Fruit-processing industries are concentrated mainly in southern Europe, and most of them are family businesses or SMEs. They generate vast amounts of fruit residues. Citrus processing, in particular, generates mountains of orange peel which make up some 45-65 % of the original citrus fruit weight. These residues can be processed anaerobically (as a co-substrate) to generate electricity in biogas conversion plants.

The most common strategy for orange juice companies is to create synergies or reach agreements with local waste managers, other orange juice companies, farms or urban facilities with anaerobic digesters (AD) or biogas plants capable of treating citrus residues in co-digestion with the bio-waste they usually treat (sewage sludge, manure, other substrates). Innovations in bio-waste treatment offer promising improvements to this established practice. 

Silvestre et al. (2010) studied sewage sludge anaerobic digestion with orange peel (12 % of volatile solids input) and other organic wastes in a semi-continuous anaerobic system. The biogas production from sewage sludge and orange residues increased 286 % compared with biogas production using only sewage sludge. Additionally, the organic matter removal efficiencies increased from 50 % (sewage sludge anaerobic digestion) to 68 % (sewage sludge co-digestion).

Despite the fact that orange peels provide significant biomethane potential, low pH, low micro-nutrient content and high content of essential oils (if not previously extracted), they cannot be used as the only feedstock (mono-substrate) in an AD plant. The recommended amount is 10-12 % of citrus residue in a plant.