A modular membrane supported bioreactor for anaerobic conversion of gas into liquid products including membrane module(s) having a plurality of hollow fibers packed across a cross sectional area of the membrane module in a direction transverse to the axis of the membrane module; a membrane vessel for retaining the membrane module(s) at least partially submerged in a process liquid and isolated from ambient atmosphere; and a gas supply conduit operably connected to the hollow fibers for supplying a process gas to the hollow fiber lumens. The gas supply conduit enables the formation of a biofilm on the outer surface of the hollow fiber wall by interaction of microorganisms with the process gas and the production of a liquid product that mixes with the process liquid.