NOVELTY - Process for producing a gas comprising hydrogen and a gas comprising methane, comprises: microbially fermenting a substrate in a liquid volume in a first reactor to produce a gas comprising hydrogen and an effluent; and microbially fermenting the effluent of the first reactor in a liquid volume of a second reactor to produce a gas comprising methane.    USE - The process is useful for the production of gas comprising H2 and gas comprising CH4 (claimed), where the H2 and CH4 mixtures are useful in biogas for electricity generation and transportation applications (where air pollution is an issue) and in air pollution reduction.    ADVANTAGE - The process is a two-stage process, harvesting both H2 and CH4 or of H2-CH4 mixtures. The process varies the working volume for the first stage of the reactor, with the second stage at near constant liquid volume, to dynamically accommodate the bacterial population simultaneously to changes in the performance of the bioreactor system, as well as changes in the flows and composition of the incoming feed. The biogas from the second stage is circulated through the first stage to reduce the partial pressure of H2 in the first stage and thus increase the total amount and rate of H2 produced, this results in a maximum amount of mixture of the two fuels.    DETAILED DESCRIPTION - Process for producing a gas comprising hydrogen (H2) and a gas comprising methane (CH4), comprises: microbially fermenting a substrate in a liquid volume in a first reactor to produce a gas comprising H2 and an effluent; and microbially fermenting the effluent of the first reactor in a liquid volume of a second reactor to produce a gas comprising CH4, where the ratio of H2 produced in the first reactor to CH4 produced in the second reactor is adjusted by adjusting the liquid volume of the first reactor, while keeping the liquid volume of the second reactor substantially constant, and at least a portion of the gas produced in the second reactor is recirculated through the first reactor to increase the amount of H2 produced in the first reactor over the amount of H2 produced in the absence of gas recirculation.