Digestate, a precious resource for the production of green fertilisers

The eighth edition of the Biogas Italy event reiterated the importance of digestate for carbon neutral agriculture. Biogas plants contribute not only to energy generation but also to the production of fertilisers, derived from digestate

Held in Rome on March 13 and 14, the eighth edition of Biogas Italy, the event organised annually by the Italian Biogas Consortium (CIB), this year launched the claim ‘Think Negative’, recalling the commitment to develop a ‘carbon negative agriculture to produce more consuming less’. Over a thousand stakeholders participated in the two-day event that brought together the world of biogas and agricultural biomethane, and took stock of the many measures dedicated to agriculture to ensure food security by protecting soil fertility, to stimulate the production of renewable energy and combat climate change. What the CIB proposes through its Farming for Future approach is a dynamic method that goes beyond food production, entering the broader sphere of agroecology as an engine of sustainable development and as a model to be exported globally. Precisely on these issues there are interesting food for thought in the recent report produced by Rita Di Bonito (ENEA Researcher) and Vito Pignatelli (President of ITABIA) entitled ‘Fertilisers from digestate to contribute to the objectives of the Green Deal’, the salient passages of which are outlined below.  

Need for fertilisers. In the European Union, the use of fertilizers for agriculture requires the import of quantities of nitrogen, phosphates and potassium - around 30%, 68% and 85% of total needs respectively - the cost of which has recently undergone significant increases due to the energy crisis triggered by the most recent conflicts. In fact, the production of nitrogen fertilisers is based on the use of natural gas, which in September 2022 recorded a price increase of 149% compared to the same period of the previous year, with significant effects on prices in the agricultural sector. Phosphorus also presents a growing supply criticality as it is obtained from phosphate rock, which is imported from Morocco, Russia, Syria and Algeria for about 88% and is considered to be running out. For this reason, it has been included by the EU in the list of 20 critical raw materials for which recovery and recycling strategies need to be developed. 

The fertiliser market currently amounts to around 1 billion euro, of which 70% is inorganic fertilisers and 30% organic fertilisers. The 2023 Nutrient Management Action Plan provides not only for a more efficient use of fertilisers, but also for the development of sustainable farming practices with the privileged use of organic fertilisers. In this context, particular attention is paid to technologies for the recovery of nutrients from the treatment of waste, effluent and waste materials from the agricultural, livestock and agro-industrial sectors, but also from the treatment of the organic fraction of municipal waste and from the purification of waste water. The development of new technologies for fertiliser production must in any case be compatible with the objectives of the Green Deal, which provides for a reduction in emissions of climate-changing gases with the achievement of carbon neutrality in 2050 and their reduction of at least 55% compared to 1990 values already in 2030. From this point of view, digestate, organic material obtained as a residue from the anaerobic digestion of biomass and organic waste for the production of biogas, assumes a particular value as it represents on the one hand a valuable source of organic matter for agricultural land and on the other hand the co-product of a renewable energy source with carbon neutrality, both for the production of electricity and for the obtaining - through biogas upgrading treatment - of biomethane and bio-hydrogen (EBA Statistical Report, June 2023). Biomethane, in particular, can be used for heat generation in heavy industry, for electricity generation and as an advanced biofuel in land, sea and air transport. Biomethane production has increased continuously (from 0.5 billion Sm3 in 2011 to 4.2 billion in 2022 - Source EBA, 2023) and is set to grow significantly, as the European Union has hoped to achieve a production of 35 billion Sm3 in 2030. The production of digestate from anaerobic digestion plants currently in operation in Europe is estimated at around 128 million tonnes/year and will grow significantly as a result of the expected increase in biomethane production. The digestate is used directly, by spreading on the ground, but technologies have been developed that allow both its storage and transport away from the production site, and the recovery of nutrients such as nitrogen and phosphorus with the control of climate-altering gaseous emissions.

The enhancement of digestate. In anaerobic digestion part of the organic substance is decomposed and transformed into biogas, while the mineral fraction remains almost entirely in the digestate, which is used directly as organic fertiliser being rich in nitrogen, phosphorus and potassium, especially if obtained from the digestion of animal manure. The solid component of the digestate is similar to humus and contributes to the restoration of carbon in the soil.  The recent European Union fertiliser regulation (EU 2019/2009), which came into force in July 2022, aims to harmonise fertiliser production and create a regulatory framework to define the characteristics of products that can receive the EU label and be marketed in all EU countries. The Regulation recognises organic or organo-mineral fertilisers, as well as organic soil improvers (containing only organic carbon), among the seven functional product categories (PFCs) accepted as fertilisers. Digestate is included in the eleven categories of fertiliser constituent materials (CMC), whether from crops or other sources of organic matter, with some exceptions such as sewage sludge. The Regulation lays down minimum process conditions for anaerobic digestion, maximum limits for chemical contaminants (PAHs, heavy metals), microbiological contamination limits (salmonella, coliforms) and minimum nutrient concentrations. The use of digestate on agricultural land must also take into account the Nitrates Directive (91/676/EEC) which, in order to limit water eutrophication, limits the amount of nitrogen that can be put into sensitive areas. The development of digestate treatment and transport technologies can increase its fields of application and represent an additional economic benefit to biogas production. The digestate is usually subjected to a mechanical treatment that separates the liquid and solid phases, used differently. The solid part has a dry weight of 20-40%, is rich in phosphorus (P₂O₅) and organic matter, making it suitable as a slow-release humic fertiliser, while the liquid part, with a dry matter content of 1-8%, is richer in ammonium (NH₄₊) and can be used as a fast-acting fertiliser. If dried, the solid part of the digestate can be pelletised and - although this treatment is still not widespread - its application would have enormous potential for development even in horticulture and gardening being easily stored and transportable. As for the liquid fraction of the digestate, it can be subjected to volume reduction processes, through evaporation and filtration techniques, in order to obtain suspensions with higher concentrations of nutrients and easier to transport. Also from this treatment it is possible to recover NH₄₊ from condensation vapours in the form of ammonium sulphate to be used as mineral fertiliser.

Technologies for nutrient recovery. Recovery of individual nutrients (nitrogen and phosphorus) from digestate was initially implemented to limit nitrogen dispersion which, in sensitive areas, should not exceed the limit of 250 kg/ha according to the Nitrates Directive (91/676 ECC). However, the growing demand for fertilisers has led to the development of technologies for the recovery of nitrogen and phosphorus from digestate, already used in industrial plants. Some of these are operating near urban wastewater treatment plants, rich in nitrogen and phosphorus, but the processes used are also suitable for digestate treatment. Today the mature technologies available on the market are numerous and among them we can mention PAKU, developed by Endev Company (Finland); CarboREM, developed by Greenthesis Group (Segrate, Italy); TerraNova Energy (Dusseldorf, Germany); Ostara Pearl from Evoqua (United States, Canada).

Recently, numerous innovative technologies have been developed for the recovery of nutrients from both digestate and other types of biomass, still being tested in pilot plants with TRL (Technology Readiness Level) from six upwards. These experiments have been possible thanks to huge investments in R&D also in the form of projects funded by the European Union which considers the recovery of nutrients to meet the demand for fertilisers for agriculture as a priority with a view to recycling resources and reducing climate-altering emissions.

The enhancement of digestate and its derivatives as fertiliser therefore represents an important opportunity to return to agricultural land valuable nutrients, which would otherwise risk being wasted if not to be, as in the case of nitrogen compounds, even harmful to agro-ecosystems.

The wide availability of technologies, already on the market or at an advanced stage of development, guarantees a high flexibility of use for these products and the possibility of consciously making the best choices for each specific situation, making an important contribution to the growth of the circular economy, the competitiveness of agro-energy and the sustainability of agricultural, livestock and agro-industrial production systems.