Coagulation and flocculation with dissolved air flotation (DAF) were used in this study to reduce turbidity of wastewater produced in Isfahan petroleum refinery. Poly aluminum chloride (PAC), ferric sulfate and ferric chloride were used as coagulant and also a batch DAF unit was designed and made to improve the process efficiency of turbidity removal. Coagulant dosage, pH value of the samples, recycle ratio and residence time were considered as effective factors in this study. Tests were designed and done according to statistical experimental design methods. According to the results the optimum condition for ferric chloride which was 32ppm of coagulant in pH equal to 6.2, recycle ratio of 75% and 20 minute residence time, eventuates 52% of turbidity removal. For ferric sulfate the optimum yield of 51% occurred in 29ppm of coagulant in pH equal to 6.2, while recycle ratio was 54% and 20 minute residence time. The optimum condition for PAC include using 40ppm of coagulant in pH equal to 6, recycle ratio equal to 25% and 20 minute residence time, resulted 55% of turbidity removal. Aside from these tests which were done by DAF unit, performance of three natural coagulant aids with poly aluminum chloride, as coagulant, by Jar test was examined. These included starch, buckthorn and tragacanth. Results showed that these coagulant aids were not able to reduce turbidity of this wastewater, and in some cases increased the turbidity. Keywords: Oily Wastewater Treatment, Turbidity Reduction, Coagulation/Flocculation, Dissolved Air Flotation.
