Continuous polyhydroxybutyrate production from biogas in an innovative two‐stage bioreactor configuration

Abstract

Biogas biorefineries have opened up new horizons beyond heat and electricityproduction in the anaerobic digestion sector. Added‐value products such aspolyhydroxyalkanoates (PHAs), which are environmentally benign and potentialcandidates to replace conventional plastics, can be generated from biogas. Thiswork investigated the potential of an innovative two‐stage growth‐accumulationsystem for the continuous production of biogas‐based polyhydroxybutyrate (PHB)usingMethylocystis hirsutaCSC1 as cell factory. The system comprised twoturbulent bioreactors in series to enhance methane and oxygen mass transfer: acontinuous stirred tank reactor (CSTR) and a bubble column bioreactor (BCB) withinternal gas recirculation. The CSTR was devoted to methanotrophic growth undernitrogen balanced growth conditions and the BCB targeted PHB productionunder nitrogen limiting conditions. Two different operational approaches underdifferent nitrogen loading rates and dilution rates were investigated. A balancednitrogen loading rate along with a dilution rate (D) of 0.3 day−1resulted in the moststable operating conditions and a PHB productivity of ~53 g PHB m−3day−1.However, higher PHB productivities (~127 g PHB m−3day−1)wereachievedusingnitrogen excess at a D = 0.2 day−1. Overall, the high PHB contents (up to 48% w/w)obtained in the CSTR under theoretically nutrient balanced conditions and the poorprocess stability challenged the hypothetical advantages conferred by multistagevs single‐stage process configurations for long‐term PHB production.