Opportunities and challenges in the anaerobic co-digestion of municipal sewage sludge and fruit and vegetable wastes: A review

To overcome the anthropogenic impacts resulting from the constant growth of the world population, industries like wastewater treatment plants and food processing must be adapted to follow this tendency and suppress the associated challenges and needs arising from this global situation. In the context of the circular economy, optimising the biogas produced in the anaerobic digesters during sludge stabilisation is fundamental to ensure the energy self-sufficiency of wastewater treatment plants. The anaerobic co-digestion of two or more substrates is a viable option to overcome the drawbacks of mono-digestion and to improve methane production. Considering the economic viability of the anaerobic digestion system, it is of major importance to address the potential of the spare capacity existing in the anaerobic reactors by incorporating readily biodegradable wastes.This review compiles the works published between 2015 and 2023 regarding the anaerobic co-digestion of municipal sewage sludge with fruit and vegetable wastes, presenting a critical analysis of the gaps and future perspectives on this topic. The results demonstrated the effectiveness of co-digestion to correct imbalances from mono-digestion, and in some works, mono-digestion of fruit and vegetable wastes obtained higher biogas yields than the mono-digestion of municipal sewage sludge. For instance, the increment in biogas production varied from around 4 up to 500% when comparing mono-digestion with co-digestion trials. In two studies, the biogas production from fruit and vegetable wastes was 13 and 94% higher than in the trials with only municipal sewage sludge. Multi-stage systems enhanced anaerobic digestion due to the improvement in the early stages of the process, namely the hydrolysis which is usually the limiting phase. A significant number of the experimental trials (9 out of 20 studies) were performed in batch reactors due to easy operation and maintenance, revealing the gap of studies with other types of reactors like the continuous stirred tank reactor, which is the most used in wastewater treatment plants. The reduced number of articles within this scope (20) demonstrates that there is still potential to improve the knowledge and fill in the existing gaps, aiming to increase the energetic self-sufficiency of wastewater treatment plants.