Response to Detection of Cyanotoxins in Raw and Finished Drinking Water

A. Recommended Public Notification

Public notification may be required if cyanotoxins are detected in raw or finished drinking water. A Public Water System should consider providing public notification (i.e., a water use advisory) under the following circumstances:

  1. If cyanotoxins are detected only in raw water, public notification is not recommended by U.S. EPA. Raw water monitoring should continue according to the above protocol.
  2. If cyanotoxins are detected in finished water at a level below the health advisories, public notification is not recommended by U.S. EPA. Finished water monitoring should continue according to the above protocol.
  3. If cyanotoxins are detected and confirmed in finished water at a level at or above the health advisories, a Public Water System should issue a Do-Not-Drink/Do-Not-Boil advisory within 24 hours of receiving the results of the confirmation sample. Suggested public notification language can be found in Appendix E.
  4. If a Do-Not-Drink/Do-Not-Boil advisory is issued, a PWS should issue a second notice removing the advisory after two consecutive finished water samples are below the HALs.

B. Treatment for Cyanotoxins

U.S. EPA provides an overview of treatment strategies to remove or oxidize cyanotoxins found in raw water in Recommendations for Public Water Systems to Manage Cyanotoxins in Drinking Water (U.S. EPA, 2015a), and it provides more detailed cyanotoxin treatment information in Water Treatment Optimization for Cyanotoxins (Version 1.0) (U.S. EPA, 2016).

A public Water System that is vulnerable to cyanotoxin contamination is encouraged to assess the adequacy of treatment and, if necessary, make adjustments based on past experience treating cyanotoxins or recommendations by U.S. EPA. The specific treatment required may depend on the type of cyanotoxin present, the level of cyanotoxins in the raw water, and whether the cyanotoxins are intracellular or extracellular. Existing surface water treatment may be effective in reducing cyanotoxins to an acceptable level.

For example, conventional surface water treatment, (coagulation, flocculation, sedimentation, and filtration) has been shown to effectively remove intracellular cyanotoxins but has limited ability to remove extracellular toxins. In some cases, treatment adjustments or the addition of treatment processes may be needed.

Public Water Systems with existing surface water treatment facilities may be able to make treatment adjustments to respond sufficiently to cyanotoxins. U.S. EPA lists the following commonly used treatment strategies for reducing or eliminating cyanotoxins:

  1. Remove intact cells containing intracellular cyanotoxins first
  2. Minimize pre-oxidation of raw water to reduce the risk of releasing intracellular cyanotoxins
  3. Add or increase the use of powdered activated carbon to adsorb dissolved or extracellular cyanotoxins
  4. Increase post-chlorination to oxidize certain extracellular cyanotoxins (Caution: this could potentially increase the formation of Disinfection Byproducts)

If adjustments to existing treatment facilities, such as conventional filtration, do not sufficiently reduce cyanotoxin levels or a Public Water System does not have the ability to make treatment adjustments to respond to cyanotoxins, it may be able to shut down the intake and rely on another source, unaffected by cyanotoxins, until the threat from cyanotoxins has passed. If a water system is unable to shut down the source or treat to acceptable levels, it must consider installing additional treatment facilities. U.S. EPA suggests the below treatment options for removing intracellular and extracellular cyanotoxins.

Treatment Options for Removing Intracellular Cyanotoxins

  1. Dissolved Air Flotation
  2. Microfiltration and Ultrafiltration

The key to treatment of intracellular cyanotoxins is to remove cyanobacteria cells and to limit the release of toxins from the cells. This typically requires a water system to limit algaecide application to source water and to stop or limit pre-oxidation during treatment. A water system must consider, however, how changes in pre-oxidation affect other treatment goals.

Treatment Options for Removing Extracellular Cyanotoxins

  1. Oxidation
  2. Granular Activated Carbon
  3. Biological Filtration
  4. Nanofiltration and Reverse Osmosis
  5. Ultraviolet Light with Hydrogen Peroxide

Once cyanotoxins have been released by cyanobacteria, cell removal does little to reduce toxins dissolved in the water. Therefore, conventional surface water treatment, microfiltration, and ultrafiltration are no longer effective. Extracellular, or dissolved, cyanotoxins must be removed through treatment processes such as adsorption, molecular filtration, or oxidation. Oxidation by chlorine, potassium permanganate, ozone, and UV vary in their effectiveness to remove a given cyanotoxin. General effectiveness of oxidizing agents on various cyanotoxins is described in U.S. EPA’s Water Treatment Optimization for Cyanotoxins (Version 1.0) (U.S. EPA, 2016).

A Public Water System can consult with the Division of Drinking Water to review the adequacy of treatment for cyanotoxins or to plan to install additional treatment. Note: the installation of treatment for cyanotoxins requires plan approval and an operating permit from the Division of Drinking Water.