Evaluation of a tannin-based coagulant on the decolorization of synthetic effluents

Natural-based coagulants have demonstrated significant advantages over conventional ones. In this work, the treatment of dyehouse effluents by coagulation-flocculation-sedimentation was studied using a commercial tannin-derived coagulant and compared with the common use of iron salt. Two aqueous solutions simulating textile dyehouse effluents were prepared using an azo dye (Direct Blue 85): E1, containing only dye and salts, and E2, with also three auxiliary dyeing chemicals. The treatment of E1 and E2 was optimized in batch mode (Jar tests) in terms of coagulant dosages and pH, in order to maximize color removal and find an acceptable settling velocity of flocs. Although the pH of the effluents is around 8, acidic conditions (pH 4-5) were experimentally found to be optimal for their total decolorization, at minimum coagulant dosages. In these conditions, iron sulfate provided decolorization using much lower dosages than those required for the tannin-coagulant. At pH 4, for instance, tannin-coagulant dosage required to find 100% color removal and easily settleable flocs was 3-times higher than that of iron(III) sulfate. However, at neutral and alkaline conditions, which are of major relevance in practical terms, the tannin-coagulant was much more efficient than the iron coagulant. At pH 8 (natural pH of E2), total decolorization was achieved using 180 mg L-1 of tannin-coagulant, while dosages up to 240 mg L-1 of iron sulfate were unable to provide more than 20% of decolorisation and settleable flocs. Continuous mode experiments, conducted in optimum conditions, confirmed the performance levels observed in jar tests.