Four different end-of-pipe waste-treatment processes applicable to mechanical pulp and paper manufacture were modelled. Calculated costs for an average mill were capital $34-$44 million, operating cost $3.5-$6 million/year and discounted (10 years) $60-$85 million. Compared with mill reported values, capital and operating costs of activated sludge treatment (AST) were higher by 17 and 29%, respectively; those for aerated stabilization basin (ASB) were higher by 27 and 180%. Major variables affecting the costs were BOD and TSS levels and the wastewater-to-pulp ratio. It was concluded that ASB is more economical than AST and that anaerobic treatment plus AST could be advantageous at high BOD levels.