Sewage sludge: a source of renewable energy and resources for circular bioeconomy

A tremendous amount of wastewater is treated at different municipal and industrial plants. Traditionally, wastewater treatment plants (WWTPs) were designed for water purification and pollutant removal; however, this generic process generates huge amounts of waste in the form of sewage sludge, which is indiscriminately disposed of, buried in a landfill, incinerated, or used for agricultural purposes. This process has led to significant environmental effects like the release of greenhouse gases and various contamination of the ecosystem and public health. Stringent environmental regulations and public outcry have led to a paradigm shift in waste management systems, and have given way to the use, reuse, and recovery of energy and resources from sewage sludge for environmental sustainability, socioeconomic improvement, public acceptability, and circular bioeconomy. This review elucidates the types of energy and resources that can be recovered from sewage sludge. The outcome of this review revealed that conventional methods for disposing of sewage sludge are unsustainable and contribute significantly to the redistribution of contaminants back into the ecosystem. Hence, we have proposed sustainable technologies, both existing and nascent, that are more environmentally friendly, with the circular bioeconomy in mind. This will serve to raise awareness about these technologies, their pros and cons, and the importance of environmental sustainability.