NOVELTY - A modular membrane bioreactor comprises membrane modules having several hollow fibers, each of hollow fibers include a gas permeable hollow fiber wall defining a hollow fiber lumen and an outer surface, a membrane vessel for retaining the membrane modules, and a gas supply conduit operably connected to the hollow fibers for supplying a gas to the hollow fiber lumens that enables the formation of biofilm on the outer surface of the hollow fiber wall by interaction of microorganisms with the gas and the production of liquid product.    USE - The modular membrane bioreactor is useful for converting gas, preferably syngas enriched with carbon monoxide or a mixture of carbon dioxide and hydrogen into liquid product e.g. biofuel such as ethanol, n-butanol, hexanol, acetic acid, butyric acid and their combinations under anaerobic condition (all claimed).    ADVANTAGE - The conversion of gas into liquid under anaerobic condition can be performed effectively and economically, in large amount. The method is eco-friendly.    DETAILED DESCRIPTION - A modular membrane bioreactor comprises membrane modules having several hollow fibers packed across a cross sectional area of the membrane module in a direction transverse to the axis of the membrane module, each of several hollow fibers includes a gas permeable hollow fiber wall defining a hollow fiber lumen and an outer surface, a membrane vessel for retaining the membrane modules at least partially submerged in a liquid and isolated from ambient atmosphere, and a gas supply conduit operably connected to the hollow fibers for supplying a gas to the hollow fiber lumens that enables the formation of biofilm on the outer surface of the hollow fiber wall by interaction of microorganisms with the gas and the production of liquid product. An INDEPENDENT CLAIM is included for bioreaction method which involves retaining a liquid in a membrane vessel under anaerobic condition, maintaining a membrane module at least partially submerged in the liquid, growing a biofilm on the outer surface of the hollow fibers, and passing a gas into the hollow fiber lumens through the hollow fiber wall to interact with the biofilm and generating a liquid product.