Background & objectives: Drinking water treatment is very important owing to the importance of drinking water in human health. One of the most important methods to remove turbidity and organic substances from water sources is using proper coagulants to coagulate the water impurities. This study aimed to investigate the efficiency of coagulants i.e. Ferric Chloride (FeCl₃) and poly-ferric sulfate (PFS) on removing turbidity and reducing organic material levels from raw water supplied from Yamchi reservoir to Ardabil water treatment plant.

Methods: Present experimental-analytical study was conducted by using Jar Testing equipment. Samples of the study were taken from the conduit feeding the water treatment plant in four seasons during 2013-2014. Two coagulants namely ferric chloride and poly-ferric sulfate were tested on 48 samples taken in 4 seasons of the year with the turbidity levels of 5.67, 12.7, 15.32, and 31.35 NTU.

Results: Based on the results of this study PFS was more efficient than FeCl₃ in terms of dosage of coagulant, size of coagulum, deposition time and production of sludge with higher viscosity. More specifically, PFS was able to remove 7% more dissolved organic carbon (DOC) from the samples than FeCl₃; whereas decrease in value of pH was found to be lower by using PFS than FeCl₃.

Conclusion: Based on the results, PFS can be used as a suitable and economic alternative for ferric chloride in water treatment plant of Ardabil City in terms of less coagulant dosage and high organic compounds removal efficiency.