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Seattle’s water supply is extremely well protected from contamination. The water supplied by Seattle Public Utilities meets all current drinking water standards and is safe to drink. The Cedar River source water, and to a lesser extent on the Tolt, do experience naturally occurring, seasonal algae blooms. Typically the blooms occur in the late spring, but due to a number of environmental factors such as sunlight and temperature, blooms can occur at unexpected times of the year. For similar reasons some blooms are more intense than others. Although the algae we see in our supplies are not associated with health concerns, they can create tastes and odors. These are well controlled with current ozone treatment.
Since the Cedar supply is unfiltered, customers who filter water at home may experience their filters clogging sooner than usual during an algal bloom. Your filter may appear to be covered with clay or plaster. To help alleviate filter clogging, an inexpensive pre-filter can be installed and periodically removed, cleaned with a brush or just replaced with a new one.