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

Abstract

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 day in 60% of the iterations). Combined with thermal hydrolysis, this would lead to a VFA-yield of 23-44 g COD person equivalent day. 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.

Control tools to selectively produce purple bacteria for microbial protein in raceway reactors

“…An adapted VFA-based medium as a proxy for fermented wastewater was used for all experiments, as we argued in a previous study that fermentation prior to protein production will favor the microbial selectivity. 4,13 The COD concentration was increased to 3 g L -1 and contained a defined mixture of acetate, propionate and butyrate on a 1/1/1 COD basis. PNSB grown photoheterotrophically on this VFA mixture have a biomass yield that approximate 1 g COD biomass g -1 COD removed .…”

“…3 Mitigation can be brought about by upgrading wastewater resources to microbial protein or single-cell protein, which is the use of microorganisms as animal feed ingredient. 4,5 Resource recovery from food and beverage wastewater is preferred. Brewery wastewater is a key target as it offers a relatively high chemical oxygen demand (COD) concentration (800-9000 mg COD L -1 ) and easiness to prevent fecal contamination.…”

“…Upgrading wastewater resources to microbial protein requires either chemo- or photoheterotrophic microorganisms to convert the organic carbon as well as non-axenic production conditions, as it is cost-wise redundant to sterilize vast amounts of water. 4 Chemoheterotrophs, also known as aerobic heterotrophic bacteria (AHB; i.e. aerobic activated sludge), make use of oxidation reactions for energy generation.…”

Purple bacteria as added-value protein ingredient in shrimp feed:Litopenaeus vannameigrowth performance, and tolerance againstVibrioand ammonia stress

“…Purple non-sulfur bacteria (PNSB) are promising microorganisms that can be used as source of microbial protein (Alloul et al, 2018; Alloul et al, 2019). There biomass shows high potential as nutritious feed as it is rich in essential vitamins and carotenoid pigments (Sasaki et al, 1998).…”

Rhodopseudomonas palustris-based conversion of organic acids to hydrogen using plasmonic nanoparticles and near-infrared light

“…It is well reported in the literature that volatile fatty acids (VFAs) such as acetic acid can be easily produced by the anaerobic digestion of municipal sludge, food waste, etc. [6][7][8][9] These can be converted to molecular hydrogen and carbon dioxide by photofermentation, a biological process in which photosynthetic bacteria metabolize organic acids. Photofermentation is typical for photosynthetic bacteria such as purple non-sulfur bacteria (PNSB), which produce nitrogenase, the key enzyme for hydrogen generation, under nitrogen-limited conditions.…”