There is a lot of talk these days about the electrification of vehicles, ethanol-based power and solar power – but until fairly recently, all three were considered unable to compete economically with oil-based technologies.

That notion has been debunked, but it remains difficult to convince the general public that these could become cost competitive. Some experts believe that’s also the case with anaerobic digestion.

Presenting “Emerging Strategies for Increasing the Profitability of Mixed Feedstock Anaerobic Digesters” recently was Mark Mba-Wright, associate professor of mechanical engineering at Iowa State University.

The first question tackled was “How are we going to make these digesters profitable?”

“We are so early. We are just getting started” with this technology, Mba-Wright said. In the grand scale, he’s correct – the first digester used to capture energy wasn’t recorded until 1897. Although vast improvements have occurred since then, he noted that one of the basic ways to improve digester productivity is to mix the inputs, but those using them need to make sure they’re not losing the beneficial microbial community by doing so.

Mba-Wright said that solids recycling has increased microbial retention but has also led to the accumulation of solids and contaminants. So engineers moved to filter reactors, which immobilized microbes but suffered from plugging issues. Then came the upflow anaerobic sludge blanket, which solved the plugging issue but microbe mobility can be a challenge. New digesters try to address all previous challenges: mixing, microbe stability, solids mobility, etc.

“They are more complex and currently more expensive, but they are the best options for maximizing productivity,” he stated.

Less than 20% of American biogas is upgraded to biomethane, a renewable natural gas. “As we look for higher value products, I predict this percentage will increase over time,” Mba-Wright said.

The International Energy Agency expects more diverse feedstocks – including more usage of crops – as anaerobic digestion becomes more economical.

Some feedstocks have significantly higher methane potential than others. For example, food waste has three times the energy potential of dairy manure. Other high potential inputs include pine wood, used vegetable oil, cheese whey and Festulolium and tall fescue grasses.

“When you mix them together, some feedstocks tend to boost the productivity of the digester in a very significant way – like food waste and grass,” Mba-Wright said.

What kind of digester is used also impacts profitability. There are advantages and disadvantages to each:

  • Covered lagoons have very low operating costs but aren’t suitable for colder regions
  • Complete mix digesters are suitable for different ambient conditions and can tolerate different feedstocks but have a higher energy demand for mixing
  • Plug flow digesters have low operating costs and can digest high solid feedstock but solids can also settle on the bottoms
  • Fixed film digesters have a very low hydraulic retention time and are smaller in size but feedstock can get plugged by solids

Mba-Wright said, “Covered lagoons and complete mixers have proven themselves to be safe, reliable options – but it depends on scale. Mixing is much easier with a smaller digester, for example” – which may make sense for smaller operations.

As research and applications for anaerobic digesters move forward, Mba-Wright and others are already projecting significant decreases in costs.

“We’re so early and there’s so much potential,” he reiterated. “We need to find added value from all these streams that are flowing to the digesters.” Some that he listed included carbon capture, fertilizer replacement and renewable natural gas.

He concluded that for small and medium sized farms, the challenge for many of these innovations is on the risk side rather than the economic side. “They may not be as willing to risk investing in something that hasn’t fully been proven yet, unlike large companies,” he added.

By paying attention to innovations, however, farmers can be on the cutting edge of this technology.

by Courtney Llewellyn