Problematic algae, species that do not settle or are not easily removed by water treatment processes, are common in many water treatment plants (WTPs). We recorded algae in the water that overflowed from sedimentation basins (SBs) to filtration basins of WTPs in South Korea. Diatoms were common and other algae were not discernible in the flocs. Our field observation indicated that long and needle-shaped algae were less likely to settle and more likely to be present in overflow water. Many diatom cells or colonies that were extremely deformed from spherical had high overflow rates. Another long alga, Phormidium sp. (Cyanobacteria), originated from periphytic biofilms attached to SB walls. Algae that form long cells or colonies are less compact and less likely to settle as poor flocs. Species that overflowed the basin also clogged the sand filters, leading to a need for repeated backwashing, thus limiting the production of clean water. Species that clogged the sand filters included the needle-shaped diatom Synedra acus and the discoid diatom Stephanodiscus hantzschii f. tenuis. We also observed two cases where S. acus clogged WTP filters, requiring frequent backwashing that resulted in reduced production of drinking water and economic loss.

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