Agri-food waste valorization through anaerobic co-digestion: fish and strawberry residues

• Strawberry and fish wastes were subjected to anaerobic co-digestion.
• The mixture was highly biodegradable (83% in total volatile solids).
• The organic loading rate was optimal at 2.73 kg total volatile solids/(m3 d).
• The methane production rate at the highest organic loading rate was 13.7 mL/h.
• Chloride from fish waste was the main factor that inhibited the process at high loads.

The generation of fish waste is an environmental problem mainly associated to the canning industry in many Mediterranean countries. The application of anaerobic digestion is not advisable, however, due to the organic matter deficit in the chemical oxygen demand:nitrogen:phosphorus ratio (COD:N:P) of this waste, which entails the destabilization of the process. The co-digestion of fish waste with residual strawberry extrudate was evaluated at laboratory scale under mesophilic conditions. Strawberry waste increased the organic matter concentration in the mixture and diluted the inhibitory compounds contained in the fish waste, such as chloride, nitrogen and phosphorus. Co-digestion improved the stability of the treatment, while biodegradability was found to be 83% in total volatile solids. Moreover, the methane production yield reached a mean value of 120 mL/g total volatile solids (at 1 atm, 0 °C) for an organic loading rate in the range of 22.8–50.6 kg waste mixture/(m3 d), while the digestate was rich in nutrients, which might enable it to be used as an organic amendment in agriculture.