Background: Since coagulation and flocculation are essential processes for turbidity removal from drinking water, many studies have been conducted on different coagulant. This study was done to compare the efficacy of three coagulants of polyaluminum chloride, aluminum sulfate and ferric chloride in turbidity removal from drinking water.

  Materials and Methods: The present study -a pilot scale experiment- was conducted in the laboratory of water and wastewater chemistry of Zahedan public health school in 2007. Laboratory experiments were performed using distilled water containing synthetic Caoline. Eight turbidity levels (10-80 NTU), five pH levels (5.5, 6.0, 6.5, 7.0 and 7.5) and five coagulants doses (5, 10, 20, 30 and 40 mg/l) were used for jar test. Rapid mixing for all experiments was 380 rpm for one minute and slow mixing was 30 rpm for 20 minutes. At the end of every experiment, residual turbidity was measured by turbidity meter. Removal efficiency for various conditions was determined by covariance and regression analysis.

  Results: The results indicated that an increase of aluminum sulfate dose up to 40 mg/l can increase removal efficiency to 99.9%. The highest removal efficiency for ferric chloride occurred at feed dose of 20 mg/l and primary turbidity of 10 NTU, while for PAC the highest removal efficiency (99%) occurred at 40 mg/l. Effect of pH in turbidity removal for all experiments was similar.

  Conclusion: This study highlighted that the best coagulant for turbidity removal (pH= 5.5-7.5) is aluminum sulfate, because as the coagulant dose rises up, the removal efficiency increases significantly. Efficiency removal also depends on the initial turbidity.