The degree of delignification achieved across medium consistency O reactors is dependent upon any number of variables, e.g. O charge, alkali charge, pH, consistency, dissolved solids, temperature, and pressure.Although each variable has some cost associated with its manipulation, O and alkali costs are typically the most significant, and are also the easiest to quantify.In a mill environment, optimization of chem. addition must consider costs, equipment constraints, and final pulp properties.O addition was evaluated as a function of wash filtrate residual alkali.Cost and pulp strength data were then combined with this information to develop a chem. addition control strategy to be followed by mill operating personnel.