A closed-loop valorization of the waste biomass through two-stage anaerobic digestion and digestate exploitation

This review investigates the two-stage anaerobic digestion (TSAD) of organic waste to produce high-value products according to circular economy principles. The novelty of the study is the coupling of energy-carrying production including H 2 and CH 4 , with the digestate treatments through closed-loop valorization. The review's key findings highlight that in TSAD, the energy and digestate qualities can be improved through pre-treatments and co-digestion. Pre-treatments allow for the increase of soluble organic matter available for microorganisms. Co-digestion controls the optimal ranges of carbon-nitrogen ratios, nutrient balances, pH, and water contents, by increasing the TSAD efficiency without consuming resources like water or mineral nutrients. Digestate management is investigated for the whole, liquid, and solid fraction of the residue demonstrating a high yield in nutrient recovery and char production. In this study, TSAD is not only considered a biochemical process but a sequential biorefinery that requires optimization to be scaled up at full scale. The bottlenecks of TSAD emerged in the explorative energetic, environmental, and economic assessments, which point out the significant economic and environmental costs associated with transporting, storing, pre-treating biomasses, and dewatering and managing digestate. • Two-stage anaerobic digestion (TSAD) combined energy and digestate productions. • Nutrient balance is fundamental for energy and digestate productions. • Pre-treatments and co-digestion in TSAD improve nutrient balance and efficiency. • Valorization of digestate was considered as whole, liquid, and solid fractions. • Energetic, environmental, and economic aspects boost the optimization of TSAD.