Feasibility Maintaining of the Efficiency of Orumieh Water Treatment Plant in the Case of Floating

Orumieh WTP was established based on nominal capacity of 18000 m3/d in 1962. By the last extension, its capacity was increased up to 30,000 m3/d. The raw water is transmitted from the sited intake on the Shahre Chaie River from the sited intake by gravity channel toward the WTP. The different units of treatment plant include manually coarse bar screen, grit chamber, accelerators, (5 units), gravity rapid sand filters (8 units), pressure rapid sand filters (7 units) and gas chlorination unit.One of the major problem of treatment plant is the disturbance of system in floating conditions, because of extreme increase in river turbidity. In such cases, sludge production increases extraordinarily and the influent of filter is a high turbid water that reduces the filter run time, So that the required water for filter backwash will be higher than total treated water production. It causes the plant shut downing and /or considerable reduction in plant reception. During 2003, 19 times the plant was stopped operation and 57 time it was operated in lower capacity that in spring 2004, the values were 6 and 18 days respectively. This problem causes lack of water and frequent water interruption in large areas of the city. For solving this problem, establishment a pretreatment unit is necessary. For this achievement, the potential of plain sedimentation in turbidity reduction was evaluated. Several samples were taken in floating conditions, and then they were analyzed by settling column.The results showed that in high turbidities, the efficiency of turbidity removal is 66.1% in 80 min and it is 79.74% in 90 min. for 75% TSS removal the SOR was 35 m3/ m2.day and retention time was 87 min. Using the factor 0.7 for SOR and 1.5 for retention time, an off-line sedimentation unit (65m×26m) was designed near intake site.