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Wastewater chlorination adds chlorine compounds to wastewater that produce “free chlorine,” which destroys pathogens by damaging cell membranes, disrupting cellular respiration, and rendering enzymes non-functional. Despite some environmental concerns, chlorination remains popular due to its effectiveness, reliability, and cost-efficiency.
In the modern world, almost all of us take clean water for granted. But there is a fascinating and complex process behind every glass, one that keeps people healthy. Of these processes, the best-known and widely-used water disinfection method worldwide is chlorination.
Whenever you wash dishes, take a shower, or flush a toilet, you create wastewater. This water contains many hazardous contaminants and must be treated before it can properly re-enter the environment. Wastewater is produced from two main sources, residential and commercial.
Residential wastewater comes from daily home-related activities. Commercial wastewater, however, is that which originates from establishments such as auto repair shops, beauty salons, manufacturing plants and furniture refinishing businesses. This commercial wastewater generally includes more hazardous chemicals than its residential counterpart, so its treatment is even more important.
Pathogenic microorganisms, such as viruses, bacteria, and protozoa, are some of the most harmful contaminants of untreated wastewater that can lead to waterborne diseases. These include:
If not appropriately disinfected, these pathogens can pollute potable water sources, recreational waters, and natural ecosystems, creating very real threats to public health. In fact, waterborne illness was the norm in the United States until the 1900s when chlorination began as a common practice.
For example the first discovery of Chlorine was in Sweden in 1744. Interestingly, early humans believed that the odor of the water was a carrier of the disease. It was not until 1835 that chlorine was first used to remove odors from water.
The real breakthrough didn’t occur until 1890 when chlorine was identified as an effective disinfectant, reducing the spread of waterborne illness. This finding prompted chlorination across Great Britain, then the United States (1908), and Canada (1912).
Today, chlorination is the most widely used method of water disinfection at the global level and has been effectively protecting public health for over 100 years.
Chlorination works by adding chlorine or chlorine-containing compounds to water. When chlorine is added to water, it produces hypochlorous acid (HOCl) and hypochlorite ions (OCl-), collectively known as “free chlorine.” These powerful oxidizing agents destroy pathogens through several mechanisms:
The effectiveness of chlorination depends on several factors:
Three main types of chlorine are used in wastewater treatment:
When chlorine gas is added to water, it hydrolyzes rapidly:
Cl₂ + H₂O → HOCl + H⁺ + Cl⁻
The reaction with water:
NaOCl → Na⁺ + OCl⁻
The reaction with water:
Ca(OCl)₂ → Ca²⁺ + 2OCl⁻
Chlorination can be integrated at various stages of the wastewater treatment process:
Applied at the beginning of treatment to:
Applied after sedimentation and before filtration to:
The most common application point, after filtration as the final step. This ensures water is properly disinfected before discharge or distribution.
Sometimes performed within the distribution system to maintain required residual chlorine levels throughout the network.
One of the most important concepts in chlorination is “breakpoint chlorination”. When chlorine is first added to water, it reacts with various reducing agents and ammonia to form chloramines. As more chlorine is added, a point is reached where all these reactions are complete, and any additional chlorine remains as free chlorine.
The breakpoint chlorination process follows these steps:
Understanding and achieving breakpoint chlorination is essential for effective water disinfection, as free chlorine is a more powerful disinfectant than combined chlorine (chloramines).
Chlorination offers numerous benefits beyond pathogen removal:
While chlorination effectively disinfects wastewater, residual chlorine can harm aquatic life when discharged into natural water bodies. This necessitates dechlorination—the removal of excess chlorine before final discharge.
Dechlorination involves adding reducing agents such as:
These agents react with free chlorine, converting it to chloride ions (Cl⁻) that are harmless to aquatic organisms. This process ensures that treated wastewater can be safely returned to the environment without ecological damage.
A significant concern with chlorination is the formation of disinfection by-products when chlorine reacts with organic matter in wastewater. The most concerning DBPs include:
These by-products have been associated with potential health risks, including increased risk of bladder and colon cancer with long-term exposure. However, health authorities generally agree that the benefits of chlorination in preventing waterborne diseases outweigh these risks.
Chlorine and its by-products can affect aquatic ecosystems when discharged into natural water bodies. Even at low concentrations, residual chlorine can be toxic to fish and other aquatic organisms. This environmental concern underscores the importance of proper dechlorination before discharge.
Working with chlorine requires strict adherence to safety protocols:
Municipal water facilities typically use chlorination as the final disinfection step before discharge. The process must balance effective pathogen removal with controlling DBP formation. Advanced systems often incorporate dechlorination to protect receiving waters.
Industries such as chemical processing, petrochemical production, metal finishing, and food processing generate wastewater with specific contaminants. Other industries with significant wastewater challenges include:
Chlorination in these contexts addresses:
In hospitals, laboratories, and other sanitation-critical facilities, chlorination plays a vital role in addressing potentially hazardous biological contaminants. These applications typically require higher chlorine dosages and more rigorous monitoring.
Chlorination has been pivotal in making drinking water safe. Before the widespread adoption of chlorination in the early 20th century, waterborne diseases like typhoid, dysentery, and cholera were common. Since then, these diseases have virtually disappeared in developed countries with chlorinated water supplies.
Beyond pathogen removal, chlorination of drinking water also helps reduce iron and lead levels, which can cause taste issues. It also ensures that some residual disinfectant remains in the water from the treatment facility to the consumer’s tap.
While chlorination remains one of the most effective and popular disinfection methods, alternatives include:
Many facilities now implement multi-barrier approaches, combining chlorination with other methods to maximize effectiveness while minimizing drawbacks.
The field of wastewater chlorination continues to evolve with several emerging trends:
Wastewater chlorination represents a critical component of modern water treatment infrastructure. Despite challenges related to by-product formation and potential environmental impacts, it remains the most widely used disinfection method due to its effectiveness, reliability, and cost-efficiency.
If you would like to learn more about chlorination or how we can help with your wastewater monitoring needs, do not hesitate to contact the world-class team at Atlas Scientific.
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