TECHNOLOGY FOCUS - BIOLOGY - Preferred Process: The process (P1) involves recirculating at least a part (19a, 22) of the residual biomass, produced in the methanogenic fermentation, to the acetogenic fermentation environment (12) or to the first hydrolysis and acidogenic fermentation environment (11); and further involves filtering the product (15) obtained in the hydrolytic-acidogenic environment (11), preferably with membrane filter, upstream of the subsequent acetogenic fermentation environment (12), the filtration step is adapted to reducing the concentration of salts and/or ammonia nitrogen entering into the environment (12). The residual biomass coming from the methanogenic fermentation is divided into: a first flow (19) subjected to a microfiltration process, with obtainment of a concentrate (22) which is recirculated to the acetogenic fermentation environment (12) or to the first hydrolysis and acidogenic fermentation environment (11); a second flow (20) subjected to a pressing step, with obtainment of biomass with high dry matter percentage. The filtrate (23) obtained from the microfiltration process is subjected to a reverse osmosis step, with obtainment of a concentrate (31) containing nitrogen. The liquid filtrate obtained in the pressing step is further microfiltrated and subjected to the reverse osmosis process. The concentrate (31) containing nitrogen, obtained via reverse osmosis, is subjected to a treatment for recovering the nitrogen by means of cold saturation with carbon dioxide in a carbonation tower (33), with obtainment of a solution containing nitrogen fixed in stable ammonium bicarbonate form. The cold saturation in the carbonation tower (33) is operated with at least a part of the biogas containing carbon dioxide obtained in the methanation and/or acetogenic fermentation environment. The biogas flow produced in the methanation process is used for combustion in a boiler or cogenerator, and at least a part of CO2 contained in the exhaust gases of the boiler or cogenerator is separated for use in the carbonation tower (33) for the nitrogen recovery. The concentrated solution (35) containing ammonium bicarbonate, obtained from the recovery of the nitrogen contained in the concentrate (31), is used as additive for the solid biomass obtained via pressing. Preferred Plant: The plant (T1) comprises at least one hydrolysis and acidogenic fermentation reactor (11); at least one anaerobic acetogenic fermentation reactor (12); at least one anaerobic methanogenic fermentation reactor (13); where the reactors (11, 12, 13) are separated in order to provide different fermentation operating conditions. The plant comprises a line (22, 19a) for recirculating at least a part of the residual biomass as is or filtered, from the methanogenic reactor (13) to the first acetogenic fermentation reactor (12), or to the hydrolysis-acidogenic step (11), the latter operating in anaerobic conditions. EXAMPLE - No suitable example is given.