From Kentucky Soybean Promotion Board

As if owning and operating eight chicken barns wasn't enough, Campbellsville farmer John McLean is turning waste into wattage–literally. McLean owns two very different anaerobic digesters. Anaerobic digestion is a series if biological processes in which microorganisms break down biomass in the absence of oxygen. One of the end products is biogas, which is combusted to generate electricity and heat, or can be processed into renewable natural gas and transportation fuels.

McLean's secondary digester can hold 2.5 million gallons of material, while his more sophisticated primary digester holds 270,000 gallons. Surprisingly, he doesn't have any trouble keeping the digesters "fed."

"In addition to any excess chicken litter that we may have, we have a permit to take non-human waste including feedstocks from corporations with food waste," McLean explained. "We don't accept scraps, because we don't currently have a separator to remove plastics, which are non-biodegradable."

McLean's education is in economics, and he says that when he first got his chicken barns up and running in 2003, one of his biggest expenses was energy. "When I saw how much energy, how many BTUs, the broiler houses use, I immediately tried to find ways to cut costs." After a great deal of research, he started construction on his first digester in 2011. "I served as the general contractor on this job," he said. "I wanted to have my hands on it from day one."

Since that "day one," McLean says he has learned a great deal about anaerobic digesters. "Eighty to 90 percent of what I've learned about digesters has been learned by trial and error," he said. "Digesters are expensive, and they are usually invested in by a large cooperation and run by an employee. That employee doesn't have near the stake in the success of the operation as I do."

McLean said that digesters are fairly common in New York, Wisconsin and California due to their large dairy operations, but added that all digesters (and all waste fed into them) isn't the same. His digester was "inoculated" with bacteria from another digester that uses poultry litter. "There isn't a website or catalog you can go to and order up the perfect blend of bacteria to digest what we are putting through this system," he said. "Plus, we practice co-digestion, which means we digest both litter and feedstock (food waste)."

He was quick to point out that in order for anaerobic digestion to happen, the right environment has to be created and maintained. "The digester is kind of like a stomach," McLean explained. "You have to geed it just the right diet for things to work correctly." The particular blend of bacteria he's using works best on a low-protein "diet," he said, and digests large quantities of simple sugars and oils.

"This is the only one of its kind in the state that I know of that digests animal waste," he said. "And even though we sell two thirds of our chicken litter, having eight houses produces a great deal of waste," McLean said. "Add in that Kentucky has a lot of food waste, and there is a market for the service that a digester provides. Everything that goes into that digester turns into renewable, sustainable, green energy and fertilizer, and it doesn't go into a landfill."

McLean was quick to say that being pro-renewable energy doesn't make him anti-coal. "Living here in Kentucky, I know that we have a lot of coal miners, and coal has been financially good for our state. There's room on the grid for energy made for all kinds of processes."

So how in the world does a digester work? In the photo below, the black plastic 'bubble' that resembles a pillow is the secondary digester, which works most effeciently in the summer months when ambient temperatures are high, accelerating the digestion process. In the photo on the facing page, the green grain bin-like structure is the primary digester, which works steadily all year round in the mesophilic temperature range, which is between 68 and 113 degrees F. The digesters' action creates biogas, which is captured and fed through the pipes noted in the photos. The gas is then "scrubbed" to remove impurities before it enters the generator (first photo).

Electricity comes out of the generator and goes through a switch gear which synchronizes it back to the power grid. McLean sells the energy created to East Kentucky Power, and says they've been good to work with. "It's a three way contract on the energy," he explained, "between East Kentucky Power, MAC Farms, and Taylor County RECC since it runs through their lines." He added that East Kentucky Power has a highly diverse energy portfolio.

McLean said that he has installed numerous required safety features to protect linemen and other workers. "If the grid goes down, it shuts my generator down," he said. "Working together, we've made sure that we won't be pushing energy to the grid in case of downed lines."

So... what about all that stuff left in the digester after the gas is used? McLean explained "that's the good stuff." Called digestate, the liquid left over after the gas is separated is incredibly nutrient-dense. "We've tried different methods of application," he said, "and found that injecting it straight into the soil is best. This method virtually eliminates the smell. We're currently doing trials here on our farm. We plan to sell the digestate at some point, but we are doing some field trials this growing season to see how much better the digestate is than regular chicken litter."

McLean noted that when he is ready to custom apply digestate commercially, it will be available only to those farmers who have it in their nutrient management plan. "We do have a waste permit that allows us to sell to anyone who has a Nutrient Management Plan which includes digestate on file with the NRCS," he said. One advantage of the use of digestate over poultry litter he pointed out is that the nutrients contained in digestate are 100 percent available to the plant upon uptake. Digestate may contain less phosphorus than traditional manures more can be applied per acre. "And it puts good micro bacteria in the soil," he said.

Modest though he may be, it certainly seems that McLean is putting that degree in economics to hard work on his farm. He's growing broilers on contract, selling poultry litter as fertilizer, contracting with companies to dispose their food waste, making renewable, sustainable green electricity with that waste (to sell back to the grid) and then taking what's left–the digestate–and preparing to offer it as custom-applied fertilizer after the field trials are completed.