The objectives of this work were to: find optimum coagulation conditions for the removal of diatoms prior to filtration; and, determine the effects of algae concentration, algae size and filter media size on filter run time. In some instances, full-scale plant studies showed that filter run times decreased sharply, independently of the diatom concentration. The reasons why rapid head loss developed irrespectively of the diatom concentration were also investigated with scanning electron microscopy. Filter run times of five Korean water treatment plants were monitored to analyze filter clogging problems from January 1997 to May 1998. Filter media were sampled from seven different filters including sand and dual media filters and sieved to calculate their effective sizes and uniformity coefficients. Includes 7 references, tables, figures.