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To find a suitable method for treating printing ink wastewater, the fundamental behavior and characteristics of two kinds of experimental flexographic printing ink wastewater were studied. The differences between the two prepared wastewaters were the use of different kinds of binders-resins and pigments and different solvent contents representative of real printing inks. Examination of the water quality characteristics of the simulated wastewaters confirmed that the wastewaters were too highly loaded with organic substances for discharge and were not biodegradable.The prepared wastewaters also showed some colloidal dispersion properties. Such properties were strongly affected by pH. As pH decreased, the negative electrical charge of the particles was neutralized, resulting in their aggregation. Therefore, the present study focused on optimizing flocculation processes to destabilize the dispersions. Inorganic electrolytes were found to be more efficient flocculants than the organic polyelectrolytes tested. Even when added in very small quantity, inorganic electrolytes caused extensive neutralization of particle charges and consequent floc formation. The optimum flocculant concentration was determined on the basis of electrokinetic (zeta) potential and turbidity measurements.Mixing intensity, expressed as the mean mixing velocity gradient (G), and time of mixing (t) were investigated, and the optimum Gt product (Gt OPT ϭ 4.45 ϫ 10 4 ) was determined using turbidity values of supernatant water and sedimentation velocity of obtained flocs as criteria for flocculation efficiency. Flocculation was shown to be a simple and efficient treatment from an economic and technical point of view. It was capable of removing a substantial portion of the organic content, completely removing color and turbidity. Water Environ. Res., 72, 680 (2000).

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