Effects of Mixing Condition and Filtration Velocity on Turbidity Removal in a Contact Roughing Filter

Slow sand filtrations have been widely used for water treatment in small communities, however their capacity is often limited by high turbidity in the raw water. For this reason, several pre-treatment facilities were required for a slow sand filter. Turbidity removal from the highly turbid raw water was investigated in roughing filters as a pre-treatment process. The roughing filters followed by rapid mixing tank were operated in the form of a contact filtration. In several jar tests, the predetermined optimum aluminium sulfate(alum) doses for turbid water of 30 and 120 NTU were 30 and 50 mg/L, respectively. At the optimum alum dose, physically optimum parameters including G value of 220 sec-1 and rapid mixing time of 3 minutes were applied to the contact filtration system. Without addition of alum, the filtrate turbidity from the roughing filters, packed respectively with different media such as sand, porous diatomite ball and gravel, was in the range of 5～30 NTU at filtration velocities of 30 and 50 m/day. However, the application of a contact filtration to roughing filters showed stably lower filtrate turbidity below 1.0 NTU at filtration velocity of 30 m/day. Although the filtration velocity increased to 50 m/day, filtrate turbidity was still below 1.0 NTU in both single and double layer roughing filters. At influent turbidity of 120 NTU, the filtrate turbidity was over 5 NTU in the triple layer roughing filter, which shortened the filter run time. The flocs larger than 10 ㎛, formed in the rapid mixing tank, were almost captured through the roughing filter bed, while the almost flocs smaller than 10 ㎛ remained in filtrate.