Public Water Supplies Precautions


If there is a bloom or a suspected bloom in the vicinity of a public water supply surface water intake, operators need to begin to take a few precautions to ensure proper treatment.


A PWS can observe source waters for possible bloom occurrence by visual inspection (see Identification), evaluating system effects, and recognizing bloom indicators (see Causes).


If a PWS finds that it is vulnerable to cyanotoxins, it may want to consider evaluating whether it has effective measures in place to respond to cyanotoxins in drinking water that are compatible with meeting other treatment goals. For example, the PWS may want to examine its raw water supply and treatment process to determine the likelihood of toxin release from intact cells either in the reservoir or at the raw water intake and the level of protection provided by the existing treatment. Examples of treatment evaluation questions that could be asked include:

  • Can the existing system relocate the intake to a greater depth and draw from multiple intake depths at one intake tower?
  • Is the existing treatment a conventional coagulation, clarification and filtration process that is likely to be effective for intact cell removal?
  • Can the existing system handle more frequent backwashing and more sludge in the event of a cyanobacterial bloom?
  • Are there any conditions such as pre-oxidation that could lead to cell lysis?
  • Is chlorination being operated adequately to oxidize cyanotoxins?
  • Can powdered activated carbon (PAC) be added at adequate doses?
  • Are there any advanced water treatment facilities with ozonation and (or) granular activated carbon (GAC) that can be used to effectively remove dissolved toxins?

System Effects

A bloom may impact water quality and treatment plant operations. The changes commonly associated with a cyanobacterial bloom are listed below. If a PWS determines that a bloom may be occurring through a weight of evidence evaluation of system effects, the PWS may want to proceed with raw water monitoring (Step 3), even if the plant intake does not show visual signs of a bloom being present.

  • Increased taste and odor. Some cyanobacteria can produce MIB and geosmin, which can cause taste and odor issues within the treatment plant and distribution system during cyanobacterial blooms. Other taste and odor impacts may not be associated with cyanobacteria (for example a fishy odor is typically associated with a diatom bloom, which would not produce cyanotoxins).
  • Increased pH. As cyanobacteria draw carbon dioxide out of the water during photosynthesis, pH tends to rise. Therefore, pH increases are more often observed when 12 cyanobacterial growth is expanding in water. Diurnal pH swings may also occur. As cyanobacteria draw carbon dioxide out of the water during photosynthesis, pH may increase throughout the day. Increases in pH could also be caused by green algae blooms, but routine phytoplankton monitoring would help distinguish between the two.
  • Increased turbidity. Treatment plants may experience higher turbidity, in some cases, in filter influent and effluent due to cyanobacterial growth and cell production.
  • Decreased filter run times. Treatment plants may experience shortened filter run times during cyanobacterial blooms to varying degrees depending on the species. For example, although not entirely the result of cyanobacteria, one water treatment plant reported that the average filter run time was every 24 to 48 hours in the summer – notably less than the 72 hours or more in the winter.
  • Need for increased coagulant dose. A higher coagulant dose is often needed, potentially due to increased turbidity and total organic matter in water during a cyanobacterial bloom or the tendency for some cyanobacteria to float and inhibit settling. One treatment plant reported an approximately 50 percent increase in its average alum dose in the summer.
  • Increased chlorine demand or decreased chlorine residual. The increased organic matter loading during a cyanobacterial bloom, if not adequately removed, could result in a higher chlorine demand. Treatment plants reported that the chlorine residual was decreased during algal growth events.

For further operation questions or procedures, please contact your district engineer.


This is an excerpt from the EPA Recommendations for Public Water Systems to Manage Cyanotoxins in Drinking Water, June 2015. For further detail, view their management plan here: EPA Cyanotoxin Management in Drinking Water