Approaching the circular economy: Biological, physicochemical, and electrochemical methods to valorize agro-industrial residues, wastewater, and industrial wastes

Abstract

The circular economy policy enables the use of waste as an input to generate value repeatedly or at least to have the opportunity to be used a second time to extract valuable substances such as metals, dyes, hydrogen, phosphorus, nitrogen, nutrients, and fertilizers. For this reason, the present paper discusses approaches to valorizing wastewater, communal production, and agricultural wastes. In particular, techniques are considered to valorize biomass, lignin-containing residues, wastewater with a high organic content, sewage sludge, oily sludge, mine tailings, fly ashes, and iron production wastes. The procedures evaluated include biological processes, microbial fuel cells, electrokinetics, electro-demulsification, electrodialysis, and the Fenton process; their rational application promotes the reuse of different chemical and biological compounds to eventually supply some economic sectors with raw materials in a sustainable manner, solve the problem of pollution in the environment, and generate power at the same time. Of particular and emerging interest is the valorization of iron wastes, which serves to obtain low-cost catalysts for wastewater treatment processes. Additionally, the correct characterization of the chemical composition of wastes and determination of their final use allows better results and less time to valorize any wastes considered.