Why isn't chlorine injected immediately prior to filtration?

Injecting chlorine immediately prior to filtration is avoided primarily because it can lead to an increase in trihalomethanes (THMs). THMs are organic compounds that can form when chlorine reacts with natural organic matter present in the water. This reaction typically occurs when chlorine concentration is high, particularly in the presence of various organic materials, which are often more abundant before filtration. By delaying the chlorination process until after filtration or using alternative disinfection methods, water treatment facilities can minimize the formation of THMs, thereby ensuring safer drinking water and better adherence to regulatory standards.

The other options do not directly address the core concern about chlorination prior to filtration. While factors like filter lifespan and chlorine efficacy may be relevant in different contexts, the critical issue with respect to the formation of THMs takes precedence in this scenario.