NOVELTY - Untreated acidic whey is hydrolyzed, treated and subjected to partial anaerobic digestion. Liquid of treated milk proteins is added to chamber (3). Nutrient solution and nitrogen source partially provided from treated milk proteins are added to perform acetogenesis and methanogenesis processes. Effluent is transferred to chamber (4), nutrient solution and nitrogen source are added and methanogenesis process is performed. During all the process, biogas composition from each chamber is analyzed and collected.    USE - Anaerobic digestion of acidic whey used for producing biogas.    ADVANTAGE - The method enables anaerobic digestion of acidic whey in short time.    DETAILED DESCRIPTION - Untreated acidic whey is injected in chamber (1) with organic loading rate 6-7 kg VS/m3.day and temperature and pH is adjusted to 37-38 degrees C and 4.2-4.5, respectively. Acidic whey and sedimentation of 90 wt% or more of milk proteins in bulk liquid of acidic whey is hydrolyzed. Milk proteins sediment and 8-12 vol.% bulk liquid are separated and transferred to chamber (2). The temperature, pH and hydraulic retention time are adjusted to 37-38 degrees C, 7.5-8.1 and 5-7 days, respectively. The milk protein is treated and subjected to partial anaerobic digestion in chamber (2). The remaining bulk liquid is transferred from chamber (1) to chamber (3) and liquid of the treated milk proteins from the chamber (2) is added to the chamber (3). The temperature, pH and hydraulic retention time are adjusted in the chamber (3) to 37-38 degrees C, 6.6-7.2 and 3-4 days, respectively. Nutrient solution and nitrogen source partially provided from treated milk proteins are added to perform acetogenesis and methanogenesis processes in the chamber (3). Effluent is transferred from the chamber (3) to chamber (4), nutrient solution and nitrogen source are added and temperature, pH and hydraulic retention time are adjusted in chamber (4) to 37-38 degrees C, 7.2-7.8 and 3-4 days, respectively to perform methanogenesis process in chamber (4). During all the process, biogas composition from each chamber is analyzed and collected.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view explaining the anaerobic digestion of acidic whey.    Chambers (1,2,3,4)