Biorefinery of Sewage Sludge: Overview of Possible Value-Added Products and Applicable Process Technologies

The disposal of excess biological sewage sludge from wastewater treatment is a growing environmental issue due to the vast quantities generated worldwide. For many years, sludge disposal has been considered an inevitable "nuisance" linked to the main task of wastewater treatment; recently, the issues relating to the presence of emerging pollutants in sludge have contributed to severe limitations on traditional disposal routes and increased process costs. As a consequence of the water industry's approach to sustainability and circular economy principles, the investigation into options for sludge-embedded resource recovery has been increasing over time. Among these, the definition of biorefinery encompasses many processes used to process sewage sludge for biofuel and/or resource recovery. In addition to the "conventional" recovery of biogas and nutrients, biopolymers (PHAs and EPS), proteins, enzymes, biopesticides, and biofuels are among the "new" sludge extraction products that are being studied. Not all of the proposed technologies are, however, mature for industrial exploitation as yet. This paper presents an overview of possible product recovery from sludge biorefinery, with emphasis on recent developments, and on the benefits and limitations of applicable technologies for ensuring sustainability and environmental efficiency, through an analysis of the current literature.