NOVELTY - Preparation of biogas involves adding waste to container, adding water and struvite powder, heating, cooling, filtering, collecting filtrate (A), adding waste to container, adding activated carbon, stirring, aging, filtering, collecting activated carbon, adding filtrate to container, decompressing, collecting mixed viscous liquid, adding activated carbon to reactor, introducing superheated steam, stirring, mixing liquid in the reactor with mixed viscous liquid, soaking filtrate (A) in mixed solution, adding ammonium bicarbonate, stirring, fermenting obtained mixture, and filtering.    USE - Preparation of biogas using furfural production waste and waste liquid (claimed).    ADVANTAGE - The method enables economical preparation of biogas with heat value of 28-32 MJ/m3, by simple and environmentally-friendly process.    DETAILED DESCRIPTION - Preparation of biogas involves adding 4-5 kg waste to a container, adding water to submerge waste to 3-5 cm, adding 200-300 g 80-90-mesh struvite powder, stirring for 40-50 minutes, heating to 80-90 degrees C, adjusting pH to 7-7.5 using 40-45% aqueous sodium hydroxide, cooling to room temperature, filtering, collecting filtrate (A), adding 6-9 L waste to the container, adding 800-900 g activated carbon, stirring at 110-120 degrees C for 1-2 hours, aging for 20-30 minutes, filtering reaction mixture when hot, collecting activated carbon, adding filtrate to the container provided with a ceramic intermediate film, reducing pressure of the bottom of the container to 400-500 Pa, terminating decompression process when no liquid dripping, collecting ceramic membrane unit mixed viscous liquid, adding activated carbon to a reactor, introducing superheated steam at 110-115 degrees C, discharging air inside the reactor, introducing nitrogen under pressure of 0.1-0.15 MPa, terminating addition of nitrogen, stirring for 9-11 hours, relieving to standard atmospheric pressure, collecting liquid in the reactor, mixing obtained liquid with mixed viscous liquid, soaking filtrate (A) in obtained mixed solution, adding ammonium bicarbonate in an amount of 2-4 %mass with respect to filtrate (A), stirring, pouring obtained mixture (B) into the bottom of a digester provided with a stirrer, adding 35-40 vol.% resultant mixture to normal biogas digester, adding water in an amount of 70-75 vol.% with respect to the mixture, stirring for 10-15 minutes, sealing the digester, controlling fermentation tank internal temperature of 30-35 degrees C, stirring at 300-350 rpm for 5-7 minutes for every 4-6 days, fermenting for 45-50 days, and filtering using biogas filter. The amount of mixture (B) is 30-35 vol.% with respect to digester.