Explain the chemical pathway by which free chlorine reacts with natural organic matter to form trihalomethanes, and state a primary control strategy.

Free chlorine, used as a disinfectant in water treatment, reacts with natural organic matter (NOM) present in raw water to form trihalomethanes (THMs). This chemical pathway involves a series of steps. Natural organic matter is a complex mixture of organic compounds, such as humic and fulvic acids, originating from the decomposition of plant and animal material in natural waters. These NOM molecules contain various electron-rich functional groups, including activated aromatic rings and sites with double bonds or alkyl chains, which serve as precursors for THMs. The first step involves the electrophilic attack of free chlorine on these NOM precursors. Free chlorine exists in water primarily as hypochlorous acid (HOCl) and hypochlorite ion (OCl-). HOCl, being an electrophile, attacks the electron-rich sites on the NOM molecule. This initial reaction is often an electrophilic substitution, where a hydrogen atom on the NOM is replaced by a chlorine atom, leading to the formation of chlorinated organic intermediates. For example, HOCl can add to double bonds or substitute onto aromatic rings within the NOM structure. Among the various chlorinated organic intermediates, those possessing or able to isomerize to structures....