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Anaerobically digested dairy fiber: A valuable product

John R. Lamont and George C. Elliott Published on 24 November 2015
Manure

This summer, the EPA proposed a new set of regulations aimed at curbing methane emissions. This announcement has drawn a sharper focus on methane, a greenhouse gas almost 25 times more powerful than carbon dioxide.

Until recently, discussions about methane emissions have been overshadowed by carbon dioxide. This broadening of greenhouse gas concerns should be of great interest to the dairy industry as animal agriculture contributes significantly to total methane emissions.

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Anaerobic digestion systems to extract methane and reduce odor from manure have been used since the 1970s and have seen vast improvements in the ensuing decades. Profitable operation of an anaerobic digester involves complex economics. Selling methane as a biofuel generates additional income for farmers, utilizing this carbon-rich greenhouse gas rather than losing it to the atmosphere.

Methane extraction from manure shows great promise as a supplementary revenue source for dairy farmers, but revenue generated by energy production alone is often not enough to offset the capital costs of constructing and operating anaerobic digesters.

Also, the value of energy produced can be inconsistent because it depends on market conditions and government policies. Other sources of revenue are needed, such as tipping fees for organic wastes and value-added products derived from byproducts.

The recent interest in anaerobic digestion as a manure management solution has led to a search for creative new uses of anaerobic digestate byproducts. The primary product produced through anaerobic digestion is methane gas, which is used as a biofuel for energy production.

Anaerobic digestion also produces liquid and solid byproducts, which are often land applied after anaerobic digestion is complete.The solid fraction, also known as anaerobically digested dairy fiber, is a light, spongy, fibrous material with very little odor. Anaerobic digestion stabilizes the fiber, keeping it from breaking down further.

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Little work has been done to find value-added uses for the solid fraction of anaerobic digestate. Currently, digested fiber is used similarly to manure – as a soil amendment or as animal bedding. Digested fiber has also recently been evaluated as a feedstock for cellulosic ethanol production. Using digested fiber as a component of potting mixes may be an even more lucrative option.

Digested fiber and peat are formed under similar anaerobic conditions and have similar physical properties. Anaerobic digesters have even been referred to as “short-term renewable peat bogs.” If digested fiber were proven as a high-quality potting mix component, it could be a value-added product to add to dairy profits. The demand for digested fiber from growers would be an added incentive for dairy farmers to adopt the more sustainable anaerobic manure digestion systems.

Peat has been one of the most important components of potting mixes for decades due to its good porosity, water-holding capacity and adaptability to a wide range of crops, potting mix blends, fertilizer regimes and horticultural chemicals. Recent concerns over rising costs and the environmental impact of extracting peat from peat bogs has led some to question the sustainability of the wide-scale use of peat in potting mixes and spurred a search for more environmentally friendly alternatives.

A wide variety of agricultural byproducts such as composts, coconut husk fiber (coir), parboiled rice hull and various wood byproducts have been evaluated as potential peat replacements. While there have been successes with other peat alternatives, they are often limited by inconsistency from batch to batch and during the course of a growing season, leading to inconsistent plant growth.

Marketing anaerobically digested dairy fiber as a useful horticultural material could become yet another source of income for dairy farmers and provide a solution to some waste management problems associated with raw manure. Leaching of nutrients from accumulated manure is a significant source of non-point source water pollution.

If digested fiber were used in a potting mix, nutrients that would otherwise be lost as pollutants could be used for plant nutrition. Using digested fiber as a potting mix component could also benefit growers by offering a low-cost, local, renewable alternative to peat.

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In a recently completed research project at the University of Connecticut, anaerobically digested dairy fiber was evaluated as a component of potting mixes for greenhouse and nursery crops. For greenhouse crops, garden mums, cyclamen, poinsettia and a variety of bedding plants were grown in various digested fiber-containing mixes as well as a standard peat-perlite mix.

Preliminary analyses of these mixes showed that the alkalinity of digested fiber and the acidity of peat neutralize each other to arrive at a pH suitable for plant growth when combined in a 1-to-1 proportion. Lime is normally added to peat-based mixes both to supply plants with calcium and to raise the pH. For the digested fiber mixes, gypsum was added to supply calcium without greatly altering the pH.

For most of the crops tested, plants grown in a 2-to-2-to-1 peat-digested fiber-perlite mix grew to an equal or greater size and quality to those grown in the standard 4-to-1 peat-perlite mix. A few crops, such as pansy, had less favorable growth in digested fiber-containing mixes. This was likely due to crop-specific sensitivities to differences between mixes.

Nursery crops were also used as test subjects due to their robust nature. Representative species of woody and herbaceous nursery crops were grown in bark-peat-sand, bark-digested fiber-sand, bark-peat-perlite and bark-digested fiber-perlite mixes all with a ratio of 4-to-2-to-1. The digested fiber mixes were amended with gypsum, as in the greenhouse crop trials.

All nursery crops grown in the digested fiber-containing mix grew to be of equal or greater size and quality of those grown in the standard peat mix. Nursery crops are often grown in their pots for longer periods of time than greenhouse crops, so nursery potting mixes need to be resistant to degradation, which leads to “slumping” or “shrinkage.”

To test the resilience of digested fiber, the woody nursery crops were grown for two seasons, and the porosity, water-holding capacity and “slumping” of the mixes were measured at the beginning and the end of the trial. Both mixes experienced the same amount of total shrinkage, although the digested fiber mix had less shrinkage under drip irrigation during the first season and more shrinkage under overhead irrigation in the second season.

The results of these trials demonstrate anaerobically digested dairy fiber can be used as a 50 percent replacement for peat in greenhouse and floriculture mixes or a complete replacement for peat in nursery mixes.

Crop management decisions in these trials were made based on well-established practices for peat-based mix. Future research could help in developing management practices to optimize the use of digested fiber in potting mixes. Improvement could also be made in procedures to process digested fiber with a consistent potting mix component as a goal.

Anaerobically digested dairy fiber offers great potential to further encourage the adoption of anaerobic digestion and provide the containerized horticulture industry with a sustainable alternative to peat.  PD

George C. Elliott is an associate professor at the University of Connecticut. John R. Lamont is a Ph.D. student at the University of Connecticut.

PHOTO: Anaerobic digestion stabilizes the fiber portion of manure, keeping it from breaking down further and making it a viable replacement for peat in greenhouse, floriculture and nursery mixes. Photo by George C. Elliott.

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