Although the Upflow Anaerobic Sludge Blanket (UASB) has been widely used in wastewater treatment, it reveals clogging problems and the removal efficiency of COD was as low as 70% in the treatment of liner paper wastewater containing high concentrations of calcium ions. Calcium ions accumulated in the form of calcium carbonate and the ash content of granules reached 80%, which lowered the specific methanogenic activity of granules to 25% of the initial value. The pH was higher as the pH micro-electrode approached the core of the granule and this was why calcium carbonate was formed inside the granules. A novel process composed of UASB and CO2-stripper was suggested in this study to treat the liner paper wastewater. When the synthetic wastewater containing 5000 mg/l calcium hardness and 3000 mg/l COD was fed to the system, most of the calcium hardness was removed in the CO2-stripper and the removal efficiency of COD maintained a stable value of over 90%. (C) 2003 Elsevier Ltd. All rights reserved.