TECHNOLOGY FOCUS - BIOLOGY - Preferred Biofilm: The biofilm contains microorganisms chosen from Clostridium ragsdalei, Butyribacterium methylotrophicum, Clostridium ljungdahlii, Clostridium carboxidivorans and their combinations.    TECHNOLOGY FOCUS - INSTRUMENTATION AND TESTING - Preferred Bioreactor: The gas supply conduit is operably connected to the hollow fibers to supply the processed gas containing carbon monoxide or a mixture of carbon dioxide and hydrogen, and the membrane vessel retains the membrane module in the liquid. The membrane module is a one-headed membrane module comprising hollow fibers having free ends and having a gas inlet chamber to operably connect the gas supply conduit with the hollow fiber lumens. The membrane vessel is arranged to maintain a liquid surface. The free ends of the hollow fibers are closed-ended and the membrane vessel is arranged to maintain the fibers completely submerged in the liquid. The bioreactor further comprises an overflow conduit located to maintain the liquid with a liquid surface at a predetermined level and operable to remove exhaust gas and the liquid from the membrane vessel, and a gas/liquid separation tank operably connected to the overflow conduit to separate the exhaust gas from the liquid. The bioreactor further comprises a pressure control valve operably connected to the gas/liquid separation tank to control pressure in the membrane vessel. The bioreactor further comprises a product recovery system operably connected to receive the liquid from the gas/liquid separation tank, to separate the liquid product from the liquid, and to return liquid to the membrane vessel. The membrane vessel is adapted for complete filling by the liquid. The bioreactor further comprises a liquid outlet in communication with the membrane vessel to withdraw the liquid, a pressure reducing valve operably connected to the liquid outlet to receive the process liquid and to reduce the pressure of the liquid to diffuse dissolved gas from the liquid, and a gas/liquid separation tank operably connected to the pressure reducing valve to separate the diffused processed gas from the liquid. The bioreactor further comprises a product recovery system operably connected to receive the liquid from the gas/liquid separation tank, to separate the liquid product from the liquid, and to return liquid to the membrane vessel. The membrane module is two-headed membrane module having a first potted end spaced apart from a second potted end, the first potted end is operably connected to one end of the hollow fibers and the second potted end is operably connected to the other end of the hollow fibers to allow the process gas to flow through the hollow fiber lumens from the first potted end to the second potted end. The hollow fibers have a length equal to 1.015-1.15 times the distance between the first potted end and the second potted end to produce slack in the fibers. The bioreactor further comprises a liquid outlet operably connected to the membrane vessel to receive the liquid containing the liquid product, and a product recovery system operably connected to receive the liquid from the liquid outlet, to separate the liquid product from the process liquid, and to return the process liquid to the membrane vessel. The bioreactor contains several axially stacked membrane modules, liquid conduit communicating the liquid from one membrane module to an adjacent membrane module, where the membrane modules further comprise a gas inlet chamber operably connected to the first potted end and a gas outlet chamber operably connected to the second potted end, and the gas outlet chamber of membrane module communicates with the gas inlet chamber of an adjacent membrane module. The bioreactor further comprises a tube extending between the first potted end and the second potted end, the tube having a first perforated section proximate the first potted end to disperse the liquid, a second perforated section proximate the second potted end to collect the liquid, and a blocked section between the first perforated section and the second perforated section to prevent fluid flow through the tube. The membrane vessel is adapted to contain a pressure in the range of 15-100 psig.