Toxicity reduction and biodegradability enhancement of cork processing wastewaters by ozonation

Biodegradability enhancement and detoxification of cork boiling wastewater (CBW) are required for the successful implementation of biological treatment options. We studied the possibility of achieving these goals through ozonation pre-treatment by experimenting on the effect of ozone dose and pH. The CBW used had a pH of 5.81, a chemical oxygen demand (COD) of 1,865 mg L⁻¹, a biochemical oxygen demand (BOD₅) of 498 mg L⁻¹ and total phenol (TP) and tannin compounds concentrations of 523 and 399 mg L⁻¹, respectively. The ozone doses ranged from 0.27 to 2.63 for the O_3(applied)/COD₀ ratios with samples at natural pH and set to 3.33 and 9.96. Ozonation allowed the BOD₂₀/COD ratio (biodegradability index) to increase from 0.37 to 0.63 and a toxicity reduction from 3.08 to 1.24 TU (Microtox). The corresponding removals obtained were 15.2–62.0%, 38.4–83.2% and 56.7–92.1% for COD, TP and colour, respectively. The best outcome of ozonation pre-treatment requires O_3(applied)/COD₀ ratios over 1.5 and an acid pH. The increase of TP removals with ozone dose at acid pH led to biodegradability enhancement and CBW detoxification. However, for similar conditions the highest COD removals were obtained at alkaline pH due to the hydroxyl radicals’ high oxidation ability but lack of selectivity.