What two factors are essential to effectively kill pathogens?

To effectively kill pathogens, maintaining both chlorine residual and sufficient contact time is critical. Chlorine is a powerful disinfectant used extensively in water treatment processes to eliminate harmful microorganisms. For chlorine to be effective, it must remain in the water at a sufficient concentration, referred to as chlorine residual. This residual ensures that chlorine remains present long enough to interact with and inactivate pathogens.

Contact time is also crucial, as it refers to the duration that chlorine is present in the water before it is consumed or otherwise removed. Insufficient contact time may prevent the chlorine from having the opportunity to react with and effectively kill the pathogens. The combination of adequate chlorine levels and the appropriate time allows for comprehensive disinfection, leading to safer drinking water.

The other options, while they address various aspects of water quality and treatment processes, do not focus on the specific requirements for pathogen inactivation. Therefore, the emphasis on chlorine residual and contact time is what makes this choice the correct one for the question about effectively killing pathogens in water treatment.