BACKGROUND: Chemical thermo-mechanical pulp (CTMP) mills holds a large biomethane potential in their wastewater. Their broadened market has involved increased bleaching and utilization of different raw materials. Therefore, the main aim of this study was to obtain and maintain a well-functioning anaerobic digestion (AD) process, with a high methane yield and total organic carbon (TOC) reduction, when digesting CTMP wastewater, from different production protocols including shifts in raw material and bleaching. A lab-scale upflow anaerobic sludge bed (UASB) reactor was used for the tests. RESULTS: The variations in raw material (spruce, birch and aspen) and consequently in TOC-loading (3.6-6.6 kg TOC m(-3) and day(-1)) did not affect the UASB process negatively. Methane production values from 360 to 500 NmL g(-1) TOC were obtained, with the highest yield for wastewater from the production of birch- followed by aspen- and spruce pulp. The acetic acid and filtered-TOC (fTOC) reduction ranged from 90 to 95% and 61 to 73%, respectively. CONCLUSIONS: A well-functioning process maintained during shifts in raw material for pulp production which shows that AD is feasible for CTMP mills with a diversified product portfolio. Furthermore, the increased use of hardwood and bleaching will most likely increase their potential as a biomethane producer. (C) 2016 Society of Chemical Industry