First fertilizing-value typology of digestates: A decision-making tool for regulation

Area of Science:

• Agricultural Science
• Environmental Engineering
• Biotechnology

Background:

• Digestate, the residue from anaerobic digestion (AD), is a material with variable biochemical properties.
• Understanding digestate's fertilizing value is crucial for its sustainable management and application.
• Existing regulations for fertilizers present challenges for untreated digestate utilization.

Purpose of the Study:

• To develop a digestate typology based on its fertilizing value.
• To analyze the influence of feedstock and AD processes on digestate composition.
• To assess digestate compliance with current and proposed fertilizer regulations.

Main Methods:

• Compilation of a comprehensive digestate database from literature and internal sources.
• Statistical analysis of key fertilizing parameters: dry matter, volatile solids, C/N, C/Organic-N, total nitrogen (TN), total ammoniacal nitrogen (TAN), TAN/TN, total phosphorus (TP), and total potassium (TK).
• Analysis of 91 raw digestates, 34 solid fractions, and 25 liquid fractions.

Main Results:

• A digestate typology was successfully established, demonstrating a clear link between feedstock, AD process, and fertilizing value.
• No raw digestate samples met French or proposed EU fertilizer standards without post-treatment.
• The typology provides insights into potential post-treatment strategies to meet regulatory product categories.

Conclusions:

• The developed digestate typology is a novel tool for enhancing digestate management and informing policy-making.
• Digestate's fertilizing potential is highly dependent on its origin and production process.
• Achieving regulatory compliance for digestate as fertilizer requires specific management and potential post-treatment strategies.