Bio-based textile dyes and biogas recovery from olive pomace in a biorefinery approach

To reduce its environmental footprint, the textile industry is turning to sustainable materials such as natural dyes, which can be obtained from agro-industrial waste like olive pomace. This underutilised by-product of the olive oil industry holds untapped potential for bio-based products recovery. This study investigates an innovative biorefinery approach to recover natural textile dyes and biogas from olive pomace, promoting a circular bioeconomy. Dyes were extracted using a thermal method and tested on natural fabrics (wool). Reproducible, intense, brownish colours were achieved, with very high levelness and laundering fastness (level 5). Next, dye-extracted olive pomace was co-digested with pig slurry to recover biogas along with digestate for potential use as organic fertiliser. Batch tests evidenced that this fraction retained 88% of the methane potential of the raw pomace (256 vs. 290 mL CH 4 ·g -1 VS). However, semi-continuous experiments evidenced that the mono-digestion of olive pomace was unfeasible, while the co-digestion with pig slurry was stable in the long-term, with an average methane yield of 157 mL CH 4 ·g -1 VS. Quantification of mcrA and 16S rRNA genes revealed that this could be associated with a stable abundance of Bacteria and methanogenic Archaea . Conversely, mono-digestion led to a progressive decline in microbial populations, with methanogenic Archaea more sensitive to this effect, ultimately causing process failure. The proposed biorefinery would produce 140 kg of dyed fabric and 2 GJ per tonne of olives, highlighting the potential for resource recovery and zero-waste generation in olive mills, promoting the sustainability of the olive oil and textile sectors. • Natural dyes for textile applications were obtained from olive pomace. • Dyes showed high levelness and laundering resistance (level 5). • Dye-extracted pomace retained high biomethane potential (256 mL CH 4 ·g -1 VS). • Co-digestion with pig slurry ensured long-term stability and microbial resilience. • The biorefinery showed a positive energy balance (> 2 GJ per tonne of olives).