**1. Introduction**

Waste can be defined as any substance or object that has no further use and is intended to be discarded [1]. In this sense, waste production is inevitable in a society based on consumption, making waste management a huge challenge taking into account the enormous quantities of residues that are produced globally. In fact, about 2.6 billion Mg of waste were produced in the European Union (EU) during 2014, from which 41% was discarded in landfills, 36% was recycled, 10% used in earthmoving operations, 7% treated in wastewater treatment plants, and 6% incinerated either for energy production or for destruction. Based on this, in recent decades, humanity is shifting their focus of traditional waste management from the concept of "collection and disposal" in favor of pyramid-based management of the waste hierarchy in order to increase sustainability [2]. However, even when environmentally-friendly practices such as recycling and reuse are accomplished, much of the operations are performed "downcycling," meaning that the recycled product has an economic value below its original purpose. As such, the linear economy model based on the pyramidal hierarchy of wastes that we tend to use nowadays also has limitations. Actually, there are still opportunities for efficiency gains in many industrial processes, but these gains will probably be increasingly marginal and undifferentiated.

The future adoption of the concept of circular economy is, therefore, a necessary change of paradigm, in contrast to the current linear model. This new concept is increasingly viewed as a source of innovation in products, processes, and business models, opening excellent opportunities that should be seen by companies and organizations as competitive advantages in a dynamic and global market [3]. Specifically, with a circular flux in the consumption of resources, every waste generated is a potential raw material for another process, introducing novel ways of valorization and development of second and third generation products. The benefits are clear as few wastes would be generated and disposed of without treatment, potentially reducing environmental pollution.

Updated knowledge of current technologies is a crucial factor in determining the most suitable processes to valorize different types of wastes in future biorefineries. These waste biorefineries are facilities that integrate the necessary technologies in order to convert biomass feedstocks and other wastes into usable products, ensuring that circular economy transitions from theory to the real world. The available waste streams can either be transformed by technologies producing biofuels (waste-to-liquids, WtL) or energy (waste-to-energy, WtE) with both categories expected to be a key element in future waste management. Based on this, in this chapter, we briefly review the current state of main WtL and WtE technologies within a perspective of their use as tools for managing post-process residues and by-products. The review ends with a brief discussion on future developments regarding mentioned technological options.
