Algal Toxins In Your Drinking Water

What are they?

Numerous algal blooms occur every summer in the U.S., including a major one a few months ago that resulted in a “do not drink” order and major economic consequences at the public water system on Western Lake Erie in Toledo, Ohio. Cyanobacteria are among the most genetically diverse microorganisms

and they exist in almost all world locations and environmental niches, including

extremes of heat and salinity. They are given credit for converting the earth from an

anaerobic to aerobic environment by their ability to convert carbon dioxide into

oxygen by photosynthesis. Some of them produce potent toxins. The most common

algal toxins are microcystins, anatoxins, cylindrospermopsins and saxitoxins.

Microcystins are usually found at the highest concentrations. Each of the toxins

have congeners that are very similar chemical structures with differing toxicities.

Some people consume edible cyanobacteria as food or supplements. There are many

projects underway attempting to cultivate algae as a source of biofuels including oils

and alcohol, and some use sewage as a source of nutrients. Claims have been made

that smaller areas of algae can produce significantly more biofuel than corn biomass conversion and with less energy required.

There are numerous algal toxins including those that occur in the sea (red tides), and those that are of concern in fresh waters that are drinking water sources and recreational waters. We will concentrate on the fresh waters, particularly those associated with blue-green algae, which are also called cyanobacteria. The name cyanobacteria is a bit confusing because it is related to the blue-green color (cyan) of the algae and has no association with cyanide. Some argue that technically algae (eukaryotes) are not as simple as bacteria (prokaryotes), but we won’t try to settle that academic debate.

Where and how do they grow?

Cyanobacteria are commonly present in surface waters and only some species produce toxins; however they become a significant problem when they proliferate and produce unsightly biomass (algal blooms) and release toxins, especially when they die and decompose. They proliferate under excessive nitrogen and phosphorus nutrient conditions, sunlight and usually in slow moving water. The cells lyse after death and release most of the accumulated toxins into the water; decomposition depletes the oxygen in the water and produces eutrophic conditions that are detrimental to the aquatic life.

How do they get into drinking water?

Drinking water treatment plants drawing from source surface waters prone to summer algal blooms have to contend with potential algal blooms and be prepared to respond rapidly. Observable source water conditions could include color, suspended cells and filaments, surface scum and huge masses of aggregated algae. Frequently production of potent taste and odor producing compounds like geosmin and 2-methylisoborneol occur, generating musty off tastes in the drinking water that some people can detect at less than 20 ppt.

Water Treatment

Water treatment approaches include physical removal, chemical conversion and adsorption.

Conventional treatment that includes coagulation, flocculation, sedimentation and chlorine disinfection can remove most of the algal cells, but toxin removal is more problematic. It is essential to remove the algal cells by filtration prior to the addition of any oxidant such as chlorine. The oxidant will lyse the cells and release the toxins into the drinking water.

Oxidants/disinfectants such as chlorine, chloramine, chlorine dioxide, ozone, potassium permanganate and ultraviolet (UV) light are frequently available in a water treatment facility, or can be added, and provide a range of efficacies against the toxins. The following relate to Microcystin-LR, which is a common form and among the most potent:

Ozone will rapidly lyse the cells; it is effective against the toxin at ozone doses of five ppm or less and at very low concentration-time values (CT in mg-min/l). CT means concentration in mg/l x time in minutes, so for example, a concentration of one mg/l for 10 minutes would be the same as 10 mg/l for one minute. Elimination of Microcystin-LR is virtually instantaneous.
Potassium permanganate will lyse the cells and is also a very effective, rapid treatment for the toxin. The CT value for complete elimination is about 25 mg-min/l.
Free chlorine will lyse the cells and it is very effective, achieving nearly complete elimination at CT of about 60 mg-min/l. Chlorine is present in almost every surface water treatment plant and probably functions both as an oxidizing and chlorinating agent.
Chlorine dioxide is a good disinfectant and lyses cyanobacteria cells, but it has no reactivity toward the toxin.
Monochloramine has some reactivity against the cells but not towards the toxin.
UV light irradiation at high doses has a destructive effect on the cells but does not affect toxin concentrations.

Membranes such as reverse osmosis (RO), nanofiltration, ultrafiltration and microfiltration are all effective for the removal of cells. Pretreatment and frequent cleaning could be necessary. RO and nanofiltration, but not microfiltration, would be effective for toxin removal.