Investigating the effects of height, effective size and uniformity coefficient of rapid sand filters on removal of turbidity and coliform bacteria in water treatment plants of Tehran

Drinking water supply with proper quality is one of the main goals of water treatment plants. Filtration is a process which has the largest share of quality making in water treatment plants and about 20% of the cost is allocated to this process. The main indicator used for filters efficiency is water turbidity at outlet.
In this research, the effect of various heights (20-50-70-90 cm), effective sizes (0.75-1) and uniform coefficients (1-1.5) sand filter’s packing on turbidity removal and coliforms has been studied for four months from May 2010. Sampling was performed according to standard procedures and the samples were analyzed immediately in Tehran Islamic Azad University of Science Research laboratories and water treatment plant laboratory.
Finding: Results of coarse packing filter show the maximum efficiency of turbidity reduction was 83.58% for turbidity of 5 NTU and packing height of 90cm. The minimum turbidity reduction was 29.25% for turbidity of 10 NTU and packing height of 20cm. The maximum coliform reduction was about 87.92% for turbidity of 2 NTU and packing height of 90cm and the minimum coliform reduction was about 38% for turbidity of 10 NTU and packing height of 20cm. The effective height which causes maximum turbidity and coliform reduction is 90cm. The longest operation time in the context of a coarse bed was 35 hours in turbidity of 2 NTU and the shortest operation time in the context of a coarse strainer was 23 hours in turbidity of 10 NTU.
Results of fine packing filter show the maximumefficiency of turbidity reduction which was 93% for turbidity of 2 NTU and packing height of 90cm. The minimum turbidity reduction was 54.9% for turbidity of 7 NTU and packing height of 50cm. The maximum coliform reduction was about 91.92% for turbidity of 2 NTU and packing height of 90cm and the minimum coliform reduction was about 45% for turbidity of 10 NTU and packing height of 20cm. The effective height which causes maximum turbidity and coliform reduction is 90cm. According to the results, efficiency of the fine bed and coarse bed filter directly depends on outlet turbidity and operation of previous unit (sedimentation). The maximum and minimum operation times in fine bed were about 28 hours (NTU=2) and 14 hours (NTU=10) respectively.