Maragheh (city in Iran) WTP was established based on nominal capacity 17290 m3/day in 1983.By the land extension, its capacity increased up to 29500 m3/day. The raw water is transmitted from the sited intake on the Alavian River from the sited intake by gravity channel toward the Water Treatment Plant. The different units of treatment plant include manually coarse bar screen, grit chamber, accelerators (5 units), gravity rapid sand filters (8 units), and pressure rapid sand filters (7filters) and gas chlorination unit. One of the major problems is the disturbance of system in floating conditions, because of extreme increase in river turbidity. In such cases, sludge production increases extraordinary and the influent of filter is a high turbid water that reduces the filter run time, so that the required water for backwash will be higher than total treated water production. It causes the plant shut downing or reduction in plant reception. For solving this problem establishment a pre-treatment unit is necessary. For this achievement, the potential of plain sedimentation in turbidity reduction was evaluated.