What is the main benefit of using free chlorine in water treatment?

Using free chlorine in water treatment primarily offers the benefit of providing a residual disinfectant in the water distribution system. This residual helps ensure that chlorine remains present in the water as it travels through the pipes to reach consumers, effectively inhibiting the growth of harmful microorganisms along the way. The persistent presence of free chlorine is vital for maintaining water quality and safety after treatment, which is essential in preventing waterborne illnesses.

While other options relate to chlorine's effects or properties, they do not highlight the critical benefit of maintaining a disinfectant residual. For instance, while free chlorine can form trihalomethanes (THMs) which are by-products of disinfection, this is generally considered a negative aspect rather than a benefit. The stability of chloramine, in contrast, is a different topic altogether, focusing on its effectiveness and longevity as a disinfectant rather than the effectiveness of free chlorine in offering residual protection in the water supply.