Biodigester's appetite is fed by the ton

Crates filled to the brim with watermelon rinds, bushy pineapple tops, lettuce scraps and grass clippings each weigh in at nearly a ton.

One might be inclined to call this "organic waste." But not Clean World Partners: This is biodigester food.

It's feeding day for the four huge cylindrical tanks of the biodigester on the grounds of the American River Packaging plant in North Natomas. Nearly 10 tons of leftover plant matter are being dumped into the mouth of the digester and stuffed in with a garden fork.

Without breathing any greenhouse gases into the atmosphere, the biodigester will turn these scraps into electricity and less than a ton of solid compost and fertilizer.

It works like a cow's stomach, says Kathryn Oliver, environmental engineer at Clean World Partners (CWP). Taking in food, the biodigester produces carbon dioxide and methane, or "biogas." But unlike a cow's burps and farts, its gases are turned into electricity with a microturbine. And it is far less pungent.

It is about two weeks, says Oliver, "from the day that the apple goes in to when the apple is biogas."

The conversion is the work of legions of bacteria. They are the critical residents of the biodigester and the cow's (and probably your) stomach.

The bacteria naturally carry out a process called anaerobic digestion. Digestion, because they break down the food scraps. Anaerobic, because they do it without oxygen.

"From a science perspective, it's not new," says Warren Smith, co–founder and senior vice president of business development at CWP.

Anaerobic digestion has long been used to get rid of organic waste, and is common throughout Europe. Dairy farms will transfer cow manure to large lagoons, where anaerobic bacteria go at it. Water treatment plants use it to dispose of leftover sludge.

The difference with CWP's biodigester is its ability to efficiently handle lots of solid material. Most anaerobic digesters can only digest material that is 9 percent solid. In contrast, theirs can take half-solid, half-liquid material.

A carrot is one-fifth solid. To digest it with technology at water treatment plants, you would have to add a lot of water and use "really expensive blenders," explains Smith.

Instead, the CWP biodigester grinds up the material to the consistency of "really thick oatmeal," says Oliver, without adding much water. This mostly solid slurry travels through pipes to the biodigester's tanks.

Handling this high-solid material efficiently would not be possible without technology developed by University of California, Davis professor Ruihong Zhang.

Multiple types of bacteria each carry out a specific chemical step of anaerobic digestion. Some compete with each other, producing acid that others can't tolerate at high levels. Zhang's key innovation is a two-stage process, separating some of the steps into different tanks.

Josh Rapport, director of research and development at CWP, explained that the biodigester also can handle a "big spike" of food, and its bacteria live at a higher temperature than is common in other biodigesters, speeding up the conversion process.

The biodigesting process comes full circle. The end biogas turns microturbines, which generate about a third of the electricity needed to run the biodigester. Heat produced from the microturbine warms the tanks. Excess water is pumped back in.

Leftover material will be turned into fertilizers and compostable material. CWP is researching additional products that "we can make for tomorrow," says Smith.

With its voracious bacteria, the biodigester turns out to have some of the same nutritional requirements that humans do. A crate of iceberg lettuce won't produce as much biogas as a crate of spinach.

It also seems to have some of the same cravings. "The biodigester loves grease and fats," says Oliver.

• Read more articles by Julie Granka

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