An upflow anaerobic sluge blanket (UASB) reactor showed fairly good COD removal efficiency for the wastewater from a liner paper factory. Over 70% of COD removal efficiency were obtained even at 18 g CODA day loading. A considerable amount of alkalinity was produced in the reactor when sulphate was reduced to hydrogen sulphide by sulphate reducing bacteria (SRB). Calcium was efficiently removed in CO2 stripping unit where excess alkalinity was consumed to form calcium carbonate. No additional chemical such as caustic soda was necessary to facilitate the formation of calcium carbonate since the degassing of CO2 increased the pH level in the CO2 stripping unit. Calcium removal efficiency approached about 80% and the level of effluent calcium ranged from 80 to 150 mg/l. Since this system (UASB + CO2 stripping unit) used biologically produced alkalinity for the formation of calcium carbonate, it can be called a biosoftening process. Wastewater treatment using both an UASB reactor and a CO2 stripping unit showed favourable properties for the papermaking process. (C) 2002 Published by Elsevier Science Ltd.