Step 4 Pulp screening Asieve is used to remove knots and clumped together uncooked fibres from the pulp Step 5 Bleaching This is done in two stages. Firsty the pulp is treated with NaOH in the presence of O2. The NaOH removes hydrogen ions from the lignin and then the 02 breaks down the polymer. Then, the pulp is treated with CIO2 then a mixture of NaOH, 02 and peroxide and finally with Cl02 again to remove the remaining lignin.