Boosting biogas production in the wastewater treatment plants: A narrative review on co-digestion of sewage sludge with internal and external organic waste

The most common technology to achieve the energetically self-sustainable operation of wastewater treatment plants (WWTPs) is the coupling of anaerobic digesters with sewage treatment facilities for the disposal of sewage sludge (SS) and biogas production. However, biogas production in these WWTPs is currently limited. This paper reviews the key challenges affecting the anaerobic co-digestion of internal organic waste collected in WWTPs and external wastes, e.g., food waste and agro-industrial organic wastes. This work presents successful case studies that can overcome barriers associated with the operation of biogas plants, e.g., alkaline pre-treatment of fats, the application of low-cost waste-based enzymes, and co-digestion with organic fraction of municipal solid waste. In addition, the co-digestion of clean substrates with SS in temperature-phased digesters to produce safe fertilizer is reviewed. This review also evaluates the economic and environmental aspects of co-digestion. Finally, future investigations are proposed for the co-digestion of SS with sewage in tropical areas. [Display omitted] •Intensifying biogas production could overcome WWTPs' low energy-neutrality.•Foodwaste, manure, and grease trap sludge have more methane potential to energy generation.•Production of clean fertilizer and high-quality biogas can contribute to the Circular Economy.•CH4 leaks along with the waste and fertilizer transportation have a high carbon footprint.•Co-digestion of sludge with sewage by UASB presented an economic alternative in tropical regions.