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Abstract
A coagulation–centrifugation/ultrafiltration hybrid process has been performed for the treatment of a commercial oil-in-water emulsion used in metalworking operations. The effect of pre-treatment stage on membrane fouling was evaluated to ascertain the main fouling mechanism and to establish the best membrane cleaning procedure. The hybrid process was optimized in terms of the maximum ultrafiltration (UF) permeate flux. Destabilization temperature, transmembrane pressure, feed flow rate to the UF module, and coagulant salt molar concentration were selected as factors. The experiments were designed using Taguchi method, and the contribution of each factor was determined using a statistical analysis of variance. Experimental results were also discussed in terms of environmental parameters (chemical oxygen demand [COD], conductivity, pH, and turbidity) for the main process streams. The best fit to experimental data of permeate flux decline corresponded to the cake/gel layer formation model. The cleaning procedure combined alkaline and acid washings with mechanical cleaning of membrane surface, and through this combination total permeate flux recovery was achieved. Temperature was the most significant factor affecting permeate flux, followed by coagulant salt concentration, with COD reductions higher than 97.5% for all experimental conditions tested.