Enhancing methane yield and microbial resilience in olive pomace anaerobic digestion via co-digestion with pig manure

Abstract

Background: Intensive agricultural practices are increasing the generation of by-products and wastes, which require appropriate management strategies to prevent environmental pollution and recover valuable resources. Waste-to-energy technologies, such as anaerobic digestion, are gaining attention for integrating local feedstocks to produce biofuels and biofertilisers, contributing to closed nutrient cycles. In the Mediterranean region, olive pomace is very abundant, but its intrinsic characteristics hinder the production of biogas via anaerobic digestion. Concurrently, the direct application of untreated pig manure on agricultural land continues to pose significant environmental risks.

Results: This study assessed the biomethane potential of olive pomace and pig manure, along with microbial population dynamics during the transition from mono- to co-digestion. Mono-digestion of olive pomace led to complete process inhibition, while co-digestion with pig manure increased methane yield more than fivefold (from 53 to 283 mL CH₄ g^-1 VS). Co-digestion also enhanced the microbial diversity, improving the ecosystem resilience and metabolic versatility. A notable increase in the relative abundance of methanogenic archaea, particularly Methanosarcina, was observed. An energy assessment indicated that a full-scale plant co-digesting olive pomace and pig manure could not only operate without external energy consumption, but also produce excess electricity (577 MWh y^-1) and heat (1074 MWh y^-1).

Conclusions: These findings demonstrate that co-digestion can overcome the limitations of olive pomace mono-digestion, enabling the effective treatment of two challenging agro-industrial by-products. This approach aligns with circular bioeconomy principles and supports the decarbonisation of the olive oil and pig farming sectors, contributing to the energy transition. The presented approach serves as a baseline scenario, and further research should focus on recovering high-value bioproducts and advancing towards integrated biorefinery systems in rural areas.

Keywords: Agro-industrial waste; Bioenergy; Biogas; Circular bioeconomy; Energy transition; Olive oil; Renewable energy.