NOVELTY - A bioelectrochemical reactor (1) for generation of electricity, comprises an inlet (2), an outlet (3), cathodic zone (5) and anode (4). An interior of reactor defines a flow path for liquid comprising a first region that is an acidogenic region in which microbes generate more carbon dioxide than a second region. The reactor includes a gas retaining volume extending from first region to second region and retaining a gas so that carbon dioxide generated at first region is transported to second region, where it suppresses methane generation by microbes at second region.    USE - For the generation of electricity from oxidation by microbes of chemical species in a liquid.    ADVANTAGE - The portion of the flow path which is closest to the inlet forms an acidogenic zone and microbes in acidogenic zone generate significant amounts of carbon dioxide which diffuse through the gas retaining volume above the liquid and dissolves in the liquid phase further downstream in the flow path. This has the effect of reducing the pH at other locations in the reactor, suppressing methanogenic processes and transforming chemical species within the liquid, which function as fuel, into electricity more effectively. The bioelectrochemical reactor provides maximal values of open circuit voltage and closed circuit current, which is approximately 3 times than the current achieved by the prior art upflow microbial fuel cell.    DETAILED DESCRIPTION - A bioelectrochemical reactor (1) for generation of electricity from oxidation by microbes of chemical species in a liquid, comprises an inlet (2), an outlet (3), at least one cathodic zone (5), and at least one anode (4). An interior of the reactor is configured to define a flow path for liquid from the inlet to the outlet which passes between the cathodic zone and the anode. The flow path comprises a first region and a second region, where the first region is an acidogenic region in which microbes generate more carbon dioxide than the second region. The reactor includes a gas retaining volume extending from the first region to the second region and retaining a gas so that carbon dioxide generated at the first region is transported to the second region in the gas phase, where it suppresses methane generation by microbes at the second region. The anode and the cathodic zone comprise opposed parallel planar surfaces, and the interior of the reactor is configured such that liquid flows in a generally horizontal orientation between the opposed parallel planar surfaces of the anode and the cathodic zone. The anode and the cathodic zone comprise opposed parallel surfaces, and cross-sectional area of the flow path between the external face of at least one cathodic zone and at least one external face of a corresponding anode is less that the cross-sectional area of the flow path at at least one other location within the flow path. The reactor uses organics of waste water as a fuel for the generation of electricity.    DESCRIPTION OF DRAWING(S) - The figure shows a plan view of the components of the bioelectrochemical reactor, in which the path of the liquid flow is illustrated by arrows.    Bioelectrochemical reactor (1)    Inlet (2)    Outlet (3)    Anode (4)    Cathodic zone. (5)