Main Objective

Anaerobic digestion (methanisation) is a biological process that breaks down organic matter to treat organic waste, such as livestock manure (solid and liquid), sewage sludge, green waste, and agro-industrial by-products. This degradation produces biogas (methane), which is recovered as renewable energy: the gas is either used directly or burned to generate electricity.

The process also produces a residue (sludge) called digestat. This product is valuable for agriculture because it contains most of the nutrients from the waste entering the methaniser. In particular, it is rich in nitrogen (N), the primary element required by plants and the main component of synthetic fertilizers used in fields. Digestate also adds organic matter to the soil, ensuring fertility and promoting soil life. Consequently, this technology contributes to the energy and fertilizer autonomy of local territories.

The MétaMétha Project

By introducing anaerobic digestion into mixed crop-livestock farms, livestock effluents and a portion of synthetic fertilizers are replaced by digestates. The MétaMétha project aims to:

• Determine how to use digestates effectively compared to other products.
• Define the fertilizing power of digestate for crops.
• Study the capacity to maintain or increase stable humus levels in the soil.
• Assess environmental impacts (nitrates, ammonia, greenhouse gases) in comparison to other fertilizers.

The project involved spreading different products—livestock effluents, digestates, and synthetic fertilizers—on agricultural plots growing rapeseed or wheat to compare their effects. Working in "real-world" conditions allows for a holistic study of the system, capturing phenomena like greenhouse gas emissions and nitrate leaching while accounting for agricultural constraints.

On-site, researchers use scientific equipment (e.g., soil moisture probes) and collect water, soil, and gas samples for laboratory analysis.

Key Findings on Crop Yields

The MétaMétha project demonstrates the agronomic value of digestates, which achieved yields similar to those obtained with mineral fertilization for wheat and rapeseed. However, the bread-making wheat industry requires specific protein levels in the grain that could not be reached using digestates alone. To achieve sufficient protein content, a late application of mineral nitrogen (when the crop is taller) is recommended.

By combining field results with model simulations, a nitrogen balance was established for each plot to track the nitrogen provided by the products—whether it remained in the soil, entered the water as nitrates, was released into the air as gas, or was absorbed by the plant.

Main Results

• Economic Impact: By using digestates (waste already present on the farm), farmers save money by purchasing fewer fertilizers. The extra costs associated with the demanding process of spreading digestate are offset when the methaniser is located near the farm. Otherwise, the logistical constraints and increased labor time make organic products less favorable.
• Environmental Management: By following best practices—such as splitting digestate applications into reasoned doses, using appropriate equipment, and timing applications correctly—the environmental risks inherent to organic products (greenhouse gases, soil compaction, nitrates) can be limited.
• Biodiversity: Studies regarding the impact on biodiversity (such as earthworms) are still ongoing, but initial effects appear limited.