Eutrophication, the process by which water bodies receive excess nutrients like nitrogen and phosphorus, has become a growing concern for superficial freshwater systems worldwide. One of the most significant consequences of eutrophication is the occurrence of cyanobacterial blooms. These blooms, which involve the rapid growth of blue-green algae (cyanobacteria), are particularly troubling because 50-70% are toxic. These harmful algae produce cyanotoxins and release taste and odor compounds, creating severe water quality issues. The contamination of water supplies intended for drinking, recreational, and agricultural use poses significant public health and environmental hazards.
The USEPA writes this about cyanobacteria:

"Cyanobacteria, also referred to as blue-green algae, are a type of photosynthetic bacteria that naturally occur in freshwater ecosystems. However, too many nutrients such as phosphorus and nitrogen in the waterways with warm temperatures can result in conditions that lead to cyanobacterial blooms. These blooms can often be identified as floating mats of bad-smelling, decaying, and gelatinous scum in lakes, but can also form mats on the bottom of streams.

When cyanobacteria degrade, they may release toxins that can harm aquatic and human life. One such toxin (also known as a cyanotoxin) is microcystin. Although there are relatively few documented cases of severe human health effects, exposure to cyanobacteria or their toxins may produce allergic reactions such as skin rashes, eye irritations, respiratory symptoms, and in some cases gastroenteritis, liver and kidney failure, or death. The most likely pathway to exposure for humans is through accidental ingestion or inhalation during recreational activities in the water body.

Cyanobacteria can indicate how safe a water body is for recreational use. When high levels of cyanobacteria are present in a water body, the biological condition of the water resource may also be degraded, as the condition that allows for cyanobacterial growth is typically high in nutrients and low in dissolved oxygen.  Due to decreased light penetration, large cyanobacteria blooms may also affect benthic macroinvertebrates and submerged aquatic plants."

The Dangers of Microcystins

Microcystins (MCs) are particularly harmful cynatoxins produced by cyanobacteria. These toxins are cyclic heptapeptides, a chemical structure that makes them strongly hepatotoxic causing severe liver damage. Microcystins are also linked to tumor-promoting activities because they inhibit essential enzymes known as protein phosphatases. Their toxic effects have resulted in the deaths of wild and domestic animals and have been associated with various cases of human illness and even fatalities.
 

Combating Cyanobacteria: Current Methods

Various methods are being employed to address the growing threat of cyanobacteria and their toxins. These include physical, chemical, and biological techniques such as:

• Flocculation – clumping together the algae to make it easier to remove
• Sedimentation – allowing particles to settle at the bottom for removal
• Floatation – lifting particles to the surface for skimming off
• Oxidation – chemically breaking down harmful substances
• (Bio-)filtration – filtering out contaminants using natural or artificial filters

The Role of Advanced Oxidation Processes (AOP)

Among these strategies, chemical oxidation stands out as a particularly effective approach. Advanced oxidation processes (AOP) are widely used for:
· Killing cyanobacteria
· Detoxifying cyanotoxins
· Removing volatile substances released by algae
Research has shown that chemicals such as chlorine, permanganate, and UV photolysis in the presence of titanium dioxide (TiO2) can effectively oxidize microcystins, transforming them into non-toxic byproducts under the right conditions.

The Power of Ozone in Water Treatment

One of the most promising tools in water treatment for managing cyanotoxins is ozone (O3), a powerful oxidizing agent. Ozone is already widely used in treatment plants to:

• Control taste and odor problems
• Remove color
• Prevent biological growth

Ozone's effectiveness extends to the degradation of microcystins. It works through two mechanisms:
1. Direct molecular reaction: Ozone directly reacts with microcystins, breaking down their chemical bonds.
2. Indirect radical reaction: Ozone generates hydroxyl radicals (•OH), which are highly reactive and further degrade microcystins.
The unique structure of microcystins, particularly their double bonds and amine groups, makes them susceptible to oxidation by ozone.

Advantages of Ozonation in Drinking Water Treatment

Ozonation offers several advantages over other oxidation methods:

• Destructive process with fewer harmful byproducts: Ozonation produces fewer toxic byproducts than chlorine.
• Rapid oxidation: Ozone can oxidize contaminants more quickly than methods like UV/TiO2.
• No secondary pollution: Ozone does not lead to further contamination, unlike permanganate.

However, the efficiency of ozonation in removing algae and its metabolites depends on various factors, including the ozone dose and the presence of other organic materials in the water. Organic matter can compete with microcystins for ozone, potentially reducing the effectiveness of the treatment.

Conclusion

As cyanobacterial blooms threaten water quality, advanced treatment methods like ozonation are critical in mitigating their impact. While challenges remain, particularly in optimizing ozone doses and dealing with competition from other organic materials, ozone remains a promising and effective tool for degrading harmful cyanotoxins and ensuring safe drinking water. As water treatment technologies evolve, the integration of ozone-based systems will likely become even more essential in safeguarding water resources from the dangers of toxic cyanobacterial blooms.

 

References:

The Mechanisms of Ozonation on Cyanobacteria and its Toxins Removal https://www.sciencedirect.com/science/article/abs/pii/S1383586608004462#preview-section-abstract