Blue-green algae, or cyanobacteria, and other algae include a number of species of microscopic aquatic organisms that perform photosynthesis to produce energy and oxygen. They produced Earth’s earliest fossils.

Often identified as "pond scum," some are important food sources but others put out toxic compounds called cyanotoxins that are harmful to humans and other animals.

Green scum produced by and containing cyanobacteria.

Cyanobacteria also produce organisms that give water a musty odor. Reports of algae-caused unpleasant taste and odor in drinking water are common. US Environmental Protection Agency (EPA) is funding research to improve detection of cyanotoxins. Detection must be performed in an analytical laboratory.

Although they are vnvisible in surface water in low numbers, they multiply rapidly into clumps or “blooms” where water is warm, shallow and undisturbed. Blooms can look green, blue-green, yellow, brown or red.

Health effects:

According to Water Technology magazine, "Algae-containing water can be swallowed, can contact the skin or is inhaled in aerosol-like droplets while swimming or showering. Cyanotoxins can cause gastrointestinal and lung ailments; allergic responses; skin and eye irritation; liver damage; tumor growth; and neurotoxic (nerve) reactions. Effects of long-term consumption uncertain."

Regulation:

Though many countries and some US states have established mandatory or voluntary standards, the US EPA continues to monitor cyanobacteria only as an "unregulated contaminant." The World Health Organization guideline for the cyanotoxin microcystin LR in drinking water is 1 microgram per liter.

Water treatment for algae and cyanotoxins.

• Activated carbon is effective and can resolve cyanobacteria-caused taste and odor issues.
• Reverse osmosis (RO), nanofiltration and ultrafiltration will remove/reduce single-cell cyanobacteria, and may also help reduce cyanotoxins.
• Disinfection methods (such as chlorination) may kill cyanobacteria but not eliminate the cyanotoxins they release.

One pool expert writes, "The ONLY sure-fire method of destroying waterborne algae is by running the water through an ultraviolet light. UV lights sterilize the algae and prevent it from reproducing. It will clear up the water and keep it clear. When installed correctly, a UV will show outstanding results within a matter of a day or two."

Main Source: Water Technology Volume 32, Issue 10 - October 2009. Other information added from various sources.

Sources Cited by Water Technology: US EPA, US Centers for Disease Control and Prevention, New York State Department of Health, California Department of Public Health, University of California.