advantageous it is also interesting to observe in this example that the increase in efficiency of the ozone stage is not accompanied by a significant drop in the quality of the cellulose of which the degree of polymerization measured according to standard nf iso 5351 after reduction with sodium borohydride remains at very good levels this is illustrated in fig2 a softwood wood kraft pulp is treated in a known manner having a residual lignin content corresponding to a kappa number close to 27 with a deded bleaching sequence the brightness obtained is 81 9 iso this pulp after washing with water had a ph close to 7 it was then subjected to an ozone treatment with a consistency of 35 in the same device as in example 1 a quantity of ozone of 0 19 was progressively added to the pulp after this treatment the pulp was washed and its brightness measured by the usual standard methods the results of the bleaching by this final ozone stage are shown in fig3 they are similar to those obtained in example 1 this is particularly remarkable because the ph of the treatment is 7 and this according to the teaching of the prior art should lead to a rapid decomposition of the ozone and hence its loss of efficiency in this example it appears that the ozone treatment performance should be even better at a.