roll press 12 for dewatering the pressate from the press 12 is returned via line 13 from which it is used to dilute the brownstock this treatment acidifies the brownstock and reduces the amount of acid required to bring the ph of the pulp into the desired ph range the pulp consistency after passage from the twin roll press is adjusted to about 20 to 25 at the outlet of the twin roll press the acidified pulp is mixed with peracetic acid from line 14 0 5 w w on o d pulp basis and low pressure steam from line 15 the pulp temperature at this point is adjusted to 50 u00b0 55 u00b0 c respectively the pulp is then fed through a mixer feeder 16 essentially a screw feeder to a down flow retention tube 18 at the end of the down flow retention tube the pulp is pumped by a high consistency pump 20 such as a clove rotor pump to a high shear mixer 22 at the outlet of the high consistency pump 20 before the entrance to the high shear mixer 22 a compressed oxygen ozone mixture of about 12 w w ozone on oxygen is metered onto the pulp from feed 24 using a perforated gas sparget not shown and the pulp is mixed with magnesium sulfate and additional peracetic acid which are fed from reservoirs 26 and 28 respectively the ozone charge on pulp is maintained at about 0 5 w w on o d pulp basis in the high shear mixer 22 the pulp is mixed with the reactants and then fed into a plug flow tubular reactor 30 the pulp retention time.