Which statement best describes how disinfection byproducts such as trihalomethanes form?

Which statement best describes how disinfection byproducts such as trihalomethanes form?

• A. Disinfection byproducts form when chlorine is not added.
• B. Disinfection byproducts form when chlorine reacts with inorganic minerals.
• C. Disinfection byproducts form during chlorination but not with organic matter.
• D. Disinfection byproducts form when chlorine reacts with organic matter to form compounds such as trihalomethanes.

Answer: (D)

Disinfection byproducts form when chlorine interacts with organic matter already present in the water during the disinfection step. Natural organic matter, such as humic substances, provides the carbon structures that chlorine can modify, creating chlorinated organic compounds like trihalomethanes. This is why the presence of organic material is essential for THMs to form—without organic matter, chlorine mainly acts to kill microbes and does not generate these byproducts. Inorganic minerals alone don’t lead to THMs, and THMs are specifically associated with chlorine reacting with organic precursors.

Disinfection byproducts form when chlorine interacts with organic matter already present in the water during the disinfection step. Natural organic matter, such as humic substances, provides the carbon structures that chlorine can modify, creating chlorinated organic compounds like trihalomethanes. This is why the presence of organic material is essential for THMs to form—without organic matter, chlorine mainly acts to kill microbes and does not generate these byproducts. Inorganic minerals alone don’t lead to THMs, and THMs are specifically associated with chlorine reacting with organic precursors.