Trihalomethanes are formed as by-products under which condition?

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Trihalomethanes form as disinfection by-products when chlorine-based disinfectants react with natural organic matter that is present in source water. This chemical reaction during chlorination creates halogenated compounds, including the common THMs, such as chloroform. The formation is influenced by factors like the amount of organic matter, chlorine dose, contact time, and temperature, which explains why chlorinated drinking water can accumulate THMs. UV irradiation does not produce THMs because it doesn’t involve chlorine chemistry, and the other options describe processes unrelated to THM formation.