NOVELTY - Continuous production of hydrogen and methane involves preparing a feeding mixture of zootechnical effluents and agro-food wastes rich in fermentable molecules and include substance containing carbohydrates and consisting of whey, scotta or mixture of scotta/glycerol; fermenting the mixture in a bioreactor (b1), under anaerobic conditions and in darkness; subjecting fermented broth to methanogenic digestion in second bioreactor (b2) under conditions; introducing part of H2 produced in bioreactor (b1) into the second bioreactor (b2); and controlling composition of the mixture.    USE - For continuous production of hydrogen (H2) and methane (CH4) (claimed).    ADVANTAGE - The present method produces CH4 from co-digestion of zootechnical effluents with high percentages of agro-industrial wastes rich in fermentable molecules with high energy-giving content, which wastes include a substance containing carbohydrates and consisting of scotta or a mixture of scotta and glycerol. On the contrary, in the traditional anaerobic digestion (AD), co-digestion is possible only if the percentage of such wastes is kept low, typically below 10%, otherwise the concentration of volatile fatty acids would increase, with block of methanogenesis. The present method has considerably reduced digestion time, at least by 30%, with respect to the time attainable in traditional single stage AD, starting from non-digested raw matter; maintains mesophilic conditions, that is of temperatures of 30-40 degrees C, which are far lower than temperatures required by thermophilic processes; utilizes selected microbial consortia thus attaining higher yields in biogas; and is two-stage method which allows use of, in co-digestion, high percentages of agro-food or agro-industrial wastes, without creating problems in the methanogenesis and, instead, with a considerable increase in CH4 production with respect to what attainable with AD of the only effluents. Also recycling the H2 produced in the first stage or bioreactor into the second stage allows further increasing CH4 production, while eliminating at the same time the need to manage H2 leaving the plant. Actually, since the methanogenic microorganisms produce CH4 mainly from acetate, but also from H2+CO2, if the H2 produced in the first stage is transferred into the second stage, the result will be a further increase in CH4 production.    DETAILED DESCRIPTION - Continuous production of hydrogen (H2) and methane (CH4) from zootechnical effluents involves preparing in a first tank (s1), a feeding mixture containing zootechnical effluents and agro-food wastes rich in fermentable molecules and include a substance containing carbohydrates and consisting of whey, scotta or a mixture of scotta and glycerol; fermenting the mixture in a first bioreactor (b1), under anaerobic conditions, in the darkness and under controlled temperature, pH and pressure, by inoculation of mesophilic microorganisms for promoting the phase of hydrolysis, acidogenesis and acetogenesis, in order to produce H2, under acidic conditions, where the step of fermentation of the mixture in the first bioreactor (b1) is obtained by maintaining a subatmospheric pressure in the bioreactor (b1); subjecting the fermented broth to methanogenic digestion in a second bioreactor (b2), under controlled temperature and pH, by inoculation of mesophilic microorganisms for promoting the phase of methanogenesis, in order to produce CH4, under neutral pH conditions; introducing into the second bioreactor (b2), at least part of the H2 produced in the first bioreactor for promoting the production of CH4 in the second bioreactor (B2); and controlling the composition of the feeding mixture in accordance with the desired quantities of H2 and CH4 to be obtained. An INDEPENDENT CLAIM is included for plant for the continuous production of H2 and CH4 from zootechnical effluents, comprising: a first bioreactor (b1) for producing H2; a second bioreactor (b2) for producing CH4; a first tank (s1) for preparing a feeding mixture containing zootechnical effluents and agro-food wastes rich in fermentable molecules, which wastes include a substance containing carbohydrates and consisting of whey, scotta or a mixture of the scotta and glycerol, and optionally a second and third tanks containing a buffering solution for re-establishing the pH in the first bioreactor (b1) and in the second bioreactor (b2), respectively; a heater/cooler for maintaining the process temperatures in the first and second bioreactors and for maintaining a temperature in the first tank (s1) such as not to promote fermentation of the biomass contained in it; a system for controlling and measuring the parameters for the continuous plant operation, so as to automatically ensure the desired conditions of production of H2 and CH4, and comprising an electronic control unit associated with sensors arranged to generate a signal indicative of the value taken by the parameters, and units for adjusting the percentages of the ingredients of the feeding mixture; units for controlling and adjusting the pressure inside the first bioreactor (b1) for producing hydrogen, and acting on the flow rate of the gas leaving the first bioreactor (b1) so as to maintain subatmospheric pressure conditions in the first bioreactor (b1) and to introduce at least part of the H2 produced in the first bioreactor into the second bioreactor (b2) in order to promote the production of CH4 in the second bioreactor (b2).