We have been recognizing biogas as a high energy potential for many years, but the official value of digestate and its fertilizing virtues are increasingly being discussed. Whereas biogas is extracted from the organic material in the absence of oxygen in the digester, the digestate, the other output generated by the anaerobic digestion, represents between 90% and 95% of the input volume.
Substance rich in nutrients for plants, the digestate, obtained in relatively liquid or solid form, is an excellent fertilizing product for agriculture crops. It consists of a blend of nutrient – nitrogen (N), phosphorus (P) and potassium (K) – as well as non-digestible residues and dead microorganisms.
One of the challenges for farmers is the constancy of digestate composition imposed by its recognition of fertilizer, and not as a waste. This is a major barrier to the development of methanisation in agriculture. The biogas industry may be optimistic as the digestate is now recognized officially as a fertilizing residual material in France.
In Québec, since 2012, the digestate is no longer considered as a waste but as a fertilizer for agriculture with a strong market potential, although its definition is constantly evolving through the guides issued by the ministry. The 2015 Guide of FRMs clarifies the status of digestate as an FRM and has therefore put in place measures to reduce the previously required administrative burden for producers to obtain the acceptance of their digestate to market it for spreading in agriculture.
In the United States, the American Biogas Council (ABC), with the support of the biogas industry, has created the ABC Digestate Standard Testing and Certification Program (ABC DSTCP), which aims to recognize the digestate as a commodity, such as compost. In fall 2016, the ABC launched the Digestate Certification Program, which establish standards, strict tests and a quality management system in order to properly define and characterize the digestate, with the aim to make it as common as compost in the eyes of the market and the population. “What is digestate, how do you describe it to regulators or market it to customers?”, that’s the critical digestate knowledge gap addressed by the ABC DSTCP.
In France, after years of effort, the status of digestate just moved out of a waste status to a fertilizer product status. On June 13th 2017 in a decree, requirement specifications have been put in place for the placing on the market and the use of crude digestate as a fertilizer. French methanisers will now be able to sell or exchange this fertilizer product, if it meets the technical conditions, and will no longer be forced to dispose of it as a waste. This will reduce their administrative burden and provide them with excellent economic benefits.
The specifications, however, establish conditions for the use of digestate, which are admittedly restrictive, in order to avoid chemical pollution and pathogen risks:
To read more about the status of digestate as a fertilizer product in France: