Capture–Ferment–Upgrade: A Three-Step Approach for the Valorization of Sewage Organics as Commodities

This critical review outlines a roadmap for the conversion of chemical oxygen demand (COD) contained in sewage to commodities based on three-steps: capture COD as sludge, ferment it to volatile fatty acids (VFA), and upgrade VFA to products. The article analyzes the state-of-the-art of this three-step approach and discusses the bottlenecks and challenges. The potential of this approach is illustrated for the European Union’s 28 member states (EU-28) through Monte Carlo simulations. High-rate contact stabilization captures the highest amount of COD (66–86 g COD person equivalent –1 day –1 in 60% of the iterations). Combined with thermal hydrolysis, this would lead to a VFA-yield of 23–44 g COD person equivalent –1 day –1 . Upgrading VFA generated by the EU-28 would allow, in 60% of the simulations, for a yearly production of 0.2–2.0 megatonnes of esters, 0.7–1.4 megatonnes of polyhydroxyalkanoates or 0.6–2.2 megatonnes of microbial protein substituting, respectively, 20–273%, 70–140% or 21–72% of their global counterparts (i.e., petrochemical-based esters, bioplastics or fishmeal). From these flows, we conclude that sewage has a strong potential as biorefinery feedstock, although research is needed to enhance capture, fermentation and upgrading efficiencies. These developments need to be supported by economic/environmental analyses and policies that incentivize a more sustainable management of our resources.