Circular valorization of nopal (Opuntia spp.) waste through anaerobic digestion: biofertilizer quality, agronomic response and carbon footprint

Opuntia spp. is a strategic crop for arid and semi-arid regions due to its high drought tolerance and widespread cultivation; however, the management of postharvest and pruning residues remains a challenge for sustainable production systems. This study aimed to evaluate the circular valorization of nopal (Opuntia spp.) waste through semi-continuous anaerobic digestion, with a primary focus on digestate quality, agronomic relevance, and environmental implications rather than on energy maximization. Reactor operation exhibited a clearly defined pseudo-steady-state (PSS) period characterized by stable methane production (1.85–2.35 L CH? d?¹), methane concentration (65-67%), and reactor pH (?7.6), enabling reliable digestate characterization. Digestate produced during the PSS met physicochemical, maturity, and microbiological criteria established by Mexican regulatory standards, supporting its suitability for agricultural reuse. Agronomic validation showed that partial substitution of synthetic fertilizers with digestate improved crop performance when applied at optimized rates; in hydroponic lettuce, a 50% substitution level increased fresh biomass by 15% compared to the control, whereas higher substitution rates reduced growth. A cradle-to-gate carbon footprint assessment indicated a net reduction in greenhouse gas emissions, primarily driven by avoided synthetic fertilizer production. Overall, this integrated evaluation provides a proof-of-concept for the circular valorization of Opuntia-derived residues through anaerobic digestion, supporting sustainable nutrient management strategies for CAM crops and agricultural systems in arid and semi-arid regions.