TECHNOLOGY FOCUS - INSTRUMENTATION AND TESTING - Preferred Components: The system further comprises: an unit for adjusting a pH of a mixture of pretreated lignocellulosic biomass and waste activated sludge in the mixing unit and/or at a position downstream of the mixing unit but upstream of the unit for anaerobic digestion; a digestate dewatering unit having a digestate inlet connected to the digestate outlet of the unit for anaerobic digestion, a liquid fraction outlet and an outlet for dewatered digestate; and a digestate drying unit having an inlet for dewatered digestate connected to the outlet for dewatered digestate of the digestate dewatering unit and an outlet for dried digestate. The digestate drying unit has a flash steam inlet connected to the flash steam outlet of the flashing chamber. The flash steam outlet of the flashing chamber is connected to a heat exchanger that is arranged to transfer heat from the flash steam to the digestate drying unit.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Process: The process further comprises adjusting the temperature of the mixture of pretreated sludge and pretreated lignocellulosic biomass obtained in step (e) to a temperature of 30-60 degrees C, preferably 35-40 degrees C, prior to step (f). The process comprises adjusting the pH of the mixture of pretreated sludge and pretreated lignocellulosic biomass obtained in step (e), prior to step (f), such that the pH of the mixture of pretreated sludge and pretreated lignocellulosic biomass is 6-8, preferably 6.8-7.2. The mixture obtained in step (d) comprises 30-90 wt.% of slurry of pretreated lignocellulosic biomass and 10-70 wt.% of sludge. The step (b) comprises pretreating the lignocellulosic material under overpressure followed by releasing the pressure to obtain a slurry of pretreated lignocellulosic biomass and flash steam. The heat energy from the flash steam is used to heat the mixture obtained in step (d). At least part of the obtained flash steam is introduced to the mixture comprising sludge and pretreated lignocellulosic biomass obtained in step (d) or to the sludge provided in step (c). EXAMPLE - 300 ml of Waste activated sludge was acidified to a pH of 2 using 2M hydrochloric acid solution. A total of 29 ml of hydrochloric acid solution was added to the sludge. As control, 300 ml of sludge was not acidified but kept at original pH (6.25). Half of both acidified and control sludge was transferred to plastic bottles with a non gas tight lid and stored at room temperature for 6 hours. Half of both acidified and control sludge was transferred to plastic bottles with a non gas tight lid and heated for 120 minutes in an oven at 105 degrees C with intermittent manual stirring. When the temperature reached 90 degrees C, the bottles were transferred to a 90 degrees C oven and stored for 60 minutes, with manual stirring for every 15 minutes. Then the bottles were allowed to cool to room temperature, and the pH was adjusted to 7 by adding sodium hydroxide. The reaction was followed by anaerobic digestion for 62 days at 37 degrees C in 300 ml gas-tight serum bottles containing 100 ml of liquid volume (inoculum, substrate, salts and water). The gas-phase was nitrogen:carbon dioxide (80:20). Total gas and methane production was measured at eight occasions over the incubation period.