Batch 1 was treated for 48 h and batch 2 was treated for 60 h. pH was kept approximately constant during
the whole pretreatment at 5.0 ± 0.2 or 6.0 ± 0.2 (according to Table 1) by manually adding 0.5 M HCl. All reactors were well
mixed by magnetic stirrers at a constant speed of 350 rpm.