NOVELTY - Preparing fermentation residue, as a complex system for e.g. energy, comprises separating solids from fermentation residue; post-fermenting the residue; treating the anaerobic filter discharge; degrading N-substrates; separating the biomass remainder and low-molecular components; post-purifying water, ion exchanger and/or zeolites; adding mineral salt solutions through a mineral mixture; disinfecting water; concentrating the algal culture and producing the carbon dioxide for algal culture from the flue-gas wash of the combined heat and power plant (CHP) and/or from biogas cleaning.    USE - The pure water is used for rearing fish or as industrial water (claimed). The fermentation residue is useful in fish culture, and for the production of fermentation gas, fertilizers and additives for the industry of building materials, and water of different purity.    ADVANTAGE - The fermentation residue obtained in the process is of high-quality.    DETAILED DESCRIPTION - Preparation of fermentation residue, as a complex system for energy, water and nutrients, comprises separating solids (raw fibers) from the fermentation residues by means of decanters, separators or other suitable filter mechanisms; post-fermenting the fermentation residue in anaerobic fixed bed reactors for further biogas production and for C-decomposition; subsequently treating the discharge of the anaerobic filters by means of aerobic trickling filter for further C-decomposition; subsequently degrading N-substrates over a combined anaerobic/aerobic-step; separating the biomass remainder over external or immersed ultra-filtration modules; separating the low-molecular components by single- or multi-stage reverse osmosis; post-purifying water by combined cascade for the retention of residual impurities like organic compounds, ammonium or sulfide by means of activated charcoal, ion exchanger and/or zeolites; adding suitable mineral salt solutions over or through a mineral mixture for mineralizing the water for rearing fish or cattle watering tanks; disinfecting water with UV, ozone or other disinfection systems such as tetrachlorodecaoxide additives; concentrating the algal culture to algal concentrates by ultrafiltration as base for fish fodder; and producing the necessary carbon dioxide for algal culture from the flue-gas wash of the combined heat and power plant (CHP) and/or from biogas cleaning, where the low-temperature waste heat from CHP is transformed to current; the waste heat of the membrane filtration procedures (ultra-filtration and reverse osmosis) is useful for aquaculture in the produced water; and reverse osmosis concentrate is useful for algal cultures as fodder base for aquaculture.    DESCRIPTION OF DRAWING(S) - The figure shows the schematic block-diagram of the preparation of fermentation residue (Drawing includes non-English language text).