Anaerobic digestion in a biorefinery context

The need to move from a linear economy to a circular economy is favoring the adoption of a biorefinery production model, where fossil-based resources are replaced by green feedstocks and clean energy to synthesize fuels and products having similar performances than the conventional ones. In the last decades anaerobic digestion was probably one of the most diffused biotechnology to produce biogas, a gaseous blend composed mainly methane and carbon dioxide. Anaerobic digestion is a complex biological process which is affected by several operational parameters, as the temperature, the pH, the Hydraulic Retention Time, the Organic Load Rate and the inoculum choice. These parameters are able to affect not only the biogas yields but also the nature of the products. Moreover, the anaerobic digestion process can be performances in one or two stages making possible the production of methane in the mono stage mode, and of hydrogen, Volatile Fatty Acids (VFAs) and methane in the two-stage configuration. The VFAs have a higher economic value of methane and are the considered as the biological precursors to produce different high added value compounds, such as the polyhydroxyalkanoates and the single cell proteins. Finally, the main by-products from the anaerobic digestion, the digestate is very interesting as it is rich in nitrogen, phosphorous and potassium compounds which are considering the precursors of the fertilizers. The wide diffusion of the anaerobic digestion and its versatility in the production of different biofuels and bioproducts can make this technology the core process for the development of the next biorefineries, promoting the transition towards a green and sustainable production model.