The removal efficiency of conventional drinking water for picophytoplankton and the contribution of picophytoplankton to AOC were investigated in this research. The removal ratio during coagulation–sedimentation step was determined by jar test using PAC (poly-aluminium chloride). Lower coagulation pH showed better picophytoplankton removal in coagulation–sedimentation. The optimum coagulant dosage for picophytoplankton was twice or more than that for turbidity. The removal efficiency of picophytoplankton was 44–60% at lowest pH in water quality standard (5.8) and at an optimum coagulant dosage for turbidity. The removal ratio of picophytoplankton in rapid sand filtration was determined by pilot scale column experiments with sand and anthracite. The average removal percentage was 16.3% without PAC addition and chlorination before sand filtration; on the other hand it was 51.5% with PAC and chlorination. AOC increased by the chlorination of picoplankton including 6,800 cells/L of picophytoplankton was 21 μg-acetateC/L at 0.1 mg/L of residual free chlorine. The AOC was increased by the increase of residual chlorine concentration, and leveled off at 0.3 mg-Cl/L. From the result, the AOC originating from picoplankton (maximum AOC from picophytoplankton) could increase up to 155 μg-acetateC/L in this reservoir. It indicates that the removal of picoplankton (picophytoplankton) in drinking water treatment process is important from the viewpoint of AOC control.

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