NOVELTY - Producing biogas, involves (a) providing a lignocellulosic biomass, (b) pretreating the lignocellulosic biomass at a temperature of 120-250 degrees C, (c) providing a waste activated sludge, (d) mixing the slurry of pretreated lignocellulosic biomass with the sludge, (e) incubating the mixture obtained in step (d), and (f) performing anaerobic digestion of the mixture of obtained in step (e) by incubating the mixture in a digester comprising anaerobic microorganisms.    USE - The process is useful for producing biogas and digestate (claimed), which is useful as soil conditioner or fertilizer.    ADVANTAGE - The process efficiently increases the biogas production by pretreating the lignocellulosic material at high temperature (e.g. 120-250 degrees C) and low pH (e.g. 0.5-2.5) conditions, and reduces the costs and energy consumption using flash steam for drying the digestate.    DETAILED DESCRIPTION - Producing biogas, involves (a) providing a lignocellulosic biomass, (b) pretreating the lignocellulosic biomass at a temperature of 120-250 degrees C, preferably 170-220 degrees C, and at a pH of 0.5-3, preferably 1-2.55 to obtain a slurry of pretreated lignocellulosic biomass, (c) providing a waste activated sludge having a pH of 5-8, preferably 6-7.5, (d) mixing the slurry of pretreated lignocellulosic biomass with the sludge to obtain a mixture comprising sludge and pretreated lignocellulosic biomass, (e) incubating the mixture for a first incubation time to obtain a mixture of pretreated sludge and pretreated lignocellulosic biomass, and (f) performing anaerobic digestion of the mixture obtained in step (e) by incubating the mixture for a second incubation time at a temperature of 30-60 degrees C, preferably 35-40 degrees C, in a digester comprising anaerobic microorganisms to produce biogas and a digestate. The pH of the mixture obtained in step (d) is 0.5-3, and the temperature of the mixture obtained in step (d) is 75-99 degrees C. An INDEPENDENT CLAIM is included for a system for producing biogas, comprising an unit for pretreatment of a lignocellulosic biomass having a biomass inlet (2) and a pretreated biomass outlet (3), an unit for sewage treatment having a sewage inlet (5) and an sludge outlet, a mixing unit (9) for mixing of pretreated lignocellulosic biomass with sludge having an inlet for pretreated lignocellulosic biomass, an inlet for sludge and a mixture outlet, and an unit for anaerobic digestion having a substrate inlet, a biogas outlet (15) and a digestate outlet (16), where: the pretreated biomass outlet of the unit for pretreatment of a lignocellulosic biomass is connected to the inlet for pretreated lignocellulosic biomass of the mixing unit; the sludge outlet of the unit for sewage treatment is connected to the inlet for sludge of the mixing unit; the mixture outlet of the mixing unit is connected to the substrate inlet of the unit for anaerobic digestion; the unit for pretreatment of a lignocellulosic biomass comprises a pressure reactor comprising the biomass inlet and a biomass outlet; the unit for pretreatment further comprises a flashing chamber for recovery of flash steam from the lignocellulosic biomass from the pressure reactor; the flashing chamber comprises an inlet being connected to the outlet of the pressure reactor, a flash steam outlet and the pretreated biomass outlet; and the flash steam outlet is connected to a flash steam inlet of the mixing unit.    DESCRIPTION OF DRAWING(S) - The figure shows a block diagram of a system for producing biogas.    Biomass inlet (2)    Pretreated biomass outlet (3)    Sewage inlet (5)    Mixing unit (9)    Biogas outlet (15)    Digestate outlet. (16)