Ozonation Pretreatment to Enhance Coagulation–Flocculation and Biodegradability of Tannery Wastewater

Abstract

Abstract Tannery wastewater is characterized by high organic loads, low biodegradability, and the presence of recalcitrant compounds, which hinder the efficiency of conventional treatment. In this study, ozonation was evaluated as a pretreatment step to enhance coagulation-flocculation performance for real tannery wastewater collected in southern Brazil. Coagulation-flocculation experiments were carried out using aluminum sulfate at different dosages to determine the optimum operating conditions. Ozonation was performed at ozone flow rates 1.5 and 3.1 g h −1 for 20 min, both as a standalone treatment and prior to coagulation-flocculation. Treatment efficiency was evaluated in terms of TOC, COD, BOD 5 , turbidity, AOS, biodegradability index, and chromium speciation. Coagulation-flocculation alone achieved removals of approximately 35% for TOC and 56% for COD at the optimum coagulant dosage (100 mg L −1 ). Ozonation alone resulted in TOC and COD reductions of up to 19% and 46% respectively. The combined process improved treatment efficiency, particularly when ozonation at 1.5 g h −1 was applied prior to coagulation-flocculation, achieving removals of 53% for TOC, 64% for COD, 68% for turbidity, and 25% for Cr (VI). In addition, the biodegradability index increased from 0.25 in the raw effluent to 0.90 after the combined treatment, indicating substantial oxidation and transformation of recalcitrant organic matter into more biodegradable compounds. The results demonstrate that ozonation is an effective pretreatment strategy to improve coagulation-flocculation performance and enhance the biodegradability of tannery wastewater.