Waste to Energy

By Ned Madden
Produce Grower Magazine
October 15, 2013

Biogas from anaerobic digestion reduces heating, electricity bills for hydroponic vegetable growers.

Waste makes wealth…or at least energy, which is a good start.

Biogas and the anaerobic digestion (AD) technology that creates it has been commonplace for decades on dairy and hog farms for waste disposal and energy production. It’s now a viable option for indoor hydroponic and aquaponic fruits and vegetables producers interested in reducing heating and electricity bills while generating energy recovery revenue.

Biogas is gaining traction as a versatile clean energy carrier with significant potential to meet growing demand within the agriculture, power, heat, fuel and chemical markets. Biogas compares well against the major fossil fuels oil, coal and natural gas, which have significant disadvantages—they are finite and their combustion produces emissions damaging to the earth’s climate.

According to a June 2012 report from clean-tech market research firm Navigant Research, the fast-growing biogas market reached $17.3 billion in global revenue in 2011, and will nearly double in size to $33.1 billion by 2022.

Almost any organic material can be processed as AD feedstock, including biodegradables such as food and scraps, fats, oils and grease (FOG), plants, grass clippings, paper, sewage sludge, and animal and human wastes.

Biogas produced in anaerobic digesters consists of methane (50–80 percent), carbon dioxide (20–50 percent), and trace levels of other gases. Biogas methane – an attractive, abundant fuel (but also a potent atmospheric greenhouse-effect gas) – can generate electricity for use by the operation itself or for sale to utilities.CO2 – another greenhouse-effect gas – can be used internally to stimulate plant growth. If a digester is large enough, an opportunity exists to earn revenue by taking in wastes from off-site sources.

And there’s plenty of that. A major problem with all crop production: what to do about profuse amounts of spoiled, inedible or unwanted fruits and vegetables and other organic waste resulting from produce cultivation. Food waste is America’s second most prevalent material reaching landfills, according to the EPA, and only 2.5 percent of this waste material is recycled, composted or reused.

From farm to fork, about 40 percent of all the food produced in the U.S. goes uneaten, reports the Natural Resources Defense Council. That totals $165 billion worth of wasted food every year.

More than six billion pounds of fresh produce go unharvested or unsold each year, and preliminary data from a cluster of fruit and vegetable growers in California suggests that losses on the farm and in the packing stage range as high as 14–60 percent for a variety of common crops.

One alleviating solution: recycle the waste via AD.

AD it up

Two major types of systems are used for waste treatment: aerobic and anaerobic. The terms refer, respectively, to the presence and absence of oxygen. Aerobic and anaerobic microbes have different uses, but both processes help prepare waste and wastewater for decomposition by attacking organic compounds.

In contrast to aerobic degradation, which is mainly a single-species phenomenon, anaerobic degradation proceeds as a chain process in which several sequent organisms are involved. In addition to odor suppression, anaerobic digestion is desirable for its very high rates of purification.

AD has existed as a technology for more than a century. Originally a simple airtight vessel and a septic tank, AD technology gradually evolved into a temperature-controlled, completely mixed digester. Finally, it developed into today’s high-rate reactors containing a density of highly active biomass – organic matter that can be converted to valuable fuel.

Kroger on the go

A leading corporate true believer in “post-consumer” AD produce recycling is the Kroger Co., the biggest supermarket chain in the U.S. and one of the world’s largest retailers – 2,424 supermarkets and multi-department stores in 31 states under dozens of local banners (e.g., Ralphs, Food 4 Less, City Market, etc.). Kroger trucks leftover produce back to its California distribution warehouse on returning delivery trucks. An onsite anaerobic digester processes more than 55,000 tons of food waste a year, providing about 20 percent of the facility’s energy. Kroger is looking at adding similar systems to other distribution centers.

The Kroger Recovery System uses a sophisticated process to convert the carbon in organic material into a renewable source of methane. High-rate AD technology, from Boston-based FEED Resource Recovery Inc., generates methane-rich biogas, clean effluent and high concentrate fertilizer—all within a compact, modular footprint. Interchangeable modules give Kroger the option to use biogas as fuel for onsite boilers, natural gas trucks, or combined heat and power (CHP) units. Kroger says the biogas system, which began operations in May 2013, will pay for itself within five years.

AD techniques can also apply on a much smaller scale on the premises of indoor grow operations.

Chicago’s The Plant

The Plant is an “aquaponics” (hydroponics and aquaculture fish farming) urban ag facility – a 93,500-square-foot, 87-year-old former meat-packing warehouse in the middle of a Chicago “food desert” near the city’s Union Stockyards. The project was partly funded by the Illinois Department of Commerce and Economic Opportunity with a $1.5 million grant.

The Plant features an indoor vertical farm that triples as a food-business incubator and research/education space. The Plant’s aquaponics farm currently occupies 7,000 square feet in the structure’s basement. Along with its fish hatchery, The Plant mixes a hydroponic garden, commercial kitchen and a brewery. The waste from one part of the farm serves as raw material for another part in order to create a net-zero energy system. Everything is recycled.

The “heart” of The Plant’s renewable energy operation is an anaerobic digester designed by the German manufacturer Eisenmann Corp.

After it begins operating in Dec. 2013, the anaerobic digester will annually divert 5,000 tons of organic waste from landfills to produce methane-based biogas.

This biogas will be fed to a jet turbine generator that will produce electricity and heat for all 93,500 square-feet of the building’s space (including the businesses housed there), with no additional use of fossil fuels necessary.

The Plant’s renewable energy system will eventually divert more than 10,000 tons of food waste from landfills each year to meet all of its heat and power needs.

“Biogas production makes economic and ecological sense,” noted Thomas Gratz, regional sales manager for Eisenmann, which has its U.S. headquarters near Chicago.

The AD system – 170 feet long, 50 feet wide and 45 feet high – is located on the exterior at the rear of the building. The system consists of a front-end receiving area, main digester, separation area, post digester and technical container with all the pumps and controls. The steel 325-m3 (cubic meter) horizontal “plug flow” main digester is based on a proprietary Eisenmann design. The front-end receiving equipment has been custom tailored for operations at The Plant.

The post digester is a 1,500-m3 volume “continuously stirred reactor” type of tank. For solids separation after the digestion process, a screw press separator is in place.

The created biogas will be routed to a separate combined heat & power (CHP) unit supplied by The Plant. Also known as cogeneration, CHP is the simultaneous production of electricity and heat from a single fuel source such as biogas.

The most important factor for an operation looking into AD for their waste streams and unused produce is to select a technology provider that can give the best solution and service for the application on hand, according to Gratz.

“A proper technology provider will give you the much needed assistance and guidance from development, construction, startup all the way to operation and maintenance of the AD system,” Gratz told Produce Grower.

Stahlbush Island Farms

Stahlbush Island Farms, Corvallis, Ore., is an environmentally friendly farm and food processor recognized nationally for its sustainable production of fruits, vegetables, grains and legumes.

Stahlbush operates a first-of-its-kind (in North America) Biogas Plant that turns all of the farm’s agricultural byproducts (corn husks, silage, waste fruits and vegetables, etc.) into electricity. Begun in 2008, the $10 million dollar project took 14 months to complete before becoming operational in 2009.

By 2011, the Biogas Plant had reached record electricity output at 33,673 kWh (kilowatt hours are units of energy equivalent to the energy transferred or expended in one hour by one kilowatt of power).

According to farm management, the biogas power plant annually provides enough electricity for approximately 1,100 homes, nearly twice what the farm and food processing plant uses in a year. The plant supports Stahlbush Island Farms’ goal of minimizing its carbon footprint and gaining energy independence through the creation of a renewable, on-farm energy source.

“It takes several growing seasons to completely optimize the digester,” explained Stahlbush’s senior vice president of operations John Bailey, who says there is a steep learning curve early on in AD adoption.

“One of the things we learned about the digester is that the entire operation is one part science and two parts art,” Bailey said. “Our digester consumes everything from our corn stalks to blueberries. The continual variations in feedstock create a myriad of challenges for the operators who strive to maximize daily kW output. We are only now beginning to fully understand that there are many second- and third-tier benefits from the digester we did not utilize in our ROI equations.”


According to the American Biogas Council, there are more than 2,000 biogas-producing sites currently operational in the U.S. There are 1,500 digesters at wastewater treatment plants (though only 250 use the biogas they produce), 563 landfill-based energy projects (26 pipeline, 537 electricity/boiler), and 171 (U.S. EPA estimates 192) anaerobic digesters on farms.

Uptake of AD use has been strong in the U.S. agricultural sector due to federal and state incentives that have supported development. AgSTAR – a collaborative effort of the EPA, the U.S. Department of Agriculture and the U.S. Department of Energy – is a voluntary outreach and educational program that promotes the recovery and use of methane from animal manure.

AD systems can range in size and capacity from industrial scale to DIY home units.

CleanWorld Partners (CWP), Sacramento, Calif., is known as the North American leader in AD. In Dec. 2012, CWP opened in Sacramento what it calls the largest commercial-scale, high-solid AD system in the U.S. CWP said the Sacramento Biodigester converts 25 tons of food waste per day from area food processing companies, restaurants and supermarkets into renewable natural gas, electricity and soil-amendment products. By Dec. 2013, the Biodigester will be expanded to a 40,000-tons-per-year facility.

But the best solution for the expansion of AD technology is smaller systems at lower prices.

In the middle AD tier are systems like those from UK-based SEaB Energy Ltd., an international specialist working in the renewable energy and energy-from-waste sectors. The company’s Muckbuster and Flexibuster designs are compact, easy-to-install, turnkey AD systems housed in … shipping containers. The systems are modular, easily configured and scalable for use directly at smaller sites that typically generate 200-1,000 tons of waste per year.

For consumer DIY AD, Biorealis Systems based in Willow, Alaska, offers a small-scale home biogas digester, designed to process kitchen waste, animal dung or human waste (local sanitation codes permitting). They can be designed either as a manually fed bucket in a yard or as part of an integrated waste system connected to a farm or home sewage system.

Ned Madden is a California-based Controlled Environment Agriculture (CEA) consultant, marketing professional and journalist who writes about urban agriculture technology and business-related topics.

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