Treatment of cork boiling wastewater using a horizontal subsurface flow constructed wetland combined with ozonation

Cork boiling wastewaters (CBW) are strongly coloured complex aqueous solutions with high organic load of biorecalcitrant and toxic nature. The feasibility and efficiency of a CBW treatment process combining ozonation as pre- and post-treatment of a horizontal subsurface flow constructed wetland (HSFCW) was assessed. Over an extended monitoring period of 390 days, two lab-scale HSFCW units were tested; one planted with P. australis (CWP) and one unplanted-control (CWC) operated at average organic loads rates (OLR) of 5 and 10 g COD/m²/d. CWP always outperformed the control unit. The ozonation trials were run at pH values of 8.15-8.21 and 5.39-5.45 (without adjustment) at ozone to COD ratios of 0.25-0.29 and 0.24-0.59 when implemented as pre- and post-treatment, respectively. Average removals (calculated through mass balance basis) were 78-88%, 86-91%, 71-89% and 43-89% for COD, BOD₅, Total Phenols (TPh) and colour when ozonation was implemented as post-treatment. For ozonation as pre-treatment, respective figures were 77-80%, 79-92%, 78-85% and 19-73%. Regardless of the treatment scheme and OLR, ozonation was very effective in biodegradability increase (i.e., BOD₅/COD) from 0.18 to 0.42 when applied as pre-treatment, and decolourization after the HSFCW increased from 21% to 91% (post-treatment) with respective ozone consumed yields of 67-69% and 72-85%. The best results were obtained for the scheme CWP + Ozonation at OLR of 5.33 g COD/m²/d with COD reductions from 1950 mg/L to 81-88 mg/L in the effluent and TPh from 125 mg/L to 5-6 mg/L at limited ozone amounts of 0.21-0.45 g O₃/m²/d.