Basics of Addressing Drinking Water Supply Algal Blooms


If your water utility has a surface water supply, it may be susceptible to harmful algal blooms (HABs) and cyanotoxins.  Cyanotoxins are produced by bacteria that are also known as blue-green algae.  Under certain conditions, cyanotoxins can reproduce exponentially and produce blooms.  Not all algal blooms are toxic.  But, for those that contain cyanotoxins and are toxic, the water treatment must be effective so that public health and safety is protected. So here are some things to consider:

  • Do you monitor your surface supply for parameters that may indicate favorable conditions for HABs?
  • Do you know how to optimize your water treatment plant for the removal of algal toxins?
  • How are you going to avoid having to issue a health advisory (HA) for algal toxins in the water delivered to your customers?

The Regulations

In June, 2015, the US EPA published guidance for “do not drink” limits for cyanotoxins for issuing a health advisory (HA).  The Ohio EPA has adopted the US EPA guidance.  Basically, the policy says that if a sample taken 24 hours after initially detecting toxins in the finished water is above the Health Advisory limits, Ohio EPA is recommending that a Health Advisory be issued.


The ug/L or parts per billion limits are very small amounts equivalent to one second in 32 years, and there is not much room for error in the testing or the treatment necessary to remove any toxins present and to ensure that the water is safe to drink.

Getting Started

The best treatment starts with effective monitoring.  Screening the raw water may include phytoplankton identification and cell counts, phycocyanin, chlorophyll-a, pH, and taste and odor. Several Lake Erie water utilities now deploy monitoring instruments on buoys during the HAB season or warm weather months.  Here’s a map from the Great Lakes Observing System.

A utility may also observe changes in operational parameters at the water treatment plant.  The pH may increase, filter runs may shorten, chlorine demand may increase, and there may be taste and odor.  If conditions are favorable for an algal bloom, cyanotoxin monitoring may be next step.  Samples can be sent to the Ohio EPA’s Columbus Laboratory which has the capability to do the testing.

Optimizing Your Water Treatment Plant for Algal Toxin Removal

Think of your water treatment plant as a series of barriers.  Depending on the life cycle stage of the HAB, some of the algal toxins may have already been released from dying algal cells and be in solution as raw water enters the plant.  Small dosages of potassium permanganate can oxidize these toxins and also condition the water for better settling.  Too much permanganate, though, may damage or lyse the cell walls and release the toxins from the cells.

The most effective way to remove algal toxins is while they are still within the cyanobacteria cell.  If the cells are lysed, any toxins will spill out and go into solution where they are tougher to remove. Conventional coagulation/flocculation/settling can remove 90% or more of the algal cells.  “Enhanced coagulation,” that your plant may have studied for improved trihalomethanes (THM) treatment, will also improve settling of toxic algal cells.

Powdered activated carbon (PAC) can be fed to adsorb some of the algal toxins remaining in solution.  If permanganate is also fed ahead of the PAC, it should be separated from PAC, because, if fed too close together, PAC will adsorb the permanganate, reducing the effectiveness of both chemicals.  PAC can adsorb and remove 70% of the algal toxins in solution.

The filters will remove about 90% of any algal cells that did not settle out.  If the water is still over the HA limits, the final treatment step of disinfection by chlorination is very effective in oxidizing microcystin, normally the most prevalent form of algal toxins.  Depending on the pH, chlorine concentration, and detention time, about 85% of the microcystin may be oxidized.

Some Final Thoughts

If HABs are sporadic throughout the warm weather season and raw water toxins never get above 10 ppb, optimizing conventional treatment processes, as described above, can be effective enough to keep algal toxin levels below HA limits.  If HAB are frequent and levels get higher, an engineering professional should review the operation and maintenance costs and compare them to the capital cost of more advanced processes such as ozone and granular activate carbon.

Finally, keep in touch with other communities that have similar HAB issues, and be familiar with Ohio EPA’s Algal Bloom Response Strategy.  Working together, we can keep our drinking water safe from algal toxins.

Brian T. BissBissonB-Blogon, P.E., Senior Project Manager

Brian serves as a Senior Project Manager with CT Consultants after spending twenty-two years with Aqua Ohio, Inc. where he was responsible for planning, organizing, directing, staffing and controlling the engineering function. Mr. Bisson is currently assisting several clients, including the City of Sandusky with optimizing treatment plant operations to handle the harmful algal blooms of Lake Erie.  His water treatment experience ranges from conventional surface water turbidity removal water treatment plants to lime softening of groundwater supplies to excess lime/softening of surface water.  Brian can be reached at (330) 746-1200 or bbisson[at]

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