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A new concept in anaerobic digestion has recently come to fruition in the U.S. In November of 2010, a psychrophilic sequential batch reactor began operation on a dairy farm in Oregon.
This energy-efficient digester that boasts stability and maximum production is said to be replicable on a larger scale than most.
“We developed a [digester] model for the average-size farm,” says Bio-Terre’s chief operating officer, Elise Villeneuve.
The system was originally designed by researchers at Agriculture and Agri-Food Canada (AAFC).
Three consulting engineering firms recognized the opportunity in this system and decided to test it out. They formed Bio-Terre in 1998, moved the technology out of the lab to a pilot situation and then full-scale on three swine farms.
In that time, Bio-Terre refined the sequential operation of the system, identifying the certain period of time the rich organic effluent needs to remain to allow the natural bacteria to develop.
It consists of two vessels – one rests while the other is fed. Within the vessels, nutrient-rich sediment that has yet to be degraded settles naturally to the bottom of the tanks.
The top layer, which is mostly liquid and degraded material, is removed from the vessels. The vessels are not mixed.
This sequential batch process creates a longer solid retention time without increasing the hydraulic retention time. The average length of stay in each vessel is 3.5 days for a total seven-day cycle, compared to the 22-day hydraulic retention time in other digesters.
“There is a lot of liquid in most manure management approaches, and digesting liquid doesn’t give you anything but volume,” Villeneuve says. “This is where we come in. We take the high liquid content and make it work.”
This digester operates at 68 to 77°F (psychrophilic), compared to the mesophilic and thermophilic digesters, more common in the U.S., that run at 95°F and 131°F, respectively.
The lower temperature provides an environment for more stable bacteria to work simultaneously.
Because the bacteria are stable, they can also handle a higher loading of nitrogen. For example, this system can treat effluent with ammonia concentrations up to 8,000 mg/L NH3 – N, more than double the normal maximum digestible amount for other anaerobic digesters.
Highly diluted systems are at greater risk of being in a negative energy balance because it can take more energy to heat the water than the thermo-energy produced from the system itself.
Since the Bio-Terre system requires less heat to operate compared to other systems, 15 to 50 percent more methane is available for alternative uses rather than being committed to process heating.
This system is based at Lochmead Farms near Junction City, Oregon, but it is owned and operated by Revolution Energy Solutions LLC (RES). The company designs, builds, owns and operates renewable energy products around the world.
“Our system allows a dairyman to continue to produce milk while we employ a technology that enhances their existing manure management system,” states Al Tank, chief executive officer of RES.
Working within a current system is the key to sustainability. “We know it has to work within their system; if not it’s not going to work on a long-term basis,” he adds.
RES sees that potential in Bio-Terre’s low-temperature sequential batch system. “We believe this unique sequential batch reactor is really the next step to deployment of digesters on a large scale in the U.S.,” Tank says, providing four reasons why.
•Net energy balance – Because it operates below 75 to 79°F, energy is not used to produce energy. Waste heat can be gleaned from the engine and there is still excess heat throughout the year.
• Very stable system – It can operate seven days a week, 365 days a year without getting upset. This can be especially challenging in agriculture where the digester feedstuffs are not always consistent.
• High levels of automation – This system is controlled from the RES offices in Washington, D.C. “You want a system to be able to operate without personal intervention. This system allows you to do that,” Tank adds.
• Superior performance – “You get all that and it performs as well or better than any other system in terms of digestion and conversion to energy,” he says. “This system works.”
Tank met Lochmead Farms owner Buzz Gibson at a seminar in Portland, Oregon. Together, they established a long-term host site agreement at the 600-cow dairy.
Gibson’s cows are milked three times a day. They produce 2 million gallons of milk a year, which is shipped four miles to the farm’s own processing plant, Lochmead Dairy.
At the plant the milk is bottled and the cream is churned into ice cream. They also produce the Coconut Bliss product made from imported coconut milk.
On the farm, Gibson manages 3,500 acres. Blueberries, mint and hazelnuts are grown for ice cream ingredients. Clover is grown for seed and silage, corn for silage and earlage, alfalfa for hay and silage and wheat, fescue and ryegrass for feed.
The digester was placed on the farm between the collection pit and separators. It utilizes mixed flush and scrape manure from the milk cows and youngstock to feed the two glass-lined steel tanks. Flush water from the ice cream plant is also added.
According to Gibson, the digester is producing 150 kilowatt-hours of electricity and is on track to produce 180.
The digestate is separated and the solids are composted. Gibson has been composting solids for bedding in three greenhouses for the last eight years.
“The solids really look good coming off the press,” he says. There is less phosphorus and less product to deal with now that it runs through the digester. He’s also been told the odor from the dairy will change.
This is not the only renewable endeavor Gibson has undertaken. He installed solar panels on his processing plant and some retail outlets. He is also working on placing solar panels on his warehouse.
Gibson says he hopes this technology can become more commonplace throughout the country, but recognizes regulations can be a large hurdle.
“We’ve done our share trying to generate electricity,” he says. “We just need to educate the people who issue the permits to make them realize this is a big source of electricity.”
According to Tank, even if the technology is suitable, digesters are not for everybody. “Not every farm can connect to the grid or meet the full range of other dynamics that come into play in a digester system,” Tank says. PD