A Conceptual Model for the Recovery of Volatile Fatty Acids from Anaerobically Digested Sludge Liquors

Volatile fatty acids (VFAs) are valuable intermediates with wide-ranging applications in the production of bioplastics, bioenergy, and chemicals, offering a promising route for resource recovery from wastewater treatment processes. This study proposes a conceptual model for the selective recovery of VFAs from anaerobically digested sludge liquors, aiming to transform wastewater treatment plants into biorefineries. Anaerobic digestion (AD) of sludge generates a complex mixture of organic acids, primarily acetic, propionic, and butyric acids, along with residual nutrients and solids. The proposed model integrates key components such as phase separation, in-line monitoring, and selective extraction techniques to maximize VFA yield while maintaining system stability. The conceptual framework is designed around four main stages: (1) acidogenic fermentation optimization, (2) solid-liquid separation to concentrate VFAs, (3) selective extraction using membrane contactors or ion exchange resins, and (4) downstream valorization through microbial conversion or chemical upgrading. Emphasis is placed on manipulating operational parameters such as pH, retention time, temperature, and carbon-to-nitrogen (C/N) ratio to favor acidogenesis over methanogenesis. Additionally, the model explores the role of microbial community engineering and inhibitors to suppress methane formation and redirect metabolic pathways toward VFA accumulation. Energy and material balances are evaluated to ensure economic feasibility and sustainability. Advanced separation technologies such as forward osmosis, electrodialysis, and solvent extraction are incorporated into the model based on their selectivity and compatibility with sludge matrices. The potential integration with existing treatment infrastructure is also considered to reduce capital expenditure and operational complexity. This conceptual model not only enhances the circular economy potential of wastewater treatment plants but also aligns with climate action goals by minimizing greenhouse gas emissions and recovering valuable carbon streams. Future research directions include pilot-scale validation, techno-economic assessments, and lifecycle analysis to refine and implement the model in diverse treatment scenarios.