Two-stage treatment of pulp bleaching wastewater by Fenton and biological processes to remove recalcitrant pollutants

In this work, Fenton and aerobic biological processes were successfully coupled to treat pulp bleaching wastewater. The goal was to find a treatment solution to simultaneously maximise the removal of recalcitrant pollutants (adsorbable organic halides – AOX) and organic load (COD) from that wastewater. To that end, two treatment setups were studied: (i) aerobic biological process (activated sludge) followed by Fenton; and (ii) Fenton followed by aerobic biological process. Treatment by biological process followed by Fenton allowed for an overall removal of AOX and COD of 89.5 % and 82.5 %, respectively, at the optimised Fenton conditions determined by Response Surface Methodology: 489 mM H2O2, 9.3 mM Fe2+, and 49.2 min of treatment. Under the inverse treatment setup, an overall removal of 89.6 % AOX and 71.9 % COD was attained. Moreover, it was proven that the microbiological activity was not greatly affected by neither the initial organic load, nor the existence of Fenton chemicals and transformation products in the wastewater composition. Although the treatment sequence Biological → Fenton presented maximum overall performance, operating costs associated with this solution were over 2-fold higher than the inverse sequence, owing to the higher demand of chemicals.