A review on wastewater sludge valorisation and its challenges in the context of circular economy

The use of wastewater sludge as a source for energy and resource recovery is a good alternative for its management considering the legislation requirements and the circular economy principles. Recognizing sludge as a resource, not as a waste, has made researchers consider the recovery of valuable components from sludge, such as carbon and nutrients. The energy that can be obtained from wastewater sludge may be a sustainable solution to fulfill present and future energy requirements. This review discusses about the types of sludge produced by wastewater treatment plants (WWTPs), the technologies that can be implemented in the water and sludge line to reduce the sludge amount, as well as the conventional treatment and disposal methods. Moreover, the technologies that can be used to recover resources and energy in the context of circular economy are also presented, with a focus on Technology Readiness Level (TRL) and a critical overview of positive and negative aspects in implementation. A detailed description of some urban biorefineries aimed at the recovery of cellulose and nutrients and the production of bioplastics is further reported. The study ends with a discussion on the need for an appropriate methodological approach when proposing “end-of-waste” criteria for wastewater sludge derived products.