A critical review on different strategies to enhance mass transfer in methanotrophic bioreactors

Methanotrophs are bacteria with the ability to directly utilize methane as a substrate to produce different useful biomaterials (e.g., lipids, proteins, bioplastics, methanol). However, due to solubility and mass transfer limitations of methane, the methanotroph biotechnology remains almost entirely in laboratory/research scales. To address this, studies in recent years have focused on increasing the methane volumetric mass transfer coefficient (k_La) through modifications of bioreactor design and operations. This review critically analyzed these advances, aiming to identify the most suitable strategy for methane bioconversion. The modifications were categorized, i.e., mechanical agitation, feed gas introduction system, and cultivation medium modification [including addition of a non-aqueous phase (NAP)], and their effectiveness was related to k_La and/or productivity metrics (PM) (i.e., yield, elimination capacity, and remediation efficiency). Studies that determined both k_La and PM were assessed using a new metric (∆PM/∆k_La, with ∆ as the relative difference/change between implemented strategy and control). This metric, which was independently determined for each study, allowed cross-study comparison of the effectiveness of the different modifications/strategies. A positively high ∆PM/∆k_La indicates greater emphasis on PM improvement (or biological performance) over k_La enhancement (or physical attributes) of the bioreactor system. While the majority of strategies resulted in a positive ∆PM/∆k_La, two-phase partitioning bioreactors (TPPBs) exhibited the highest potential (i.e., highest ∆PM/∆k_La). The effectiveness of this strategy relies on the homogeneous dispersion of the added NAP. Although TPPB operation is more complex than conventional bioreactors, this review suggests that future research should consider TPPB strategies to address its complexity and economic viability.