Digestate of biogas plants, despite its value as a source of organic carbon, a complex of macro-and micronutrients necessary for plant growth, has a number of features that limit its use and commercialization.
The main feature that reduces the competitiveness of the digestate with commercial fertilizers is the variability of its physicochemical characteristics due to a combination of different factors, such as variability of feedstock, variability of the mixture of different raw materials and volumes of its introduction into bioreactors during the year, complexity of anaerobic digestion process control, residual (post-fermentation) biological and chemical activity of digestate.
When applying digestate to the fields owned by a company that operates a biogas plant, standardization of the quality of digestate products is not critical, although it would allow better management of soil condition and, consequently, crop yields. For free circulation, regardless of the producer or consumer, digestate products need to be standardized. This will increase the competitiveness of the digestate in the fertilizers market, as well as expand the list of market niches and consumer groups.
Standardization of the quality of the digestate may include:
- setting requirements for the types of input raw materials;
- categorization of products from it by purpose (for example, fertilizer, soil improver or growing medium);
- categorization by content of organic carbon, nutrients, phase state of digestate products, etc.;
- setting limits on the concentration of certain chemical elements and compounds (heavy metals, persistent organic pollutants, etc.);
- setting restrictions on the presence of pathogenic microflora and weed seeds;
- establishment of requirements for the control of the technological cycle of formation, movement and storage of digestate and products from it.
To begin with, there is a need for the object of standardization to be clearly defined. Currently, the term “digestate” is not introduced in Ukrainian legislation, although some regulations use its definitions in different terms. There are also no special requirements for standardizing the quality of digestate or products from it.
The Law of Ukraine №287-VIII “On animal by-products not intended for human consumption”, in particular, Article 19 states that residues formed as a result of conversion of by-products of animal origin for biogas or compost may be placed on the market and used as organic fertilizers or soil improver. Given that most biogas plants in Ukraine, in addition to animal by-products, use plant raw materials, and in some cases also by-products of the food industry, the provisions of this Law will not apply to digestate formed from such mixtures of raw materials. At the same time, the Law does not set special requirements for standardizing the quality of such residues.
In the draft Order of the Ministry of Agrarian Policy of July 9, 2019 “On approval of the List of substances (ingredients, components) that may be used in organic production and which are allowed for use in limited quantities”, which will have the status of a bylaw to the current Law of Ukraine №2496 -VIII “On the basic principles and requirements for organic production, circulation and labeling of organic products”, the following terms are used:
- composted or fermented mixture of substances of plant origin is a product obtained by mixing substances of plant origin that have undergone composting or anaerobic fermentation for the production of biogas;
- biogas digestate containing animal by-products processed together with materials of plant or animal origin (animal by-products (including wild animal by-products) of category III and the content of the gastrointestinal tract of category II (categories II and III in accordance with the Law of Ukraine “On animal by-products not intended for human consumption”) should not be obtained from inorganic livestock).
These definitions cover a fairly wide range of raw materials used for biogas production, exeptwastewater sludge. However, the exception for livestock by-products significantly limits the coverage of digestate flows of biogas plants in Ukraine, as most of them use manure (litter) obtained from inorganic livestock. Anaerobic fermentation of only substances of plant origin is practiced less frequently, and in fact is limited in Ukraine to single biogas plants on maize silage. If the List of Substances is complied with, the digestate (fermented mixture) can be certified as an organic fertilizer for organic farming. In Ukraine, such certification is carried out by Organic Standard LLC.
Formally, the digestate of biogas plants can be considered as an organic fertilizer, and therefore fall under the Law of Ukraine 86/95-VR “On Pesticides and Agrochemicals“. However, in practice, the commercialization of digestate as an organic fertilizer will require state 2-stage testing and state registration. Given the variability of the digestate composition, this can not be considered as a promising way.
Thus, the Ukrainian legislation does not formulate either a generalized term for “digestate”, which would cover the flows of digestate formed from all types of raw materials of plant and animal origin, or special requirements for the standardization of such digestate or products from it. Given the cumulative increase in the annual generation of digestate in Ukraine (according to UABio estimates, about 2 million tons of digestate were formed at existing biogas plants in 2019) and the need to promote free circulation and commercialization, the issue of standardizing the quality of digestate products needs to be addressed nationally. Given the movement of Ukraine towards the harmonization of legislation with the EU, it would be logical to harmonize issues related to the standardization of the quality of the digestate.
The quality of the digestate in the EU can be confirmed by appropriate certificates. Over the last 25 years, compost and digestate quality assurance schemes have been introduced in some EU countries (KBVÖ in Austria, Vlaco in Belgium, BGK in Germany, and CIC in Italy). At the European level, there is a scheme of voluntary certification of compost and digestate ECN-QAS. It aims to establish a common basis for existing quality assurance schemes in Europe and to support Member States in setting quality standards and developing their own quality assurance schemes for compost and digestate. The requirements under the ECN-QAS scheme include:
- regular assessment of compost and digestate production in the plants by the national quality assurance organisation;
- regular sample taking and analysis of the final product for relevant quality parameters from independent, acknowledged laboratories, coupled with evaluation of the results by the national quality assurance organisation;
- documentation by the national quality assurance organisation with information about the quality properties of the product, legal requirements, the necessary compost and digestate declaration and information about use and application rates according to good practice;
- awarding of the ECN-QAS Quality Label to composting or digestion plants by the certified national quality assurance organisation.
To be available on the market, any type of fertilizer in the EU must comply with the requirements of Regulation (EU) 2019/1009 of 5 June 2019. The main feature of this Regulation is the categorization of products by functional purpose(a total of 20 types, combined into 7 groups of functional products: 1) fertilizer; 2) liming material; 3) soil improver; 4) growing medium; 5) inhibitor; 6) plant biostimulant; 7) fertilising product blend) and the material component(a total of 11 types of products: 1) virgin material substances and mixtures; 2) plants, plant parts and plant extracts; 3) compost; 4) fresh crop digestate; 5) digestate other than fresh crop digestate; 6) food industry by-products; 7) microorganisms; 8) nutrient polymers; 9) polymers other than nutrient polymers; 10) derived products within the meaning of Regulation (EC) (1069/2009; 11) by-products within the meaning of Directive 2008/98/EC).
The Regulation set requirements for both different types of functional products and for products categorized by material component. Regarding the digestate itself, 2 types are clearly defined, covering almost all types of raw materials of animal and plant origin for biogas production, except domestic and industrial wastewater sludges, mixed solid waste, and certain categories of livestock by-products, which pose risks to human and animal health.
Requirements for digestate relate to the origin of raw materials, the content of additives (necessary to regulate the biochemical process), clear demarcation of processing lines of raw materials and digestate (including storage of digestate) within the biogas plant, control of organic matter decomposition (with combination of thermophilic fermentation at 55⁰С, pasteurization and/or composting), the content of macroscopic impurities (glass, metal or plastic), residual biochemical activity (expressed through oxygen demand or residual biogas potential). According to their functional purpose, both types of digestate can be classified as organic fertilizer (solid and liquid forms), organic soil improver and growing medium.
The requirements under the ECN-QAS certification scheme for digestate and the requirements of Regulation (EC) 2019/1009 for organic fertilizer, soil improver and growing medium are given in Table 1. As can be seen, given the minimum organic content requirements for obtaining the ECN-QAS certificate or the labeling of CE for free circulation on the EU market as an organic fertilizer or soil improver only a solid form of digestate (solid fraction after separation, dried and/or granular digestate) or liquid concentrated forms can be considered.
Table1 – The requirements under the ECN-QAS certification scheme for digestate and the requirements of Regulation (EC) 2019/1009 for organic fertilizer, soil improver and growing medium
 Agrochemicals – organic, mineral and bacterial fertilizers, chemical ameliorants, plant growth regulators and other substances used to increase soil fertility, crop yields and improve the quality of crop products
Author: Petro Kucheruk – expert of Bioenergy Association of Ukraine