Abstract: The disclosure herein describes a bioreactor for the biochemical treatment of liquids containing organic matter. It comprises a container with an inlet for receiving liquid to be treated with at least one horizontal plate to support a bed of microorganism cells which react with the organic matter to form a volatile gas. An aperture is provided in the plate to provide liquid flow communication between upper and lower compartments on either sides of the plate. A blockage element extends over the aperture to prevent the liquid and the microorganism cells from returning through the aperture to the lower compartment. An outlet is provided for releasing the gas from the container while a further outlet is used for releasing treated liquid.