NOVELTY - Microbial electrolysis cell (600) comprises: a high surface-area brush anode (602); a cathode (604); a power source operatively connected to brush anode and cathode; an anion exchange membrane (606) separating brush anode from cathode; and a reaction chamber containing the brush anode, the cathode and the anion exchange membrane, where a separation distance between the brush anode and the cathode is not greater than 1.5 cm, and the anion exchange membrane operatively separates the reaction chamber into an anode chamber containing the brush anode and a cathode chamber containing the cathode.    USE - The microbial electrolysis cell is useful in a multi-module microbial electrolysis cell treatment apparatus for producing hydrogen from a fluid stream (all claimed), and caustic soda, which are used in energy-conversion applications, or as a chemical precursor, a versatile commodity chemical or a carbon-free energy source, where the produced caustic soda is used as a byproduct in many food and beverage production plants.    ADVANTAGE - The microbial electrolysis cell: achieves high current densities at relatively low applied voltages; utilizes electrons produced at the anode for the purpose of generating chemical products at the cathode and not as a significant power source external to the microbial electrolysis cell; provides pure hydrogen at large scale with less hydrogen loss to biological activity at the anode; reduces Ohmic losses, thus allowing for faster ion movement and thus higher rates of caustic production at a given applied voltage; may use 70% less electricity and produce 80% less solid sludge as compared to aeration methods for the treatment of wastewater, while delivering identical or improved treatment performance; is low-maintenance, energy efficient and may be assembled without the use of exotic materials; provides a high-surface area for bacterial growth, translating into efficient contaminant breakdown in the wastewater stream; and is easily scaled to treat high volumes of wastewater.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for:    (1) a multi-module microbial electrolysis cell treatment apparatus comprising a treatment tank having an inlet for influent and an outlet for effluent, and many cylindrical microbial electrolysis cells operatively disposed within the treatment tank; and    (2) producing hydrogen from a fluid stream comprising organic matter in the microbial electrolysis cell, comprising providing at least one microbial electrolysis cell, contacting the fluid stream with anode-respiring bacteria disposed on the anode, contacting the cathode with a cathode solution comprising water and removing, from the cathode chamber, a gaseous composition comprising at least 98% hydrogen.    DESCRIPTION OF DRAWING(S) - The figure illustrates a top view of and a perspective view of a microbial electrolysis cell.    Microbial electrolysis cell (600)    Anode (602)    Cathode (604)    Membrane (606)