NOVELTY - Manufacturing a fibrous solid substrate, comprises: e.g. (a) providing a biomass material comprising solid and liquid parts from a biogas fermenter following an anaerobic fermentation and biogas production; and (b) subjecting the biomass material to at least one separation step resulting in the provision of a fibrous solid fraction comprising organic and inorganic nitrogen parts, and at least one liquid fraction comprising solid and liquid organic and inorganic phosphor-containing parts.    USE - The method is useful for: manufacturing a fibrous solid substrate, which is useful for cultivating fungal cells (claimed); and producing renewable energy.    ADVANTAGE - The method: recycles the spent fungal substrate back to a biogas fermenter (claimed); provides biomass energy that has a long-term, commercial future; can upgrade, sanitize and reuse the spent biomass materials having a reduced organic nitrogen content; utilizes improved and more cost effective pretreatment steps; facilitates efficient biomass processing and an increased production of renewable energy from processing and anaerobic fermentation of a wide variety of biomass materials; and reduces the content of inorganic nitrogen compounds in a fibrous solid fraction.    DETAILED DESCRIPTION - Manufacturing a fibrous solid substrate, comprises: (a) providing a biomass material comprising solid and liquid parts from a biogas fermenter following an anaerobic fermentation and biogas production; (b) subjecting the biomass material to at least one separation step resulting in the provision of a fibrous solid fraction comprising organic and inorganic nitrogen parts and at least one liquid fraction comprising solid and liquid organic and inorganic phosphor-containing parts; (c) subjecting the fibrous solid fraction to a sanitation treatment comprising at least one sanitation step, where the sanitation treatment reduces or eliminates viable microorganisms present in the fibrous solid fraction, and/or reduces the contents of volatile nitrogen-containing compounds and/or precursor volatile compounds present in the fibrous solid fraction; and (d) obtaining a fibrous solid fraction comprising organic and inorganic nitrogen parts having a reduced content of volatile nitrogen-containing compounds suitable for use as a fibrous solid substrate for cultivating fungal cells; and (e) optionally adding to the fibrous solid fraction at least one solid and/or liquid supplemental nutrient substrate compositions, thus generating a fibrous solid substrate suitable for cultivating fungal cells.