The selection of a coagulant-flocculant agent which, based on the maximum chemical oxygen demand removal, warrants the best performance of the removal system for a very complex high-load chemical-pharmaceutical industry wastewater, is described. A total of 23 coagulants/flocculants was tested, including salts, poly-hidroxyaluminates, synthetic polymers as well as natural gums. In a second stage, some mixing aspects were studied. The effects of the specific impeller, the agitation speed during the coagulation and flocculation stages, and the absence or presence of baffles were evaluated in a six-place agitation system. The conventional impellers were replaced by the following types of propellers: Rushton, marine, A310 (Lightning), three flat blades, 45° inclined six blades, and conventional flat blade propeller. It was demonstrated that the appropriate coagulation-flocculation system is capable of diminishing the COD, the apparent color and the dissolved solids up to 40.6, 25.6 and 39.4%, respectively. The best results were observed when using BL-5086, guar gum, Niad II-3, Niad II-4 and locus beam gum. The impeller performance was highly dependent on the agitation speed for each fixed system. With respect to the mixing aspects, it was shown that the selection of the right propeller for the coagulation and flocculation stages is crucial in determining the quality of the treated water, as well as the quantity and quality of the residual sludges generated in the process.

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