Semi-dry biofilm reactors for efficient gas phase bioprocessing applications

Abstract

Traditional liquid-phase bioprocessing systems face significant challenges in achieving efficient degradation of airborne volatile organic compounds (VOCs) and maximizing biofuel production, largely due to complex mass transfer resistances and kinetic limitations inherent in gas-liquid-solid reaction environments. These barriers result in reduced efficiency in VOC removal and restrict the potential productivity of biofuel fermentation processes. To address these challenges, this research explores innovative gas-phase bioprocessing strategies using biofilm reactors, aiming to enhance the efficiency of bioprocessing and overcome the limitations associated with liquid media. The initial part of the study focuses on VOC biofiltration and biodegradation, processes typically constrained by factors such as VOC aqueous solubility, mass transfer resistance across multiple layers, and cellular metabolic kinetics. Recent advancements in this field are analyzed to introduce a simplified evaluation framework that leverages inherent mass transfer and kinetic parameters to establish a universal space-time productivity (effectiveness) index. This index provides a standardized metric for comparing process efficiencies across various reaction systems, allowing for an objective assessment of different engineering designs. Such a standardized evaluation not only reveals the limitations of current technologies but also offers guidance for future research efforts to address critical engineering constraints, thus advancing the field of VOC biofiltration. Building on this analysis, the research presents an innovative “dry” biofilm reactor system to enhance VOC degradation efficiency by directly exposing Pseudomonas putida F1 biofilms, supported on carbonized cellulosic fibers, to gaseous VOC substrates. By eliminating the need for bulk aqueous-phase media, this approach removes aqueous-phase mass transfer resistance, facilitating more efficient VOC capture and degradation. Using toluene as a model VOC, the biofilm system achieved a specific growth rate of 0.425 day⁻¹ under optimal conditions (300 p.p.m. toluene and 80% relative humidity). Long-term degradation tests conducted in a tubular packed bed reactor demonstrated a toluene degradation rate of 2.5 mg gDCW⁻¹ h⁻¹ during the initial growth phase. Importantly, the biofilm retained its biodegradation activity in the stationary phase, achieving a toluene degradation rate of 1.9 mg gDCW⁻¹ h⁻¹, with simultaneous CO₂ release at 6.4 mg gDCW⁻¹ h⁻¹, indicating complete carbon conversion of the substrate. Operated without any bulk liquid medium phase, the biofilm achieved direct degradation of gas‐phase VOC at rates of about one order of magnitude higher than what has been previously reported for liquid culturing or immobilized cells. These findings demonstrate the potential of dry biofilm reactors to efficiently degrade VOCs without requiring large amounts of water, representing a significant advancement in sustainable air pollution control. The second part of this study addresses the productivity limitations in biofuel fermentation, particularly in ethanol production. Traditional liquid-phase fermentation has reached its productivity ceiling, prompting a shift to alternative methods. In this research, a hollow fiber membrane (HFM)-supported biofilm reactor was developed to enable ethanol biosynthesis under gas-phase conditions. This design minimizes reaction volume and allows in situ ethanol recovery, significantly enhancing productivity. Under optimal operating conditions, the gas-phase reactor achieved a volumetric ethanol productivity of 37.5 g LBiomass⁻¹ h⁻¹ during the microbial growth phase and maintained productivity at 19.1 g LBiomass⁻¹ h⁻¹ during the stationary phase, with specific productivity rates of 0.58 g gDCW⁻¹ h⁻¹ and 0.047 g gDCW⁻¹ h⁻¹, respectively. Long-term operations demonstrated stable ethanol production, underscoring the potential of gas-phase bioprocessing to surpass the productivity limitations of liquid-phase systems. This liquid-free approach presents substantial potential for enhancing the efficiency and scalability of biofuel production systems. In summary, gas-phase bioprocessing using biofilm reactors offers a transformative solution for both environmental remediation and biofuel production. By eliminating mass transfer limitations inherent in liquid-phase systems, the proposed gas-phase strategies provide an efficient and sustainable pathway for VOC degradation and biofuel synthesis, opening new avenues for advancing bioprocessing technology.