Production of biopolymer using a membrane-based bioreactor
Methane is a potent greenhouse gas produced from natural gas reservoirs, landfills, and anaerobic digesters. Some methanotrophic bacteria produce intracellular polyhydroxybutyrate (PHB) in the absence of nitrogen and presence of methane and oxygen. In this study, Colorado School of Mines (CSM) and Mango Materials (MM) collaborated on a Phase I NASA grant, investigating the production of PHB in a novel membrane-based bioreactor. Direct gas sparging is MM’s current method of gas delivery, but one of NASA’s potential applications for this system would be in microgravity where gas bubbles will not rise, making mass transport and exchange/resupply of gas almost impossible. Membrane contactors may be a management solution for liquid and gas separation and gas exchange. In this study we designed, built, and operated a bioreactor that uses hydrophobic microporous membranes for gas exchange. The membranes’ oxygen gas flux and mass transfer coefficient (KLa) were characterized and compared using liquid flows of 2 liters per minute (LPM) and oxygen gas flows of 0.5 LPM. The highest observed oxygen flux during this test was 340 mg/(min*m2) for the flat sheet CLARCOR (QP952) membrane, and the highest rate of transfer, KLa, was 23.88 hr–1 for the Liqui-Cel 2.5 x 8.0 hollow fiber module (X50 fibers, 1.4 m2 membrane area). During tests and growth trials, water vapor diffused through the membrane pores into the gas channels and condensed, causing membrane wetting that impaired gas transfer by plugging pore spaces, which was mitigated by drying the membranes and membrane modules of any collected water. Bacteria slightly fouled the membranes during growth trials and reduced gas transfer. However, wetting impacted membrane gas transfer more than fouling during this study. Bench-scale growth trials were inoculated with a consortium of type II methanotrophic bacteria dominated by the Methylocystis genus. Successful growth trials used parallel membrane module configuration. Growth trials with the flat sheet membrane QP952 produced bacteria with 22% PHB per dry cell mass. Growth trials using hollow fiber modules from Minntec (FiberFlo MCV-030) produced bacteria with 46.9% PHB per dry cell mass. These results confirmed that membranes are capable of providing sufficient gas transfer for growth and production of PHB by methanotrophic bacteria.
membrane contactor, methanotrophs, polyhydroxybutyrate, methane, biopolymer, oxygen flux
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Colorado School of Mines. Arthur Lakes Library