Cyanobacteria in Drinking Water

Cyanobacteria are microscopic organisms found naturally in lakes, streams and ponds. Once known as blue-green algae, they are actually photosynthetic bacteria.

When present at low density they do not cause recreational or aesthetic problems. However, when conditions are optimal for their growth, these organisms can sometimes multiply to form harmful cyanobacteria blooms (HCBs) that can potentially produce toxins capable of affecting liver and kidney functions or the central nervous system, or causing skin irritations.

In addition to producing toxins, cyanobacteria can pose other treatment challenges for public water suppliers, including taste and odor issues and shortened filter run times. While not regulated as drinking water contaminants, some cyanobacteria toxins have been included among the parameters that certain public water systems using surface water have been required to monitor under the Unregulated Contaminants Monitoring Rule beginning in 2018.

In 2015, USEPA issued health advisory levels for two cyanobacterial toxins. The information below is provided to guide drinking water suppliers in preventing, identifying and responding to cyanobacteria blooms.

More information is available in the Cyanobacteria and Drinking Water: Guidance for Public Water Systems fact sheet and the EPA webpage Cyanobacterial toxins monitored under the Fourth Unregulated Contaminant Monitoring Rule (USEPA).