davis Department of Biological and Agricultural Engineering. She is also the chief technology advisor to CleanWorld, a private company with which UC Davis partnered to produce the machine.

“The digester is basically a vertical hammer mill,” Zhang says, referring to a machine that crushes and shreds material through the repeated blows of little hammers.

READ is composed of four tanks. Three of them are for breaking up and pulverizing waste. The digester first separates any plastic that does not belong. It then grinds the food waste and organic material into a paste, which is pumped into the first tank.

“The first tank has bacteria that break food waste down into organic acid,” Zhang says. “Then (it) goes into the second tank, which has high density microbes that convert organic matter into gas.”

The fourth tank is where the digestate, or leftover organic matter, is stored.

“The leftovers have all the nutrients,” Zhang says.

Zhang developed this technology 10 years ago at UC Davis and was able to turn the plans into action with the help of CleanWorld, which specializes in biodigesters.  The university took over full operation of the digester in early 2018, with Zhang as a liaison between CleanWorld and UC Davis, and is now investing more into the operation.

It’s also an easy way for local restaurants, farms and communities to dispose of waste for between $35 and $52 per ton.

UC Davis is a non-profit, but Zhang says her operation “is a full business model.”

“Economically, we’re not getting any money back,” she says. “We got funding from [the] state to create these. It’s not about money, it’s about the show and tell, and making it work.”

Even though Zhang and her team are not making money, that’s not to say others couldn’t.

“This is a great example of taking the technology forward and making a commercial business,” she says.

Tomatoes on the ground after harvest
At a Woodland, Calif., commercial farm, tomatoes that were left behind after harvest remain on the ground as a way to fertilize the ground for next year’s crop. Whether a synthetic or biofertilizer, farms will use nutrients to boost their crop production over the course of a season. (Katherine Baker/Columbia University)

Zhang and her team give the fertilizer they make to area farms who then report the results back to them. A 2-year-long study on digestate fertilized tomatoes yielded results comparable to tomatoes fertilized with UAN-32, a popular synthetic fertilizer, which is 32% nitrogen.

“Yields for the digestate fertilizers were equal to the UAN-32 and even higher in the case of the digestate concentrate,” Edalati says. “The digestate fertilized tomatoes had higher soluble sugar content than the UAN-32 tomatoes.

“UAN-32 is good for providing nitrogen, but does not give you anything else. Plants also need more than just nitrogen. Digestate can provide that.”

Zhang and her team want to ship the digestate farther than they are currently able to.

“The digestate is valuable, but transporting it is not necessarily economically viable,” Edalati says of the liquid fertilizer. “The pellets would be one way to be able to transport nutrients far away at a cheaper cost.”

The difficulty lies in nitrogen content. The liquid digestate is 5% to 6% nitrogen – an essential component to fertilizer – whereas the pellet form is only 0.1 percent to 0.3 percent nitrogen. “You’d have to transfer a lot more of this to get the same amount of nitrogen,” Edalati says.

Neither, though, compare to UAN-32, the popular synthetic commercial fertilizer.

“You can literally apply a couple hundred milliliters of UAN-32 versus hundreds of gallons of (digestate),” Edalati says. “That’s the challenge.”

Edalati holds up the liquid digestate, or biofertilizer, in glass jars. This made it easy to see the differences in color and particulates, from various stages in the anerobic process. (Justin Rex/Texas Tech University)

Thankfully, though, they have a virtually endless supply of test material while they work out how to make their fertilizer more nitrogen-rich.

“Ice cream, Muscle Milk, tomato paste and cut tomatoes from Campbell’s,” he says with a laugh, listing some of the more frequent items from which he’s made fertilizer. “All of those used to go to a landfill. Now they come here.”

“Coffee, too,” Zhang adds. “We put a lot of coffee in here. Maybe those bacteria love it and get energized.”


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