Anaerobic Digestate


Guest post by Daniel Nguyen, Review Program Technical Supervisor at Organic Materials Review InstituteIf you have questions pertaining to this article, please contact author.

Anaerobic digestion is becoming increasingly common in agricultural operations as a means of reducing waste materials while generating additional revenue. Commercial facilities utilize controlled microbial degradation to process feedstock materials such as food waste and manure. In addition to diverting these materials from landfills and other disposal methods, anaerobic digestion facilities generate biogas, which can be used as an energy source, and digestate which can be used as a fertilizer or soil amendment. OMRI has received and reviewed multiple applications for commercial digestate products to be used as inputs in organic agriculture.

Manure is one of the most common feedstocks used to create anaerobic digestate products. As of January 2018, the AgSTAR Livestock Anaerobic Digester Database lists 265 livestock anaerobic digester facilities located across the United States. When used in organic production, special considerations exist for farmers using any materials derived from manure. In order to reduce the potential for pathogens in harvested crops, the USDA National Organic Program (NOP) regulations require that manure must either be processed in a prescribed manner, or that the organic farmer must observe a minimum number of days between application of manure and harvest of the crop. Some of the processes prescribed by the NOP include:

- Composting so that all feedstocks are heated to a minimum of 131°F for a minimum of 3 days; or

- Heating, without combustion, to a minimum temperature of 150°F for at least 1 hour, or to 165°F with a drying step to establish a maximum moisture level of 12%. (NOP 5021)

Unlike aerobic composting processes, anaerobic digestion occurs in an oxygen-free environment that does not typically reach the high temperatures described in the NOP regulations. Additionally, the resulting anaerobic digestate is often too wet to qualify for the drying requirements identified in the drying step. Without these specific processing steps, anaerobic digestate derived from manure can only be applied by farmers with a required interval before harvest. These intervals are:

- Applied to land used for a crop not intended for human consumption,

- Applied to land 120 days prior to the harvest of products whose edible portion has direct contact with the soil surface or soil particles or,

- Applied to land 90 days prior to the harvest of products whose edible portion does not have direct contact with the soil surface or soil particles. (7 CFR §205.203(1)(i)-(iii))

On April 5, 2016, the National Organic Standards Board (NOSB) received a petition to amend the standards. Specifically, the petitioner asked the NOSB to consider anaerobic digestate, processed at non-thermophilic (lower) temperatures over a period of 21 days, as a non-restricted material separate from raw manure. The petitioner asked that this specific form of processing be treated similarly to a composting process, and that the resulting anaerobic digestate could be applied to land without any pre-harvest interval restrictions.

The NOSB considered the petition, and commissioned a technical report to evaluate anaerobic digestion. The technical report indicated that the processes described in the petition and literature do not demonstrate that anaerobic digestate sufficiently reduces pathogens. Ultimately, the NOSB unanimously rejected the petition based upon the technical report findings. As a result, manure-based anaerobic digestates that do not undergo the processing steps currently described by the NOP continue to carry restrictions concerning when they can be applied to certified organic land.

OMRI supports organic integrity by providing an independent review of products intended for use in certified organic production. Acceptable products are OMRI Listed® and appear on the OMRI.org website and in printed lists. OMRI also provides technical support and training for professionals in the organic industry.

Daniel Nguyen is a Review Program Technical Supervisor with OMRI. He holds a B.S. in Biology from the University of Oregon. As a student, he assisted the testing of experimental nanotechnology for use in soil and water remediation at UC Santa Barbara. At the University of Oregon, he worked in an ecology lab where he assisted research into Costa Rican orchids.

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