Scaling up a bioelectrochemical biofactory concept for producing polyhydroxybutyrate (PHB) from purple phototrophic bacteria

Biorefineries offer a sustainable model that supports circular economy and nutrient recovery from waste feedstocks. Biorefineries were centered on microalgae for biomass and biofuel generation, but the concept has shifted toward inclusion of more versatile microorganisms to cope with diversity of waste substrates. Purple phototrophic bacteria (PPB) are particularly interesting, as they can treat wastewater while producing biomass, polyhydroxybutyrate (PHB), and carotenoids. Furthermore, PPB can utilize electrodes as extracellular electron donors, enhancing the synthesis of these products. Additionally, electrochemical moving bed reactors have been shown to improve PHB production by supporting electroactivity in planktonic cells. In this study, a photo microbial electrochemical moving bed reactor (photoME-MBR) was scaled up from 250 mL to 50 L, which constitutes the largest example for a bioelectrochemically-assisted PPB case study. The new configuration was operated under cathodic conditions to assess biomass, PHB, and carotenoid production; brewery wastewater treatment efficiency, and bioelectrochemical performance. Synthesis of value-added products at pilot scale was comparable to laboratory-scale productivity, while achieving organic pollutants removal at a rate of 136 gTOC/m3·d. Cathodic polarization significantly enhanced PHB production (100 mgPHB/gDryBiomass) by promoting extracellular electron uptake from the conductive bed. Microbial community analysis identified Rhodopseudomonas sp. and Bradyrhizobium sp. as dominant genera.