Characterization of DIgestate and evaluation of their agronomic VAlorisation – DIVA

The biogas plants representative of the French sector were determined by a literature review and meeting with experts.
Five different biogas plants were chosen: two agricultural, one household waste, one biowaste and one territorial (centralized).
The characterization of the digestate of these 5 plants was conducted by five sampling campaigns spread over about a year. The results obtained (physico-chemical, rheological, biological stability, agronomic value) were compared with standard thresholds NFU44-051, NFU44-095 and NFU42-001.
In parallel, several post-treatment processes were evaluated: for solid digestates, drying and composting; for liquid digestates, biological treatment and membrane filtration.
The agricultural use of digestate was approached by laboratory tests, field trials and modeling to determine: the nitrogen fertilizer value in the short term, the dynamics of mineralization of C and N, the risk of phytotoxicity and the behavior of C and N in fields. This study was completed by measures of ammonia and greenhouse gas emissions after spreading.
Finally, an evaluation of the operating costs and environmental impacts (LCA) of each digestate management channel was carried out on the basis of bibliographic data, measurements made in the project and on sites.

The composition of the digestates depends on the raw waste methanized but varies only slightly within a same plant. Their dehydratability depends on their MS content and on their MO/MS ratio. These rules no longer apply after phase separation that keeps NH3 and K in the liquid phase and C, P and metals in the solid phase.
Solid phase of digestate:
• Drying volatilizes 75% NH3 at 70°C and 100% above 90°C. Drying under hot air does not destroy spores of Clostridium perfringens. The one by contact-agitation sanitizes the digestate but creates powdery product.
• Composting sometimes requires addition of structuring agent or co-substrate for increase in temperature and induces significant NH3 emissions.
Liquid phase digestate:
• Nitrification avoids NH3 emissions and keeps nitrogen in the digestate. It should be better explored.
• Membrane filtration processes require significant pretreatment to limit membrane clogging. They however enable to capture more than 93% of the residual MO and 95% of the ions of digestate.
Digestate have agronomic properties intermediate between fertilizers and amendments. Their application must be controlled to limit volatilization of NH3-N which varies between 30 and 84%. In soil, digestate induce N2O emissions higher than those of mineral fertilizers. These emissions are higher for digestate with a low P/Norg. Field trials have not shown a strong effect (positive or negative) of the contribution of digestate on crop yields (fertilizer coefficient equivalent of 40-50%).
The assessment study shows that direct application of digestate is the most efficient and least impactful process for environment. Post-treatments are only interesting when export of excess nitrogen or phosphorus is required or for the marketing of the products.

Because of its scope, the DIVA project has tested different possibilities to get an overview of sectors and technologies of digestate management. The remaining locks are multiple:
Regulation. Digestates do not satisfy any of the existing standards, it is necessary to continue to apply for homologations to get advance in the legislation and better knowledge of digestate. In this sense, a work and deep questioning should be undertaken about the use of enterococci and Clostridium perfringens as indicators for which current standardized methods are problematic.
Technology. The work done and many returns of information collected on site showed that the implementation of digestate post-processing requires optimization to achieve desired product quality. Most of the processes installed on site are only poorly or not working at all. Their maintenance costs have been greatly underestimated. Moreover, it is absolutely necessary to work on the recovery of ammonia emissions from dryers and from composting for, at least, reduce emissions to the atmosphere and ensure the safety of workers and, better, capture and recycle nitrogen.
Agronomy. Beyond the necessary needs in data and modeling, it appears that field trials are still needed to test the efficiency of digestate spreading (available technologies), confirm and predict digestates value as fertilizer equivalent coefficient and validate the long-term safety of their application.
Environmental analysis. The Life Cycle Assessment method is used to assess the environmental impacts of a service, which is quantified by the means of functional units applied in a defined area. The global character of this method has raised questions on the interest of post-processing digestate, without obtaining firm answer. LCA still requires many developments for the assessment of localized impacts, which is absolutely necessary in the case of application of waste on soil.

Work in DIVA was presented in 24 international conferences, 16 public conferences and published in 4 scientific publications. However, the impact of DIVA was more visible at national level by presenting the results every year to a monitoring committee made up of stakeholders in the sector, associations, collectivities and ministries; participating in the steering committees of the CASDAR Valdipro project for the approval of agricultural digestate and by presenting of work at the BN Ferti and national conferences.

In an environmental context where the pressure is growing in terms of energy needs and fighting against climate change, biological recovery of organic matter by anaerobic digestion and biogas (methane) will experience strong development in coming years. Thus a diversification of the type of organic wastes used in anaerobic digestion is encouraged (sewage sludge, biowaste from municipalities and food industries, animal waste). However, the anaerobic digestion of wastes generates liquid and solid residues (the digestate) whose end use/disposal asks technical and environmental questions. Indeed, the potential use of digestate as a "product" and not a "waste" depends on many parameters: (i) the composition of the digestate and its adequacy to standards amendment (NF U 44-051, NF U 44-095) or fertilizer (NF U 42-001), (ii) the ability of digestate to be processed in post-treatment units when required, (iii) a better understanding of the agronomic value and risk of phytotoxicity of raw and post-processed digestates, and finally (iv) an improved knowledge of environmental impacts related to post-treatment and land application of digestate (energy consumption and greenhouse gas emissions in particular).
Recent studies show that the composition of digestate is mainly influenced by the choice of the inputs to the anaerobic digestion process and that the agronomic valorisation of the solid fraction of digestate requires a post-treatment by co-composting. Regarding the liquid fraction, it is a fraction with low concentrations of nitrogen and phosphorus, which is currently mostly spread directly into the fields. Nutrient low concentration of this fraction greatly limits the possibilities of transport which creates difficulties for recovery. The precise characterization of the raw digestates composition and the development of post-processing processes able to generate recoverable products are demanded by professional actors and state departments to ensure security and sustainability of anaerobic digestion.
The DIVA project is a consortium of 3 public research institutes, 1 private institute, 2 companies and 1 research firm working on the management and treatment of urban and/or agricultural wastes. Helped by a committee of advisers and users, which includes representatives of French agencies, ministries, and public and industrial partners, this industrial research project of 48 months focuses on several scientific and technical objectives: (i) to identify and characterize the different types of digestate and management methods currently used in France, (ii) assess the ability of digestate to be recovered directly in agriculture or to be processed through post-processing operations to achieve the status of product, (iii ) clarify the agronomic value and assess the environmental impact of the spraying of raw and processed digestate (particularly with respect to the functioning of the nitrogen biogeochemical cycle: nitrate leaching, ammonia volatilization, N2O emission), and (iv) to conduct a techno-economic and environmental assessment (LCA) of digestate management (post-treatment and application) compared with direct application.
All this work will eventually allow the establishment of a regulatory framework for an evolution of digestates from a status of "waste" to "product", including standardization, certification or provisional authorization to sell.