Sequential treatment of tannery effluent by coagulation–flocculation and sono-photo-Fenton: performance, optimisation and phytotoxicity evaluation

A comprehensive treatment approach for tannery effluent (TE) was investigated using an integral system involving coagulation–flocculation (CF) as a pre-treatment before the ultrasound combined with the photo-Fenton process (US-PFP). The pre-treatment by CF process using 2 g/l of FeCl₃ resulted in chemical oxidation demand (COD), turbidity, and colour number (CN) removal efficiencies of 56%, 99%, and 85%, respectively. The efficiency of the US-PFP was statistically evaluated, and the optimal operating conditions were determined by Box–Behnken Design. The results obtained show that the sequential process with CF as a pre-treatment and subsequent application of the US-PFP system achieves a cumulative maximum removal of 89% of COD and 98% of CN under the optimal operating conditions ([H₂O₂] = 7677 mg/l, [Fe²⁺] = 415 mg/l, and pH = 3). The phytotoxicity assessment of treated tannery samples was conducted through seed germination and root elongation tests using Medicago sativa (alfalfa) and wheat as bioindicators. The experimental results showed that the sequential application of CF pre-treatment following the US-PFP significantly reduced phytotoxicity. Overall, the sono-photo-Fenton process proves to be a viable and efficient method for treating TE.