Membrane processes as the key technology in cascaded valorization of municipal organic waste

Municipal organic waste is frequently recognized to be a valuable resource for sustainable production of material, chemicals and energy carriers apart from disposal or composting. The application of a cascaded valorization process for utilizing this resource in form of a biorefinery concept is considered to be favourable both for ecologic and economic reasons. Therefore, it is suggested to firstly extract/produce high-priced chemicals followed by the production of fuels and energy carriers. Electric power and thermal heat can be generated in a final step. Residuals of this cascade might be converted to fertilizers.

One possible pathway to produce energy carriers from raw or pretreated organic waste presented in this current work is the well-established ABE fermentation (acetone-butanol-ethanol) with a preceding hydrolysis step and subsequent biogas fermentation for the digestate. The hydrolysis step contributes to an improvement of carbohydrates conversion in the ABE fermentation by acidification of the complex substrate to easier digestible monomers like amino acids, fatty acids and monosaccharides. The gas mixture produced during hydrolysis is separated into carbon dioxide and hydrogen by membrane-based gaspermeation technology. Hydrogen from hydrolysis then is fed to both downstream fermentations resulting in a significant improvement of these process steps as it acts as metabolic intermediate and reducing agent. Thus, for ABE fermentation, hydrogen improves the conversion rate of carbon to solvents. In biogas fermentation, hydrogen enhances the ratio of methane to carbon dioxide in the biogas by internal biogas upgrading. Finally, the produced biogas is upgraded to biomethane also applying gaspermeation.

Due to the product toxicity, ABE fermentation is limited to reasonably dilute product concentrations (up to 20 g/l). Therefore, constant in-situ product separation of solvents from the fermentation broth is mandatory to sustain stable operation and high carbon conversion rates. Membrane-based pervaporation showed superior suitability for the effective and energy-efficient recovery of these products.

This work will summarize joint experimental and simulative efforts undertaken in the H2020 project WASTE2FUELS (Grant agreement No 654623) and in the Austrian national project KASAV (No 838708). Results from extensive experimental analysis of pervaporation of synthetic ABE-water-mixtures and real fermentation broth will be presented indicating the influence of operational conditions and fermentation minor components like salts, sugar and acids on separation characteristics. Furthermore, results from the experimental gas-phase separation of mixtures of methane, hydrogen and carbon dioxide will be shown (hydrolysis gas separation and biogas upgrading). As a conclusion, membrane technology will be presented as the major technology for the described concept of cascaded production of liquid and gaseous fuels and energy carriers from organic waste.