NOVELTY - The process comprises diluting fibrous solid biomass in a liquid (L) for forming a mixture (M) comprising fibrous products in liquid suspension, hydrolyzing the fibrous products in the mixture, separating dried material from moisturized suspension of the liquid, and forming a methanization substrate from the separated dried material. The biomass is diluted such that the mixture contains 5-10 mass% of dried material. The process comprises crushing the biomass before dilution, storing the crushed biomass, progressively diluting the crushed biomass, and crushing the dried material.    USE - The process is useful for making a methanization substrate from fibrous solid biomass using an installation, which is useful in a system for producing biogas (all claimed), where the biomass is obtained from straw manure.    ADVANTAGE - The process ensures effective and rapid degradation of undesirable products present in the fibrous biomass and high production of biogas in an industrial manner, and improves the biodegradability of the methanization substrate.    DETAILED DESCRIPTION - The process comprises diluting fibrous solid biomass in a liquid (L) for forming a mixture (M) comprising fibrous products in liquid suspension, hydrolyzing the fibrous products in the mixture, separating dried material from moisturized suspension of the liquid, and forming a methanization substrate from the separated dried material. The biomass is diluted such that the mixture contains 5-10 mass% of dried material. The process comprises crushing the biomass before dilution, storing the crushed biomass, progressively diluting the crushed biomass, and crushing the dried material in liquid phase before/after the separation step. The separation of the dried material and the liquid is carried out by screening, pressing and/or centrifugation. The process further comprises adding, in the liquid, liquid additives including urea or acid and/or additional products such as biomass source obtained from plant, animal or fungal origin without crushing before dilution in the liquid. The separated liquid is used for diluting the dried material and to control dryness of the substrate. INDEPENDENT CLAIMS are included for:    (1) an installation (4) for making a methanization substrate from fibrous solid biomass; and    (2) a system for producing biogas (2).    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of an installation for production of biomass.    Biogas (2)    Installation for making the substrate (4)    Dilution hole (10)    Separation phase (12)    Horizontal agitator (20)    Liquid (L)    Mixture with fibrous products. (M)