NOVELTY - Semi-continuous biological hydrogen production includes anaerobic digestion of mixture of waste in a hydrogen production reactor with an average residence time of the mixture in the hydrogen production reactor of 2-4 days, obtained through daily removal of a portion corresponding volume of mixture in the reactor; and feeding an equivalent volume of analogous fresh mixture into the reactor.    USE - Method for semi-continuous biological hydrogen production (claimed).    ADVANTAGE - The method can produce high quantities of hydrogen, with constant production of the element in time, which can be integrated with other processes for biochemical transformation of organic waste to provide a process whose end products are all useful and re-usable.    DETAILED DESCRIPTION - Semi-continuous biological hydrogen production comprises anaerobic digestion at 30-40 degrees C of a mixture of waste having not greater than 10 wt.% solids, containing biodegradable organic fractions of urban waste, both from separate refuse collection and from mechanical sorting, by vegetation waters deriving from olive oil production and by active sludge from urban sewage purification. The method is conducted with an average residence time of the mixture in the hydrogen production reactor of 2-4 days, obtained through daily removal of a portion corresponding to approximately 1/n of the volume of mixture in the reactor, where n indicates the number of days of desired average residence time, and feeding an equivalent volume of analogous fresh mixture into the reactor. INDEPENDENT CLAIMS are included for:    (1) anaerobic hydrogen production reactor (13) of the continuous stirred-tank type, equipped with inlet port(s) and outlet port(s), both gas-tight, with instruments to control pH and temperature of the reaction mixture present in the reactor, and with gas-tight ports to take samples of gas or reaction mixture; and    (2) a plant (10) for the combined production of hydrogen and methane, constituted by the reactor and a second gas-tight reactor (14) for methane production, connected in series by gas-tight ports and feed systems, so that an outlet port of the hydrogen production reactor is connected to an inlet port of the second reactor.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of an integrated plant for producing hydrogen.    Plant (10)    Mixing station (12)    Reactors (13, 14)    Station for dehydration of material (15)    Composting station (17)