Water supplies meeting treatment requirements

Water Treatment & Algal Toxins


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Algal Toxins


Lake Superior


• Excess nutrients in water bodies may result in Harmful Algal Blooms or HABs, largely made up of one or more species of cyanobacteria, also known as blue-green algae. HABs may produce toxins harmful to human and animal health.
• The U.S. EPA has created health advisory levels for specific cyanotoxins ranging from 0.3-3 micrograms per liter depending on the type of toxin and target population
• To determine the best technique to treat for algal toxins, managers need to consider the type of cyanotoxin and whether the toxin is encased within the bacterial cell or not

Where does it come from?

The growth of excess algae in surface water bodies, or Harmful Algal Blooms (HABs) are often caused by elevated nutrient levels, high water temperatures, and low water flows. These HABs can support the growth or blooms of cyanobacteria, a group of photosynthetic bacteria, some of which release a group of algal toxins called cyanotoxins.[1] IIn high doses these toxins cause human health risks and create taste and odor issues for drinking water.[2] The conditions to form cyanotoxins are not well understood and cyanotoxin levels can vary from year to year, even in the same water body.

How is it treated to meet applicable standards?

The U.S. EPA has created health advisory levels for specific cyanotoxins and some states such as Ohio have taken action to enact standards. Microcystins levels are advised by the U.S. EPA to be at or below 0.3 micrograms per liter for children under six and 1.6 micrograms per liter for people over six. Health Canada sets a maximum acceptable concentration at 1.5 micrograms per liter.[10] Similarly, the maximum contaminant level for cylindrospermopsin is set at or below 0.7 microgram per liter for children under six and 3.0 micrograms per liter for people over age six according to the U.S. EPA. [3,4]

Traditional water treatment processes can treat for cyanobacteria and toxins when they are present at low levels. During bloom periods, cyanotoxins run the risk of breaking through treatment processes and entering drinking water. Active management and monitoring are important steps to reduce this risk of contamination.[5]

Within treatment types there are options to remove cyanotoxins while they are encased in cyanobacterial cells (the intracellular stage) versus options to remove toxins which have been released from cells (the extracellular stage).[7] To determine the best technique to treat for algal toxins, managers need to consider the type of cyanotoxin and whether the toxin is in the intracellular or extracellular stage. Removing the cyanobacteria can remove the toxin in the intracellular stage, but extracellular toxins are harder to remove as they require physical removal through chemical or adsorptive means. [6,7]

The effectiveness of intracellular removal techniques varies based on the conditions of the water body and the level of cyanobacteria present. The conventional treatment of surface water, defined as the sequential use of coagulation, flocculation, sedimentation, and filtration provides generally sound cyanobacterial removal for intracellular cyanotoxins (intact cells). This method requires evaluation to reduce the cell breakthrough and release of toxins and ensure that sludge does not return cyanotoxins to the supply. [6,7] Flotation is also an effective intracellular removal technique since many cyanobacteria are buoyant. This includes Dissolved Air Flotation (DAF). [6,7]

Membranes offer an effective method in that microfiltration and ultrafiltration will efficiently remove cyanobacteria when the cells are not allowed to accumulate for long period of time. Thus, cleaning may need to be done more frequently during blooms. [6,7,8] Pre-treatment oxidation frequently runs the risk of lysing cyanobacteria and releasing toxins. Managers should make decisions to either use low doses of oxidant or use sufficiently high doses to destroy total toxins. [6,7,8]

Extracellular removal is often harder and dependent on the conditions of the bloom and water quality. Reverse osmosis and nanofiltration are effective at removing some types of extracellular toxins, but lysis is likely and not all cyanotoxins can be removed from these membrane techniques. [6,7,8] Oxidation may be an effective treatment for some cyanotoxins. Potassium permanganate, ozone, and free chlorine have been found to be efficient at removing some types of algal toxins, although this is dependent on the pH and which toxins are present in the drinking water. [6,7,8] Additionally, increasing some oxidants will create higher levels of regulated disinfectant byproducts in finished water.

Chloramines and chlorine dioxide have not been found to be effective oxidation methods. UV radiation is not an effective treatment option for cyanotoxins when used alone, however advanced oxidation which combines UV radiation with ozone or hydrogen peroxide is effective treatment for some cyanotoxins at high levels. [6,7,8]

Activated carbon adsorption is an accepted treatment technique for removing cyanotoxins in the extracellular stage. Both powdered activated carbon (PAC) and granular activated carbon (GAC) are effective based on the type of carbon, type of toxin, other water quality parameters, and pore size. [6,7] Treatment systems are estimated to cost approximately $0.45/1000 gallons of treated water.[9]GAC may need to be regenerated more often to ensure adsorption capacity during blooms. Optimized biological filtration has been shown to be effective at removing most cyanotoxins through biodegradation, but many factors cannot be easily controlled in this process such as acclimation periods and water quality. [7,8]



  1. Water Quality & Health Council, Harmful Algal Blooms: Cyanobacteria and Safe Drinking Water (2018). Accessible online: https://waterandhealth.org/safe-drinking-water/treatment/harmful-algal-blooms-cyanobacteria-and-safe-drinking-water/
  2. EPA, Harmful Algal Blooms and Drinking Water Factsheet (2016). Accessible Online: https://www.epa.gov/sites/default/files/2016-11/documents/harmful_algal_blooms_and_drinking_water_factsheet.pdf
  3. EPA, 2015 Drinking Water Health Advisories for Two Cyanobacterial Toxins (2015). Accessible Online: https://www.epa.gov/sites/default/files/2017-06/documents/cyanotoxins-fact_sheet-2015.pdf
  4. EPA, EPA Drinking Water Health Advisories for Cyanotoxins. Accessible online: https://www.epa.gov/cyanohabs/epa-drinking-water-health-advisories-cyanotoxins
  5. EPA, Managing Cyanotoxins in Public Drinking Water Systems. Accessible Online: https://www.epa.gov/ground-water-and-drinking-water/managing-cyanotoxins-public-drinking-water-systems
  6. American Water Works Association and the Water Research Foundation, Managing Cyanotoxins in Drinking Water: A Technical Guidance Manual for Drinking Water Professionals (2016). Accessible online: https://www2.illinois.gov/epa/topics/water-quality/monitoring/Documents/ManagingCyanotoxinsinDrinkingWater_Sept2016.pdf
  7. Ohio EPA, DRAFT WHITE PAPER ON CYANOTOXIN TREATMENT (2016). Accessible online: https://epa.ohio.gov/static/Portals/28/documents/hab/Algal%20Toxin%20Treatment%20White%20Paper%20FINAL%20November%202016.pdf
  8. EPA, Water Treatment Optimization for Cyanotoxins (2016). Accessible online: https://www.epa.gov/sites/default/files/2018-11/documents/water_treatment_optimization_for_cyanotoxins.pdf
  9. Association of State Drinking Water Administrators, PFAS Treatment Options and Considerations for Drinking Water Utilities (Webinar) (2018) Study by Purolite. Accessible online: https://www.asdwa.org/past-events-webinar-recordings/?mgs_158=pfas&mgi_158=14529/pfas-treatment-options-and-considerations-for-drinking-water-utilities
  10. Government of Canada, Guidelines for Canadian Drinking Water Quality – Summary Table (2020). Accessible online: https://www.canada.ca/en/health-canada/services/environmental-workplace-health/reports-publications/water-quality/guidelines-canadian-drinking-water-quality-summary-table.html